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[vsnet-grb-info 39755] LIGO/Virgo/KAGRA S250904ae: Identification of a GW compact binary merger candidate
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41689 SUBJECT: LIGO/Virgo/KAGRA S250904ae: Identification of a GW compact binary merger candidate DATE: 25/09/04 04:40:51 GMT FROM: Trang Nguyen <trang.nguyen(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250904ae during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-04 03:33:07.667 UTC (GPS time: 1440992005.667). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], and PyCBC Live [6] analysis pipelines. S250904ae is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250904ae The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1021 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3330 +/- 883 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41689. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39754] GRB 250903A: EP-FXT counterpart detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41688 SUBJECT: GRB 250903A: EP-FXT counterpart detection DATE: 25/09/04 02:56:02 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. Z. Wu (HUST), R.-Z. Li (YNAO, CAS), M. J. Liu and H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250903A (SVOM/sb25090304, Maiolino et al. GCN 41677) at 2025-09-03T18:20:54 (UTC), about 1 hour after the SVOM/ECLAIRs trigger, with an exposure time of 6300s. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Maiolino et al. GCN 41677, An et al. GCN 41679, Götz et al. GCN 41681, Xin et al. GCN 41683, GCN 41684, Pérez-Fournon et al. GCN 41685). Preliminary analysis on this source are automatically conducted, and details are listed as follows. Source 1: EPF_J234711.4-755801 RA (J2000): 356.7979 Dec (J2000): -75.9668 Flux: 2.41 x 10^-12 erg/s/cm2 (observed, 0.5-10 kev) Flux_err: 1.56 x 10^-13 erg/s/cm2 (1 sigma) The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic). Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41688. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39753] GRB 250902A: GECAM-B detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41687 SUBJECT: GRB 250902A: GECAM-B detection DATE: 25/09/04 02:48:15 GMT FROM: zhengchao_astro(a)foxmail.com Chao Zheng, Chen-Wei Wang, Shao-Lin Xiong and Yue Huang (IHEP) report on behalf of GECAM team: GECAM-B was triggered on-ground by GRB 250902A at 2025-09-02T01:30:00.550 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #41648) and SVOM/GRM (Wang et al., GCN #41653). According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of a multi-pulses with a duration (T90) of 10.5 +1.0/-1.5 s. The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecambgrb250902A.png GECAM-B localized this burst to the following position (J2000): Ra: 166.6 deg Dec: 20.5 deg Err: 0.7 deg (1-sigma, statistical only) The time-averaged spectrum from T0-2 s to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.15 +0.14/-0.14 and the cutoff energy, parameterized as Epeak, is 185 +23/-16 keV. The event fluence (10-1000 keV) in this time interval is (2.69 +/-0.19)E-05 erg/cm^2. Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS). View this GCN Circular online at https://gcn.nasa.gov/circulars/41687. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39752] GRB 250903A: LCO optical observation
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41686 SUBJECT: GRB 250903A: LCO optical observation DATE: 25/09/04 01:48:35 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> A. Saccardi (CEA/Irfu), L. P. Xin (NAOC), C. Wu (NAOC), B. Cordier (CEA/Irfu), report on behalf of the SVOM mission team: We observed the field of the GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument. Our observation started on 2025-09-03 at 23:25:00 UT (6.0 hr after the trigger) and we obtained 4x300 s exposures using the SDSS r filter. In the stacked image, we detected the optical afterglow (An et al., GCN 41679, Xin et al., GCN 41684, Pérez-Fournon et al. GCN 41685) within the errorbox of SVOM/MXT (Gotz et al., GCN 41681). We derived the following magnitude r = 20.04 +/- 0.11 mag calibrated by the Guide Star Catalog, Version 2.3.2 (GSC2.3) (STScI, 2006) and not corrected for Galactic extinction. This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/41686. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39751] GRB 250903A: LCO optical afterglow detection
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41685 SUBJECT: GRB 250903A: LCO optical afterglow detection DATE: 25/09/03 21:28:52 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL) We observed the field of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677) and SVOM/MXT (Götz et al., GCN 41681) with one of the two 1-m telescopes of the Las Cumbres Observatory Global Telescope network (LCO) equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory, Australia. The observation, of 300 seconds in the SDSS-r filter, started at 2025-09-03 19:12:07 UT, about 1.78 hr after the SVOM/ECLAIRs trigger. We clearly detect an uncatalogued source within the SVOM/ECLAIRs (Maiolino et al., GCN 41677) and SVOM/MXT (Götz et al., GCN 41681) error circles at a position consistent with the one reported by An et al. (TRT R band, GCN 41679) and Xin et al. (SVOM/VT VT_B and VT_R, GCN 41684). We measure a magnitude of r = 20.29 +/- 0.10, calibrated with the Legacy Surveys DR10 catalog and not corrected for Galactic extinction. This work makes use of observations from the Las Cumbres Observatory global telescope network program IAC2025B-008 (SGLF). View this GCN Circular online at https://gcn.nasa.gov/circulars/41685. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39750] GRB 250903A: SVOM/VT optical confirmation with VHF data
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41684 SUBJECT: GRB 250903A: SVOM/VT optical confirmation with VHF data DATE: 25/09/03 21:14:00 GMT FROM: Liping Xin at NAOC, SVOM <xlp(a)nao.cas.cn> L.P. Xin, H.L. Li, C. Wu. Y. L. Qiu, Z.H. Yao, Y.N. Ma, X.H. Han, J. Wang, W.J. Xie, Y. Xu, H.B. Cai, J. Y. Wei (NAOC), J. Palmerio (CEA), Tais Maiolino (LUPM), Claire Guépin-Detrigne (LUPM) report on behalf of the SVOM team. SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Maiolino et al. GCN 41677). SVOM/VT began observing the field automatically at 2025-09-03T18:15:57 UTC, about 52 min after the T0, with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With downlinked VHF data, the optical counterpart reported (An et al., GCN 41679) within the errorbox of SVOM/MXT (GOTZ et al., GCN 41681) was clear detected with the brightness of VT_B=20.12+/-0.03 and VT_R=19.65 +/-0.03 mag at 54.5 min after the trigger. More follow-ups are encouraged. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41684. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39749] EP250903a: SVOM/VT optical confirmation
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41683 SUBJECT: EP250903a: SVOM/VT optical confirmation DATE: 25/09/03 20:50:43 GMT FROM: Liping Xin at NAOC, SVOM <xlp(a)nao.cas.cn> L. P. Xin, H. L. Li, Y. N. Ma, Y. L. Qiu, C. Wu, Z. H. Yao, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM performed a Target of Opportunity observations of EP250903a detected by EP/WXT (Zhao et al., GCN 41674). The observations was started from 2025-09-03T15:01:54 UTC, 58 min after the EP trigger time, in VT_B (400-650 nm) and VT_R (650-1000nm) bands simultaneously. . The optical source reported (Levan et al., GCN 41680) was clear detected in VT stacked images. The brightness was VT_B=22.1+/-0.1 and VT_R=21.5+/-0.1 mag at the mid time of 1.1 hours after the trigger. Our photometry is not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41683. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39748] EP250901a: Mondy, Assy and SAO RAS optical limits
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41682 SUBJECT: EP250901a: Mondy, Assy and SAO RAS optical limits DATE: 25/09/03 20:14:49 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin (SAO RAS), A. Pozanenko (IKI), E. Klunko (ISTP), V. Kim, M. Krugov (FAI), O. Spiridonova (SAO RAS) report on behalf of GRB follow-up team and IKI-GRB-FuN: We observed the field of EP250901a (Song et al., GCNs 41627, 41651) with the 1.5m AZT-33IK telescope of the Solar Sayan Observatory (Mondy), 1.5m AZT-20 telescope of Assy Observatory and 1.0m Zeiss-1000 telescope of SAO RAS during the night of September 1. Within the refined FXT error circle (Song et al., GCN 41651) we didn't detect any new objects down to the following limits. UT_start t_mid-T0, h exp, s filter up.lim telescope FWHM 13:51:06 2.250997 30 x 120 R 23.2 Mondy, AZT-33IK 2".0 16:17:59 4.837000 61 x 60 r 22.8(AB) Assy, AZT-20 2".6 16:57:33 5.231111 14 x 120 Rc 22.4 SAO, Zeiss-1000 2".1 These preliminary results are in an agreement with the limits from MASTER (Lipunov et al., GCN 41629) and Mephisto (Liu et al., GCN 41631). In the stacked R band image of AZT-33IR within FXT error circle localization (Song et al., GCN 41651) we detected two objects, both presented in the PanSTARRS DR1 images and PS1 catalog, the 1st one is also visible in z-band image of the Legacy Survey DR10. obj. R. A. Decl. (J2000) R_mag +/- err #1 18:12:10.03 +18:42:25.0 22.5 +/- 0.2 #2 18:12:09.78 +18:42:28.3 23.2 +/- 0.3 Also in the stacked image of AZT-33IK we detected one object near the border of XRT circle (SWIFT J181210.9+184216; Evans et al., GCN 41644) with the coordinates of R. A. (J2000) = 18:12:11.02 Decl. (J2000) = +18:42:23.1 with uncertainness of +/- 0".4 in both coordinates, and brightness of R = 22.39 +/- 0.16. This object is presented in the PanSTARRS DR1 images and catalog. The field was calibrated with nearby stars from the PanSTARRS DR1 catalog and not corrected for Galactic extinction. RA DEC r err R err 273.042629860 18.705792060 18.284 0.004 18.048 0.009 273.048977320 18.703099150 17.400 0.004 17.204 0.008 273.046825830 18.696310020 19.130 0.017 18.940 0.017 View this GCN Circular online at https://gcn.nasa.gov/circulars/41682. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39747] Subject GRB 250903A: SVOM/MXT afterglow detection
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41681 SUBJECT: Subject GRB 250903A: SVOM/MXT afterglow detection DATE: 25/09/03 19:02:33 GMT FROM: Diego Gotz at CEA <diego.gotz(a)cea.fr> D. Götz (CEA), M. Moita (CEA) F. Robinet (IJCLab), P. Maggi (ObAS), H. Goto (Kanazawa Univ./CEA), C. Plasse (CEA) , C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team: GRB 250903A (Maiolino et al. GCN# 41677) was observed by SVOM/MXT after an automatic SVOM slew, but due to Earth constraints the observation started at 2025-09-03T18:30:41, about 66 minutes after Tb. Using the on board automatic detection software, MXT detects a faint source at the coordinates R.A. (J2000) = 23h47m21s Dec (J2000) = -75d58m43s with a 90% C.L. radius of 3 arc min (including 25 arc sec systematic error added in quadrature). This position is at 3.1 arc min from the ECLAIRs position and 0.5 arc min from the optical counterpart reported by An et al. (GCN# 41679). We hence identify this source as the X-ray afterglow of GRB 250903A. Further analysis will be perfromed once the full X-band data set is received. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE. The SVOM point of contact for this burst is Tais Maiolino: tais.maiolino(a)lupm.in2p3.fr. Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst. View this GCN Circular online at https://gcn.nasa.gov/circulars/41681. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39746] EP250903a: LCO candidate counterpart discovery
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41680 SUBJECT: EP250903a: LCO candidate counterpart discovery DATE: 25/09/03 18:47:51 GMT FROM: Andrew Levan at Radboud University <a.levan(a)astro.ru.nl> A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radbound), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration: We observed the location of EP250903a (Zhao et al. GCN 41674) with the Sinistro instrument mounted on the 1-m telescopes at the LCO Siding Springs Observatory. 3 x 300 s exposures were obtained in the SDSS r filter at a start time of 14:44:15 UT on 2025-09-03 (~41 minutes post trigger). Within the WXT and FXT error circles we locate a new source in our stacked r-band image with r~21 mag (AB) calibrated against nearby SkyMapper stars and not corrected by Galactic extinction, at a location of RA(J2000): 22:36:27.75 DEC(J2000): -49:51:25.6 The error is about 0.5” in each coordinate. We note the transient is offset ~1” from a Legacy survey object with magnitude r= 21.49 mag which has a photometric redshift of 0.24 +/- 0.11. In the legacy images the source appears to extend underneath the new source. Thus, our photometry likely includes some contribution from the host galaxy. View this GCN Circular online at https://gcn.nasa.gov/circulars/41680. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39745] GRB 250903A: TRT optical counterpart detection
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41679 SUBJECT: GRB 250903A: TRT optical counterpart detection DATE: 25/09/03 18:46:15 GMT FROM: J. An <jiean0813(a)foxmail.com> J. An (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC) report on behalf of a large collaboration: We observed the field of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 17:34:50 UTC on 2025-09-03, i.e., ~10 min after the SVOM/ECLAIRs trigger. A series of R-band frames were obtained. An uncatalogued and varying optical source is detected within the SVOM/ECLAIRs error circle (Maiolino et al., GCN 41677), at coordinates R.A. (J2000) = 23:47:11.67 Dec. (J2000) = -75:58:00.21 with an uncertainty of ~ 0.8 arcsec. Preliminary photometry shows the source has R ~ 19.0 mag at 13.35 min post-trigger, calibrated with Legacy Survey DR10 catalog and not corrected for Galactic extinction. We think the source is the optical counterpart of GRB 250903A. View this GCN Circular online at https://gcn.nasa.gov/circulars/41679. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39744] GRB 250902B: AstroSat CZTI detection
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41678 SUBJECT: GRB 250902B: AstroSat CZTI detection DATE: 25/09/03 18:16:21 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> M. Tembhurnikar (IUCAA), A. Arya, (IITB), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a GRB 250902B which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 41660), and SVOM GRM (Wang et. al., GCN Circ. 41668). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-09-02 18:45:45.00 UTC. The measured peak count rate associated with the burst is 47 (+17, -10) counts/s above the background in the combined data of two quadrants (out of four), with a total of 297 (+122, -145) counts. The local mean background count rate was 152 (+2, -2) counts/s. Using cumulative rates, we measure a T90 of 17.2 (+2.7, -7.6) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-09-02 18:45:44.68 UTC. The measured peak count rate associated with the burst is 365 (+71, -49) counts/s above the background in the combined data of all quadrants, with a total of 1924 (+390, -422) counts. The local mean background count rate was 1405 (+5, -6) counts/s. We measure a T90 of 15.4 (+3.2, -2.4) s from the cumulative Veto light curve. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/41678. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39743] GRB 250903A: SVOM detection of a burst
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41677 SUBJECT: GRB 250903A: SVOM detection of a burst DATE: 25/09/03 18:13:40 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Tais Maiolino (LUPM), Claire Guépin-Detrigne (LUPM), Nicolas Dagoneau (CEA/Irfu), Stéphane Schanne (CEA/Irfu), Miguel Moita (CEA/Irfu) report on behalf of the SVOM mission team: At 2025-09-03T17:24:56 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250903A (SVOM burst-id sb25090304). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network. The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 12 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 16.17 in the [5-20] keV energy band over a time window of 20.48 seconds starting at 2025-09-03T17:24:43. The localization of the best alert is R.A., Dec. 356.9689, -75.9460 degrees: R.A. (J2000) = 23h47m52.54s Dec. (J2000) = -75d56m45.52s with a 90% confidence level (C.L.) radius of 5.17 arcmin (including systematic error of 2 arcmin added in quadrature). This burst was also detected by SVOM/GRM with a significance of 15.80. The SVOM/GRM light curve showed a multi-peak structure with a T90 duration of about 27 (-11/+4)s. SVOM slewed to the burst. No X-ray observation has been performed by SVOM/MXT for the time being. No optical observation has been performed by SVOM/VT for the time being. We notice the presence of BLLac 2FGL J2351.6-7558 located at 12 arcmin from the ECLAIRs position. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. The Burst Advocate (BA) on shift for this alert is Tais Maiolino: tais.maiolino(a)lupm.in2p3.fr. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/41677. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39742] EP250903a: Global MASTER-Net observations report
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41676 SUBJECT: EP250903a: Global MASTER-Net observations report DATE: 25/09/03 17:18:36 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the EP250903a ( EP Team et al., GCN 41674) errorbox 4803 sec after notice time and 10720 sec after trigger time at 2025-09-03 17:01:41 UT, with upper limit up to 15.9 mag. Observations started at twilight. The observations began at zenith distance = 60 deg. The sun altitude is -8.9 deg. The galactic latitude b = -56 deg., longitude l = 342 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2983979 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 10751 | 2025-09-03 17:01:41 | MASTER-SAAO | (22h 31m 47.53s , -49d 57m 38.2s) | C | 60 | 15.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41676. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39741] EP250903a: TRT optical observations
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41675 SUBJECT: EP250903a: TRT optical observations DATE: 25/09/03 17:08:34 GMT FROM: J. An <jiean0813(a)foxmail.com> J. An (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration: We observed the field of EP250903a (Zhao et al., GCN 41674), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 14:14:31 UTC on 2025-09-03, i.e., ~ 11.5 min after the EP/WXT trigger and a series of frames in the R band were obtained. No uncatalogued source is detected in our stacked image within or at the border of the EP-FXT error circle, down to a 5-sigma limiting magnitude of R > 20.1 (Vega) at ~ 1.21 hr post-trigger, calibrated with SKYMAPPER DR2 catalog and not corrected for Galactic extinction. The two relatively bright catalogued sources/stars within or at the border of the EP-FXT error circle do not show variability in brightness. Our result is consistent with that by GOTO (O'Neill et al., GCN 41672). We note that there are faint galaxies within the EP-FXT error circle from Legacy Survey, so deep follow-ups are encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41675. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39740] EP250903a: Einstein Probe detection of an X-ray transient
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41674 SUBJECT: EP250903a: Einstein Probe detection of an X-ray transient DATE: 25/09/03 15:41:06 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> T. Zhao, Y. J. Song(NAO, CAS), Y. J. Zhang(THU), H. Sun(NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250903a, which was also followed by GOTO (O'Neill et al., GCN 41672). The transient triggered EP-WXT (ID: 01709201159) at 2025-09-03T14:03:01 (UTC). The WXT position of the source is R.A. = 339.120 deg, DEC = -49.863 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 339.1097 deg, DEC = -49.8572 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). There are several stars within the FXT error circle. The stellar flare origin for this event can not be fully ruled out at this time. Further information will be updated when the telemetry data is received. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41674. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39739] EP 250827B: Detection of the fading supernova counterpart by LCO.
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41673 SUBJECT: EP 250827B: Detection of the fading supernova counterpart by LCO. DATE: 25/09/03 15:34:43 GMT FROM: ankur ghosh at CAPP, University of Johannesburg <ghosh.ankur1994(a)gmail.com> Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Rahul Gupta (NASA GSFC), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES)on behalf of a larger collaboration. We observed the field of the EP250827B/AT 2025wkm (Schroeder et al., GCN 41635) in V, r filter of the 0.4-m SCICAM QHY600 at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Haleakala Observatory, Hawai . The 0.4 m SCICAM QHY600 is equipped with 9576 x 6388 pixel CCD (FOV: 1.9 x 1.2 degrees, scale: 0.74 arcsec/pixel) but we only used the FOV of 30 x 30 arcmin for our observation. Observations began on September 03, 2025, starting 7.32 days after the GRB trigger. Further observations are currently ongoing. We clearly detect the optical transient (OT) reported by GCNs (Schroder et al., GCN 41635; Hall et al., GCN 41636; Hall et al., GCN 41641; Eyles-Ferris et al., GCN 41655; Ma et al., GCN 41667; Eyles-Ferris et al., GCN 41669, He et al,. GCN 41671) in our r band image. Eyles-Ferris et al., GCN 41655 reported the detection of the rise of the associated Ic-BL supernova, with a measured magnitude of 18.91 at 5.61 days after the explosion. The source subsequently faded by ~1 mag within 1.71 days. |Date| |UTstart| |t-T0 (days)| |Exp (sec)| |Filter| |Magnitude| -------------------------------------------------------------------------------------------------- 2025-09-03 13:58:49.728 7.32 3 x 900 r r = 19.81 +/- 0.05 The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41673. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39738] EP/WXT01709201159: GOTO optical follow-up
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41672 SUBJECT: EP/WXT01709201159: GOTO optical follow-up DATE: 25/09/03 15:13:59 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz(a)bham.ac.uk> D. O'Neill, M. E. Wortley, B. P. Gompertz, A. Kumar, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration: We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the EP/WXT trigger number 01709201159. A targeted observation was performed at Sept. 3 2025 14:10:33 UT, (+0.13h post trigger). The images were taken in the GOTO L-band (400-700 nm) and consisted of 4x90s exposures. Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. We do not identify any candidate optical counterparts within the EP/WXT localisation uncertainty region after the trigger time, down to a 3-sigma limiting magnitude of L > 20.0 AB mag. Additionally, we do not see any evidence for stellar variability consistent with the WXT uncertainty region. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and were not corrected for Galactic extinction. GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/41672. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39737] EP250827b / AT 2025wkm: TRT optical observations
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41671 SUBJECT: EP250827b / AT 2025wkm: TRT optical observations DATE: 25/09/03 15:06:38 GMT FROM: L. B. He at NAOC <helb(a)bao.ac.cn> L.B. He (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Y. Fu (HUST), J. An, X. Liu, Z.P. Zhu, S.Q. Jiang, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang, D. Xu (NAOC) report on behalf of a large collaboration: We followed up the optical transient AT 2025wkm, which is potentially associated with EP250827b (Schroeder et al., GCN 41635). Our first epoch was carried out at the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Fresno, California, U.S.A (SRO). And we obtained several frames in the R band. The counterpart (Schroder et al., GCN 41635; Hall et al., GCN 41636; Hall et al., GCN 41641; Eyles-Ferris et al., GCN 41655; Ma et al., GCN 41667; Eyles-Ferris et al., GCN 41669) was clearly detected in the stacked image, with R = 19.62 +/- 0.04 (Vega), calibrated with Pan-STARRS DR2 catalog and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41671. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39736] EP250827b: NOT spectroscopic observations and AT 2025wkm classification as a broad-lined type-Ic supernova
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41670 SUBJECT: EP250827b: NOT spectroscopic observations and AT 2025wkm classification as a broad-lined type-Ic supernova DATE: 25/09/03 13:24:21 GMT FROM: Gregory Corcoran at University College Dublin <gregory.corcoran(a)ucdconnect.ie> G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), R. A. J. Eyles-Ferris (Leicester), V. Vuolteenaho (NOT and Oulu Univ.), M. Pursiainen (Warwick), I. Worssam (Birmingham) report: We carried out two spectroscopic observations of the optical transient AT 2025wkm (Schroeder et al., GCN 41635) likely associated with the Einstein Probe transient EP250827b, using the ALFOSC spectrograph mounted on the Nordic Optical Telescope (NOT). The observations started on 2025-09-02 at 00:31:54 and 2025-09-03 at 04:37:19 UT for the two epochs, respectively (5.76 and 6.93 days after the EP trigger). Both observations consisted of 4 x 900 s exposures and cover the wavelength range 3800-9500 AA. Both spectra show features consistent with broad-lined type Ic supernovae at a comparable phase, though with an excess of flux on the blue end, providing a good match to the super-luminous SN 2011kl (associated with GRB 111209A; Greiner et al. 2015, doi:10.1038/nature14579) near peak. The brightness of the source, at an absolute magnitude of r = -18.91 and still rising (Eyles-Ferris & Starling, GCN 41669), is currently consistent with classification as both a normal Ic-BL SN and a super-luminous SN. Between the two epochs, we also identify no significant spectral evolution consistent with the constant colour observed by Eyles-Ferris & Starling (GCN 41669). Further photometric and spectroscopic observations are planned and encouraged to better characterize this interesting source. View this GCN Circular online at https://gcn.nasa.gov/circulars/41670. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39735] EP250827b: continued Liverpool Telescope observations
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41669 SUBJECT: EP250827b: continued Liverpool Telescope observations DATE: 25/09/03 12:19:38 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris and R. L. C. Starling (U of Leicester) report on behalf of a larger collaboration: We observed the position of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635) with the 2m Liverpool Telescope using the IO:O instrument. We obtained a series of exposures in the SDSS u’g’r’i’z’ filters starting at 2025-09-03 01:45:07 UT, approximately 162.5 hours after the X-ray detection. We performed image subtraction on the stacked g’r’i’z’ images using reference images from Pan-STARRS. AT 2025wkm is clearly detected in all our subtractions and appears to be currently rising in all bands. We measure r’ = 19.90 +/- 0.05, calibrated to Pan-STARRS and not corrected for Galactic extinction, which corresponds to an absolute magnitude of -18.91 adopting the redshift of 0.120 of Sevilla and Ho (GCN 41639). We note no significant change in colour (g’ - r’ ~ 0) when compared with our earlier epoch photometry (Eyles-Ferris and Starling, GCN 41655). Further observations are planned. View this GCN Circular online at https://gcn.nasa.gov/circulars/41669. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39734] GRB 250902B: SVOM/GRM observation
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41668 SUBJECT: GRB 250902B: SVOM/GRM observation DATE: 25/09/03 11:23:57 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Feliu Lacreu (IAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a burst GRB 250902B (SVOM trigger reference: sb25090205) at 2025-09-02T18:45:36.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #41660). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 17.4 +1.3/-0.7 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250902B.png In addition, the position of this burst, as determined by Fermi/GBM (RA = 82.3, Dec = -5.0, GCN #41660), is located at about 85 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With the localization from Fermi/GBM, the time-averaged spectrum from T0-10 to T0+20 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.85 +0.18/-0.17 and the cutoff energy, parameterized as Epeak, is 146 +21/-16 keV. The event fluence (10-1000 keV) in this time interval is (1.07 +0.08/-0.07)E-05 erg/cm^2. The 'Amati' relation diagram of GRB 250902B is shown at: https://www.bursthub.cn//admin/static/svgrb250902B_amati.png The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/41668. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39733] EP250827b: SVOM/VT observation of AT 2025wkm
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41667 SUBJECT: EP250827b: SVOM/VT observation of AT 2025wkm DATE: 25/09/03 05:04:52 GMT FROM: Yinuo Ma <mayn(a)bao.ac.cn> Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM performed a Target of Opportunity observation of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635). SVOM/VT began observing the field at 2025-09-02T09:14:40 UTC, 6.12 days after the X-ray detection, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X-band data availible, the source consistent with AT 2025wkm (Schroeder et al., GCN 41635; Hall et al., GCN 41636; Sevilla & Ho, GCN 41639; Hall et al., GCN 41641; Eyles-Ferris & Starling, GCN 41655) was clearly detected in both VT_B and VT_R. However, the source in VT_R was contaminated by the blooming of bright star, so the magnitude of VT_B is: mid time (day) | exposure time (s) | band | mag (AB) | mag err ---------------|-------------------|------|----------|-------- 6.24 | 37*70 | VT_B | 19.81 | 0.03 Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41667. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39732] LIGO/VIRGO/KAGRA S250818k: No evidence for radio variability of AT2025ulz in MeerKAT 3 GHz data
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41666 SUBJECT: LIGO/VIRGO/KAGRA S250818k: No evidence for radio variability of AT2025ulz in MeerKAT 3 GHz data DATE: 25/09/03 04:23:58 GMT FROM: Kunal Mooley at NRAO,Caltech <kmooley(a)caltech.edu> Lauren Rhodes (McGill U.), Oleg Smirnov (Rhodes U. / SARAO), Kunal Mooley (IIT Kanpur, Caltech), Patrick Woudt (U. Cape Town) We reprocessed both MeerKAT S-band (3 GHz) datasets of the AT2025ulz (Stein et al., GCN 41414) field reported previously (Bruni et al. GCNs 41594, 41500) in the context of the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440). Our reprocessing reveals that in both datasets, from Aug 28 (10 days post-GW-event) and Aug 21 (4 days post-GW-event), the source has a constant integrated flux density of ~90+/10 uJy. Thus, we do not find any evidence for radio variability. Further, the extended nature of the radio source indicates that the emission is host galaxy dominated. We caution all users of the SARAO Science Data Processor (SDP) continuum pipeline that the pipeline results must be carefully assessed before dissemination across the astronomical community. In the Aug 28 SDP images, there were large-scale artefacts, likely RFI-induced, that may be responsible for the excess flux measurement reported in GCN 41594. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. View this GCN Circular online at https://gcn.nasa.gov/circulars/41666. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39731] GRB 250831A (SVOM trigger sb25083102): KAIT confirmation of stellar flare
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41665 SUBJECT: GRB 250831A (SVOM trigger sb25083102): KAIT confirmation of stellar flare DATE: 25/09/03 04:11:08 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of GRB 250831A (SVOM trigger sb25083102; Xie et al., GCN 41614) starting at 09:32:33, 180 seconds after the burst. Observations were performed in 3 x 3 tiling mode, and a set of clear (roughly R) filter images were obtained. The first image covered the proposed flare star G 272-61 (Wu et al., GCN 41615; Magnani et al., GCN 41616; Xie et al., GCN 41618; Li et al., GCN 41654) was at 09:36:55, 442 seconds after the burst. Although the star was saturated in KAIT images, photometry with a large aperture clearly shows the magnitude decreased from 9.2 mag to 10.2 mag (faded by 1.0 mag) about 2 hours later. This is similar to the light curve reported by SVOM/VT, confirming this event is due to a stellar flare. View this GCN Circular online at https://gcn.nasa.gov/circulars/41665. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39730] Fermi GRB 250902B: Global MASTER-Net observations report
by GCN Circulars 03 Sep '25

03 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41664 SUBJECT: Fermi GRB 250902B: Global MASTER-Net observations report DATE: 25/09/03 03:31:08 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 250902B ( Fermi GBM team, GCN 41660) errorbox 18838 sec after notice time and 18874 sec after trigger time at 2025-09-03 00:00:08 UT, with upper limit up to 18.6 mag. The observations began at zenith distance = 77 deg. The sun altitude is -59.1 deg. The galactic latitude b = -20 deg., longitude l = 208 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2983003 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 18905 | 2025-09-03 00:00:08 | MASTER-SAAO | (05h 20m 30.13s , -04d 32m 16.8s) | C | 60 | 17.4 | 18905 | 2025-09-03 00:00:08 | MASTER-SAAO | (05h 22m 16.39s , -04d 47m 44.4s) | C | 60 | 13.4 | 19743 | 2025-09-03 00:14:07 | MASTER-SAAO | (05h 20m 31.93s , -04d 32m 23.9s) | C | 60 | 17.7 | 19743 | 2025-09-03 00:14:07 | MASTER-SAAO | (05h 22m 18.01s , -04d 47m 48.8s) | C | 60 | 16.7 | 19962 | 2025-09-03 00:17:46 | MASTER-SAAO | (05h 27m 44.14s , -04d 30m 25.1s) | C | 60 | 17.8 | 19962 | 2025-09-03 00:17:46 | MASTER-SAAO | (05h 29m 30.52s , -04d 45m 52.0s) | C | 60 | 16.8 | 20055 | 2025-09-03 00:19:19 | MASTER-SAAO | (05h 35m 39.60s , -04d 30m 43.8s) | C | 60 | 18.0 | 20055 | 2025-09-03 00:19:19 | MASTER-SAAO | (05h 37m 26.17s , -04d 46m 09.6s) | C | 60 | 15.7 | 20368 | 2025-09-03 00:24:32 | MASTER-SAAO | (05h 20m 25.60s , -04d 32m 23.1s) | C | 60 | 17.8 | 20368 | 2025-09-03 00:24:32 | MASTER-SAAO | (05h 22m 12.33s , -04d 47m 48.4s) | C | 60 | 17.1 | 20590 | 2025-09-03 00:28:14 | MASTER-SAAO | (05h 27m 42.05s , -04d 30m 33.7s) | C | 60 | 17.8 | 20590 | 2025-09-03 00:28:14 | MASTER-SAAO | (05h 29m 28.33s , -04d 45m 56.0s) | C | 60 | 17.1 | 20814 | 2025-09-03 00:31:58 | MASTER-SAAO | (05h 35m 47.86s , -04d 30m 20.8s) | C | 60 | 17.9 | 20814 | 2025-09-03 00:31:58 | MASTER-SAAO | (05h 37m 33.96s , -04d 45m 44.0s) | C | 60 | 16.9 | 21125 | 2025-09-03 00:37:08 | MASTER-SAAO | (05h 20m 30.36s , -04d 31m 31.8s) | C | 60 | 17.8 | 21125 | 2025-09-03 00:37:08 | MASTER-SAAO | (05h 22m 17.39s , -04d 46m 55.0s) | C | 60 | 17.4 | 21217 | 2025-09-03 00:38:41 | MASTER-SAAO | (05h 27m 38.90s , -04d 32m 11.0s) | C | 60 | 17.8 | 21218 | 2025-09-03 00:38:41 | MASTER-SAAO | (05h 29m 25.88s , -04d 47m 33.9s) | C | 60 | 17.3 | 21436 | 2025-09-03 00:42:19 | MASTER-SAAO | (05h 35m 42.83s , -04d 30m 02.1s) | C | 60 | 18.1 | 21436 | 2025-09-03 00:42:19 | MASTER-SAAO | (05h 37m 29.66s , -04d 45m 25.2s) | C | 60 | 17.6 | 21623 | 2025-09-03 00:45:27 | MASTER-SAAO | (05h 28m 42.64s , -06d 25m 09.3s) | C | 60 | 17.8 | 21623 | 2025-09-03 00:45:27 | MASTER-SAAO | (05h 30m 29.96s , -06d 40m 32.5s) | C | 60 | 17.5 | 21717 | 2025-09-03 00:47:00 | MASTER-SAAO | (05h 36m 44.94s , -06d 23m 48.0s) | C | 60 | 18.3 | 21717 | 2025-09-03 00:47:00 | MASTER-SAAO | (05h 38m 32.18s , -06d 39m 10.9s) | C | 60 | 17.8 | 21811 | 2025-09-03 00:48:35 | MASTER-SAAO | (05h 20m 28.96s , -04d 31m 31.0s) | C | 60 | 17.8 | 21811 | 2025-09-03 00:48:35 | MASTER-SAAO | (05h 22m 16.10s , -04d 46m 53.7s) | C | 60 | 17.6 | 21905 | 2025-09-03 00:50:09 | MASTER-SAAO | (05h 27m 37.83s , -04d 32m 12.9s) | C | 60 | 17.8 | 21905 | 2025-09-03 00:50:09 | MASTER-SAAO | (05h 29m 24.93s , -04d 47m 35.3s) | C | 60 | 17.6 | 22202 | 2025-09-03 00:55:06 | MASTER-SAAO | (05h 21m 31.95s , -06d 24m 31.1s) | C | 60 | 17.9 | 22202 | 2025-09-03 00:55:06 | MASTER-SAAO | (05h 23m 19.48s , -06d 39m 55.5s) | C | 60 | 17.9 | 22295 | 2025-09-03 00:56:39 | MASTER-SAAO | (05h 35m 39.88s , -04d 30m 50.9s) | C | 60 | 18.2 | 22295 | 2025-09-03 00:56:39 | MASTER-SAAO | (05h 37m 26.70s , -04d 46m 15.3s) | C | 60 | 17.9 | 22481 | 2025-09-03 00:59:44 | MASTER-SAAO | (05h 28m 48.06s , -06d 25m 09.7s) | C | 60 | 18.0 | 22481 | 2025-09-03 00:59:44 | MASTER-SAAO | (05h 30m 35.49s , -06d 40m 34.0s) | C | 60 | 17.9 | 22579 | 2025-09-03 01:01:23 | MASTER-SAAO | (05h 36m 44.70s , -06d 25m 51.1s) | C | 60 | 18.6 | 22579 | 2025-09-03 01:01:23 | MASTER-SAAO | (05h 38m 32.64s , -06d 41m 14.1s) | C | 60 | 18.4 | 22672 | 2025-09-03 01:02:56 | MASTER-SAAO | (05h 20m 30.08s , -04d 32m 05.7s) | C | 60 | 18.0 | 22672 | 2025-09-03 01:02:56 | MASTER-SAAO | (05h 22m 17.93s , -04d 47m 28.7s) | C | 60 | 18.0 | 22765 | 2025-09-03 01:04:29 | MASTER-SAAO | (05h 27m 40.12s , -04d 30m 40.7s) | C | 60 | 18.0 | 22765 | 2025-09-03 01:04:29 | MASTER-SAAO | (05h 29m 27.84s , -04d 46m 03.6s) | C | 60 | 17.9 | 23049 | 2025-09-03 01:09:13 | MASTER-SAAO | (05h 21m 26.00s , -06d 25m 32.3s) | C | 60 | 18.0 | 23049 | 2025-09-03 01:09:13 | MASTER-SAAO | (05h 23m 14.50s , -06d 40m 55.7s) | C | 60 | 18.0 | 23143 | 2025-09-03 01:10:46 | MASTER-SAAO | (05h 35m 38.42s , -04d 29m 57.8s) | C | 60 | 18.2 | 23143 | 2025-09-03 01:10:46 | MASTER-SAAO | (05h 37m 26.26s , -04d 45m 21.1s) | C | 60 | 18.3 | 23236 | 2025-09-03 01:12:19 | MASTER-SAAO | (05h 28m 47.39s , -06d 25m 15.1s) | C | 60 | 17.9 | 23236 | 2025-09-03 01:12:19 | MASTER-SAAO | (05h 30m 35.79s , -06d 40m 38.5s) | C | 60 | 17.9 | 23329 | 2025-09-03 01:13:52 | MASTER-SAAO | (05h 36m 43.44s , -06d 25m 56.1s) | C | 60 | 18.5 | 23329 | 2025-09-03 01:13:52 | MASTER-SAAO | (05h 38m 31.77s , -06d 41m 19.5s) | C | 60 | 18.6 | 23422 | 2025-09-03 01:15:26 | MASTER-SAAO | (05h 29m 30.58s , -04d 47m 30.8s) | C | 60 | 18.0 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41664. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39729] Fermi-LAT Gamma-ray Observations of IceCube-250831A
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41663 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-250831A DATE: 25/09/02 19:46:44 GMT FROM: Leo Pfeiffer at University of Würzburg <pfeiffer.leo(a)gmail.com> L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg) and S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC250831A neutrino event (GCN 41620) with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 2025-08-31 at 18:00:45.82 UTC (T0) with J2000 position RA = 105.64 (+0.51, -0.48) deg, Decl. = 5.04 (+0.61, -0.50) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC250831A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546). We searched for the existence of intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (>5sigma) new excess emission (> 100 MeV) within the IC250831A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250831A best-fit position, the >100 MeV flux upper limit (95% confidence) is <2.6e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <1.6e-08 (<7.1e-08) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this analysis, the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer at uni-wuerzburg.de). The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden. View this GCN Circular online at https://gcn.nasa.gov/circulars/41663. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39728] GRB 250902A: Fermi GBM Detection
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41662 SUBJECT: GRB 250902A: Fermi GBM Detection DATE: 25/09/02 19:12:27 GMT FROM: sumanbala2210(a)gmail.com S. Bala (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 01:30:00.49 UT on 02 September 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250902A (trigger 778469405/250902062). The Fermi-GBM position was reported in GCN 41648 (Fermi GBM Team 2025). The angle from the Fermi LAT boresight is 78 degrees. The GBM light curve consists of a bright pulse with a duration (T90) of about 8.4 s (50-300 keV). The time-averaged spectrum from T0-1.0 to T0+17.4 s is best fit by Band function with a Band function with Epeak = 257 +/- 13 keV, alpha = -1.25 +/- 0.02, and beta = -2.31 +/- 0.1. A power law function with an exponential high-energy cutoff fits the spectrum equally well with a with power law index = -1.28 +/- 0.01 and Epeak = 294 +/- 10 keV. The event fluence (10-1000 keV) in this time interval is (3.06 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+3.5 s in the 10-1000 keV band is 43.5 +/- 0.5 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" View this GCN Circular online at https://gcn.nasa.gov/circulars/41662. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39727] GRB 250901A: Swift/UVOT Upper Limit
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41661 SUBJECT: GRB 250901A: Swift/UVOT Upper Limit DATE: 25/09/02 18:56:52 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250901A 2.6 ks after the SVOM/ECLAIRs trigger (Piron et al., GCN Circ. 41634). We do not detect any afterglow consistent with the XRT position (Osborne et al., GCN Circ. 41645) or optical position (Xin et al., GCN Circ. 41646; Malesani et al., GCN Circ. 41649). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 2629 4244 1589 >21.01 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.023 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41661. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39726] GRB 250902B: Fermi GBM Final Real-time Localization
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41660 SUBJECT: GRB 250902B: Fermi GBM Final Real-time Localization DATE: 25/09/02 18:56:35 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 18:45:34 UT on 2 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250902B (trigger 778531539.099344 / 250902782). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 82.3, Dec = -5.0 (J2000 degrees, equivalent to J2000 05h 29m, -5d 00'), with a statistical uncertainty of 1.4 degrees. The angle from the Fermi LAT boresight is 20.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902782/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902782/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902782/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41660. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39725] IceCube-250831A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41659 SUBJECT: IceCube-250831A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/09/02 15:58:24 GMT FROM: Alicia Mand at IceCube/UW-Madison <aemand(a)wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-250831A (https://gcn.nasa.gov/circulars/41620) in a time range of 1000 seconds centered on the alert event time (2025-08-31 17:52:25.820 UTC to 2025-08-31 18:09:05.820 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-250831A. We report a p-value of 1.00 in this time window. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-250831A is 1.4e-01 GeV cm^-2 in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 3e+02 GeV and 1e+05 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-08-30 18:00:45.820 UTC to 2025-09-01 18:00:45.820 UTC). In this case, we report a p-value of 0.23, consistent with no significant excess of track events. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-250831A is 1.6e-01 GeV cm^-2 in a 2 day time window. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu. [1] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021) View this GCN Circular online at https://gcn.nasa.gov/circulars/41659. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39724] GRB 250830B: Swift-XRT afterglow detection
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41658 SUBJECT: GRB 250830B: Swift-XRT afterglow detection DATE: 25/09/02 14:31:53 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has conducted further observations of the field of the MAXI-detected burst GRB 250830B. The observations now extend from T0+11.1 ks to T0+223.0 ks and have a total exposure time of 6.0 ks. . The source previously reported, "Source 1", is fading with 2.5 sigma significance and thus is believed to be the GRB afterglow. Using 2349 s of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 105.74037, -64.85289 which is equivalent to: RA (J2000): 07h 02m 57.69s Dec(J2000): -64d 51' 10.4" with an uncertainty of 3.0 arcsec (radius, 90% confidence). This position is 2.3 arcmin from the EP position. The light curve can be modelled with a power-law decay with a decay index of alpha=0.43 (+0.15, -0.16). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.4 (+0.8, -0.5). The best-fitting absorption column is 1.5 (+6.0, -0.4) x 10^21 cm^-2, consistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.4 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.5 (+6.0, -0.4) x 10^21 cm^-2 Galactic foreground: 1.1 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.4 (+0.8, -0.5) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021858. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021858. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41658. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39723] LIGO/Virgo/KAGRA S250902av: 1 counterpart neutrino candidate event from an IceCube neutrino search
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41657 SUBJECT: LIGO/Virgo/KAGRA S250902av: 1 counterpart neutrino candidate event from an IceCube neutrino search DATE: 25/09/02 13:11:05 GMT FROM: Erik Blaufuss at University of Maryland, College Park <blaufuss(a)umd.edu> IceCube Collaboration (http://icecube.wisc.edu/) reports: We have performed a search for track-like muon neutrino candidate events detected by IceCube consistent with the sky localization of the low-significance gravitational-wave candidate event S250902av in a time range of 1000 seconds centered on the alert event time (2025-09-02T07:01:18.83 UTC to 2025-09-02T07:17:58.83 UTC) [1,2]. During this time period, IceCube was collecting good quality data. A single hypothesis test was conducted using a Bayesian approach to quantify the joint GW + neutrino event significance, which assumes a binary merger scenario and accounts for known astrophysical priors, such as GW source distance, in the statistical significance estimation [3]. One track-like event was found in spatial and temporal coincidence with the gravitational-wave candidate S250902av calculated from the map circulated in the S250902av-2-Preliminary notice. This represents an overall p-value of 5.1e-03 for the Bayesian search. The p-value measures the consistency of the observed track-like events with the known atmospheric backgrounds for this single map (not accounting for statistical trials from multiple GW events). Further details are available at https://gcn.nasa.gov/missions/icecube and at https://roc.icecube.wisc.edu/public/LvkNuTrackSearch. Properties of the coincident event are shown below: dt(s) RA(deg) Dec(deg) Angular uncertainty(deg) p-value(Bayesian) ------------------------------------------------------------------------------------- 134.47 15.51 24.37 1.54 0.00533 where: dt = Time of track event minus time of GW trigger (sec) Angular uncertainty = Angular uncertainty of track event: the radius of a circle representing 90% CL containment by area. p-value = the p-value for this specific track event from this search. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu [1] M. G. Aartsen et al 2020 ApJL 898 L10 [2] Abbasi et al. Astrophys.J. 944 (2023) 1, 80 [3] I. Bartos et al. 2019 Phys. Rev. D 100, 083017 View this GCN Circular online at https://gcn.nasa.gov/circulars/41657. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39722] LIGO/Virgo/KAGRA S250901cb: Updated Sky localization
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41656 SUBJECT: LIGO/Virgo/KAGRA S250901cb: Updated Sky localization DATE: 25/09/02 10:06:38 GMT FROM: Michael J. Williams at University of Portsmouth <michael.williams(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250901cb (GCN Circular 41638). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250901cb For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 581 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2018 +/- 482 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/41656. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39721] EP250827b: Liverpool Telescope observations
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41655 SUBJECT: EP250827b: Liverpool Telescope observations DATE: 25/09/02 08:06:57 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris and R. L. C. Starling (U of Leicester) report on behalf of a larger collaboration: We observed the position of EP250827b/AT 2025wkm (Schroeder et al., GCN 41635) with the 2m Liverpool Telescope using the IO:O instrument. We obtained a series of exposures in the SDSS u’g’r’i’z’ filters starting at 2025-09-02 02:47:47 UT, approximately 139.5 hours after the X-ray detection. We performed image subtraction on the stacked g’r’i’z’ images using reference images from Pan-STARRS. We clearly detect a source consistent with AT 2025wkm in all our subtractions and measure r’ = 19.97 +/- 0.04. Our measured magnitude compared to Schroeder et al. (GCN 41635) and Hall et al. (GCN 41636) suggests AT 2025wkm may still be rising in the r band. The colour of the source (g’ - r’ ~ 0.0) is consistent with the blue spectrum observed by Sevilla and Ho (GCN 41639). Further observations are planned. View this GCN Circular online at https://gcn.nasa.gov/circulars/41655. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39720] GRB 250831A (SVOM trigger sb25083102): confirmation of a stellar flare by SVOM/VT
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41654 SUBJECT: GRB 250831A (SVOM trigger sb25083102): confirmation of a stellar flare by SVOM/VT DATE: 25/09/02 07:31:25 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, Z. H. Yao, Y. N. Ma, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM performed an automatic slew on the soft burst triggered by SVOM/ECLAIRs (Xie et al., GCN 41614). SVOM/VT began observing the field automatically on 2025-08-31T09:34:43, i.e., 335 sec with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X band downlinked data, we find that the position of the high proper motion star G 272-61 is consistent with XRT Source 1 (Evans et al., GCN 41617). With a refined analysis, it was decreasing in brightness from 7.6 mag to 10.0 mag in VT_B during the first 0.5 hour after the trigger time, confirming this event as due to a stellar flare. Our photometry was estimated in AB magnitude. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41654. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39719] GRB 250902A: SVOM/GRM observation
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41653 SUBJECT: GRB 250902A: SVOM/GRM observation DATE: 25/09/02 07:19:01 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Xiao-Yun Zhao, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Tais Maiolino (LUPM) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a burst GRB 250902A (SVOM trigger reference: sb25090201) at 2025-09-02T01:30:01.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #41648). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 6.9 +0.5/-0.3 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250902A.png In addition, the position of this burst, as determined by Fermi/GBM (RA = 159.3, Dec = 16.3, GCN #41648), is located at about 164 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. ECLAIRs was not collecting data at the time of this burst. With the localization from Fermi/GBM, the time-averaged spectrum from T0-1 to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.66 +0.06/-0.06 and the cutoff energy, parameterized as Epeak, is 431 +128/-76 keV. The event fluence (10-1000 keV) in this time interval is (4.47 +0.10/-0.10)E-05 erg/cm^2. The 'Amati' relation diagram of GRB 250902A is shown at: https://www.bursthub.cn//admin/static/grb250902A_amati.png The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/41653. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39718] EP250828a: Early near-infrared follow-up with MIRAGE on the MDM 1.3m telescope
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41652 SUBJECT: EP250828a: Early near-infrared follow-up with MIRAGE on the MDM 1.3m telescope DATE: 25/09/02 04:39:41 GMT FROM: kde1(a)mit.edu K. De (Columbia/CCA), S. Ibrahim (Columbia), T. Suen (Columbia), M. Sitaram (Columbia), D. Schiminovich (Columbia), G. Mo (Caltech/Carnegie), K. Shin (Caltech) We observed the field of the X-ray transient EP250828a (Liu et al., GCN 41574; Aryan et al., GCN 41579; Taguchi et al., GCN 41582; Yao et al., GCN 41583; Fu et al., GCN 41584; Zheng et al., GCN 41586; Lipunov et al., GCN 41588; Bochenek et al., GCN 41590; Aryan et al., GCN 41591; Ni et al., GCN 41592) during commissioning operations of the MDM InfraRed Astronomy inGaas Explorer (MIRAGE) instrument on the MDM 1.3m telescope. Observations were carried out starting at UTC 2025-08-28 10:59:08, starting ~ 3.4 hours after the time of the X-ray detection. We obtained a series of dithered exposures in Y-band amounting to a total exposure time of 850 s. Images were aligned, stacked and calibrated to the PS1 catalog. Performing forced photometry at the position of the X-ray and optical counterpart reported in GCN 41582, 41583, 41591, 41592, we do not detect a point source to a 5 sigma limiting magnitude of > 19.1 AB mag. Analysis will be improved as further commissioning data is processed. MIRAGE is a new YJH-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the commissioning observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41652. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39717] EP250901a: refined analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41651 SUBJECT: EP250901a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/09/02 04:26:12 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. J. Song, T. Zhao, J. W. Hu, C. L. Guo, Z. X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team: The X-ray transient EP250901a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 41627), with several multi-wavelength follow-up observations (Lipunov et al., GCN 41629; Liu et al., GCN 41631; Evans et al., GCN 41644). The refined WXT data shows that the event started at T0=2025-09-01T12:02:50 (UTC) and lasted for about 150 s before the interruption of the autonomous follow-up. It has a peak flux of ~1 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.1 x 10^21 cm^-2 and a photon index of 1.02 (-0.79/+0.84). The derived average unabsorbed 0.5-4 keV flux is around 5.05 (-1.54/+2.40) x 10^(-10) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 250s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 273.0416, DEC = 18.7052 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 3.0(-0.06/+0.07) x 10^21 cm^-2 and a photon index of 2.01(-0.15/+0.17). The derived average unabsorbed 0.5-10 keV flux is around 6.03 (-0.27/+0.43) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. A historical ROSAT detection (1RXS J181210.7+184228) lies 10.1 arcsec away from the FXT position, with a flux of 3 x 10^(-13) erg/s/cm^2 at 0.5-4 keV by assumimg a power-law spectrum with NH=3x10^20 cm^-2 and photon index =1.7. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41651. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39716] GRB 250901A: EP-FXT follow-up observation
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41650 SUBJECT: GRB 250901A: EP-FXT follow-up observation DATE: 25/09/02 04:04:05 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. N. Yang (NAO, CAS), Z. Y. Liu (USTC), Y. J. Zhang (THU), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of GRB 250901A (GCN 41634) at 2025-09-01T20:03:51 (UTC), about 2 hours after the SVOM/ECLAIRs detection, with an exposure time of 5 ks. The source was also followed by Swift-XRT (GCN 41645). The optical afterglow was also detected by SVOM/VT (GCN 41646). FXT detected an uncatalogued source at R.A. = 356.1845, DEC = -15.7418 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), consistent with the position of the source reported by Swift/XRT (offset is 4.0 arcsec). The FXT spectrum of this source can be fitted with an absorbed powerlaw with the absorption fixed at the Galactic value of 1.7x10^20 cm^-2, and a photon index of 1.97(+/-0.16). The derived unabsorbed 0.5-10 keV flux is approximately 2.4(+/-0.3)x10^-12 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41650. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39715] GRB 250901A: NOT optical observations
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41649 SUBJECT: GRB 250901A: NOT optical observations DATE: 25/09/02 01:45:56 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), B. Schneider (LAM), G. Corcoran (UCD), A. Martin-Carrillo (UCD), A. Saccardi (CEA/Irfu), V. Vuolteenaho (NOT and Oulu Univ.), report on behalf of a larger collaboration: We observed the optical counterpart (Xin et al., GCN 41646; Osborne et al., GCN 41645) of GRB 250901A (Piron et al., GCN 41634) using the Nordic Optical Telescope (NOT), located in the Canary Islands, equipped with the ALFOSC camera. Observations were carried out in the r (3x300 s) and z (5x200 s) bands. In a preliminary stack of the images, the afterglow is faintly detected in both filters. At a mean epoch of 2025 September 2.00 UT (5.73 hr after the trigger), we measure an AB magnitude r = 23.1 +- 0.2, calibrated against nearby stars from the Pan-STARRS catalog, and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41649. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39714] GRB 250902A: Fermi GBM Final Real-time Localization
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41648 SUBJECT: GRB 250902A: Fermi GBM Final Real-time Localization DATE: 25/09/02 01:40:30 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 01:30:00 UT on 2 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250902A (trigger 778469405.491528 / 250902062). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 159.3, Dec = 16.3 (J2000 degrees, equivalent to J2000 10h 37m, 16d 18'), with a statistical uncertainty of 1.0 degrees. The angle from the Fermi LAT boresight is 78.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902062/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902062/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902062/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41648. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39713] IceCube-Cascade 250828A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41647 SUBJECT: IceCube-Cascade 250828A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/09/02 00:57:15 GMT FROM: Alicia Mand at IceCube/UW-Madison <aemand(a)wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-Cascade 250828A (https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/141290_67759305.amon) in a time range of 1000 seconds centered on the alert event time (2025-08-28 19:22:57.520 UTC to 2025-08-28 19:39:37.520 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, one track-like event is found within the 90% containment region of IceCube-Cascade 250828A. We find that this additional event is well described by atmospheric background expectations, with a p-value of 0.28. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-Cascade 250828A ranges from 1.3e-01 to 6.1e+00 GeV cm^-2 in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 2e+02 GeV and 3e+06 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-08-27 19:31:17.520 UTC to 2025-08-29 19:31:17.520 UTC). In this case, we report a p-value of 0.63, consistent with no significant excess of track events. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-Cascade 250828A ranges from 1.5e-01 to 6.2e+00 GeV cm^-2 in a 2 day time window. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu. [1] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021) View this GCN Circular online at https://gcn.nasa.gov/circulars/41647. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39712] GRB 250901A: SVOM/VT optical afterglow with VHF data
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41646 SUBJECT: GRB 250901A: SVOM/VT optical afterglow with VHF data DATE: 25/09/01 23:27:01 GMT FROM: Liping Xin at NAOC, SVOM <xlp(a)nao.cas.cn> L.P. Xin, H.L. Li, C. Wu. Y. L. Qiu, Z.H. Yao, Y.N. Ma, X.H. Han, J. Wang, W.J. Xie, Y. Xu, H.B. Cai, J. Y. Wei (NAOC), J. Palmerio (CEA) , F. Piron (LUPM), C. Guépin (LUPM) report on behalf of the SVOM team. SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Piron et al., GCN 41634). SVOM/VT began observing the field automatically at 2025-09-01T18:20:17.000 UTC, 355 seconds after the T0, with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With downlinked VHF data, an uncatalogued source is found in stacked images within the errorbox of SVOM/MXT (Piron et al., GCN 41634) and Swift/XRT (Osborne et al., GCN 41645) compared to Legacy survey. It is located at R.A. and DEC. 356.18453, -15.74171 deg or R.A., (J2000) = 23:44:44.28 DEC. (J2000) = -15:44:30.16 The error is about 0.5 arcseconds. The AB magnitudes were derived as follows: --------------------------------------------------------- (T-T0)_mid exptime band mag 505 sec 300 sec VT_R 19.66+/-0.03 505 sec 300 sec VT_B 21.75+/-0.05 Given the color of VT_B-VT_R~2.0 mag, it might be a medium redshift GRB (Wang et al., 2020, RAA). More follow-ups are encouraged. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41646. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39711] GRB 250901A: Swift-XRT afterglow detection
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41645 SUBJECT: GRB 250901A: Swift-XRT afterglow detection DATE: 25/09/01 22:33:36 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> J.P. Osborne (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250901A. We searched for X-ray sources in 754 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 754 s, obtained between T0+2.6 ks and T0+3.3 ks. An uncatalogued X-ray source is detected and is above the LSXPS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=356.1837, -15.7410 which is equivalent to: RA (J2000): 23h 44m 44.08s Dec(J2000): -15d 44' 27.7" with an uncertainty of 3.9 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=3.2 (+1.8, -3.0). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.5, -0.4). The best-fitting absorption column is 6 (+12, -4) x 10^20 cm^-2, consistent with the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6 (+12, -4) x 10^20 cm^-2 Galactic foreground: 1.9 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.9 (+0.5, -0.4) If the light curve continues to decay with a power-law decay index of 3.2, the count rate at T+24 hours will be 2.9 x 10^-6 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x 10^-16 (1.1 x 10^-16) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000066. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00033. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41645. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39710] EP250901a: Swift-XRT observations
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41644 SUBJECT: EP250901a: Swift-XRT observations DATE: 25/09/01 22:18:32 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250901a, collecting 2.0 ks of Photon Counting (PC) mode data between T0+6.2 ks and T0+12 ks after the trigger. We have detected 1 source. This has been automatically classified as an known X-ray source. Known X-ray sources ------------------- Source 1 (SWIFT J181210.9+184216): ================================== RA (J2000.0): 273.0457 = 18 12 10.97 Dec (J2000.0): +18.7047 = +18 42 16.9 Error: 5.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 2.1 arcmin from the Einstein Probe/WXT position. Mean rate: 0.0570 +/- 0.0097 ct s^-1 Mean flux: (2.06 +/- 0.35)e-12 erg cm^-2 s^-1 Peak rate: 0.061 +/- 0.015 ct s^-1 Peak flux: (2.21 +/- 0.54)e-12 erg cm^-2 s^-1 ECF: 3.62e-11 erg cm^-2 ct^-1 assuming NH=4.95e+21 cm^-2, gamma=2.27 determined from a spectral fit. This matches a catalogued X-ray source 1RXS J181210.7+184228 in the ROSAT/RASSFSC catalogue. Details: Separation: 12.4" from the XRT source Cat Rate: 2.0e-02 +/- 8.7e-03 ct s^-1 Cat Flux: 6.7e-13 +/- 2.8e-13 erg cm^-2 s^-1 (0.3-10 keV) so the source is 2.5-sigma above the catalogued flux. The source may be fading, at the 0.9-sigma level. A SIMBAD object `1RXS J181210.7+184228' is 12.4" away. There are 3 2MASS objects within the source's 3-sigma error radius. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00067. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41644. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39709] LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41643 SUBJECT: LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates DATE: 25/09/01 22:04:52 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), James Freeburn (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report: DECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-01T03:39 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607). We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS new transients within the LVK 99% CI area, and we report here select transients of high quality and interest: First, one matched to a NED galaxy within the 90% volume (GCN 41608): | id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | NED separation | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- | | T202509012137497m775946 | AT 2025wpv | 324.457179 | -77.996131 | 2025-09-01 03:57:43.776 | 22.6 | 0.3 | 1.16 | 1.6” Next, a transient of a known Gaia quasar (Gavras et al. 2023): | id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | Gaia separation | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- | | A202509012159035m780432 | AT 2025wpq | 329.764660 | -78.075678 | 2025-09-01 03:41:20.544 | 20.3 | 0.3 | 0.6 | 0.165” Finally, we confirm the transient reported by McMahon et al. (GCN 41640): | id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | | T202509012157542m772445 | AT 2025wpk | 329.475736 | -77.412522 | 2025-09-01 03:49:33.024 | 22.2 | 0.2 | 1.2 Further analysis is underway. We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration. View this GCN Circular online at https://gcn.nasa.gov/circulars/41643. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39708] GRB 250901A: LCO optical upper limit
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41642 SUBJECT: GRB 250901A: LCO optical upper limit DATE: 25/09/01 22:02:00 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. Turpin (CEA/Irfu), B. Cordier (CEA/Irfu), report on behalf of the SVOM mission team: We observed the field of the GRB 250901A detected by SVOM/ECLAIRs (Piron et al., GCN 41634) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument. Our observation started on 2025-09-01 at 19:22:42 UT (68 minutes after the trigger) and we obtained 3x200 s exposures using the SDSS r filter. In the stacked image, we did not detect any new optical source at the position of the Swift/XRT source #1 (https://www.swift.ac.uk/SVOM/SVOM_FIELD00033/) We derived the following upper limit r > 21.3 AB mag (3-sigma) calibrated using nearby stars from the PanSTARRS DR1 catalog and not corrected for Galactic extinction. This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/41642. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39707] EP250827b: Swift observations of AT 2025wkm
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41641 SUBJECT: EP250827b: Swift observations of AT 2025wkm DATE: 25/09/01 20:57:31 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (Carnegie Mellon U.), Tomas Ahumada (Caltech), and Robert Stein (UMD) on behalf of the ZTF collaboration: We requested Swift ToO observations of the X-ray flare EP250827b and optical transient AT 2025wkm reported by Schroeder et al. (GCN 41635). The observation was taken with a midpoint time of 2025-08-31 at 21:13:29 UT, approximately 4.62 days after the flare, with a duration of 4ks. The source was not detected with XRT with an upper limit of 2.5 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift reported by Sevilla & Ho (GCN 41639) of z = 0.1200 this gives a rough luminosity upper limit of 4.5 * 10^42 erg/s. We clearly detect a source at the position of AT 2025wkm in UVOT observations. We use a synthetic galaxy model to produce subtracted magnitudes and correct for galactic extinction. We report in the UVW1 filter, a magnitude of m= 21.7 +/- 0.4 AB and in the UVM2 filter, a magnitude of m = 22.2 +/- 0.3. We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible. View this GCN Circular online at https://gcn.nasa.gov/circulars/41641. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39706] LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41640 SUBJECT: LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates DATE: 25/09/01 20:32:52 GMT FROM: Isaac McMahon at University of Zürich <isaac.mcmahon(a)ligo.org> Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team: At 2025-09-01 03:39:18 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed four fields centered on the following ICRS coordinates: | RA | DEC | |----------|----------| | 324.165777 | -77.51123 | | 327.955221 | -77.54517 | | 323.965777 | -77.61123 | | 327.755221 | -77.64517 | These pointings cover the 90% localization region of candidate gravitational-wave event S250830bp. All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 21.8 in g, 21.8 in r, and 21.5 in i, and 20.7 in z. We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei. Poor observing conditions during observing complicated positive identification of transient candidates. However, we report one candidate below with good detections in multiple bands. The candidate is separated by 5.58 arcseconds from the center of the apparent host galaxy LEDA 236249, which does not have a reported redshift. | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- | | DESGW_3336579 | AT2025wpk | 329.476163 | -77.41256 | 22.41 | 0.24 | 21.39 | 0.09 | 21.1 | 0.06 | 20.93 | 0.14 | Further observations are ongoing and we encourage followup of the event region and the above candidate. The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41640. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39705] EP250827b: Gemini Spectroscopy of AT 2025wkm
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41639 SUBJECT: EP250827b: Gemini Spectroscopy of AT 2025wkm DATE: 25/09/01 20:27:47 GMT FROM: Anna Y Q Ho at Cornell University <ayh24(a)cornell.edu> Cassie Sevilla and Anna Ho (Cornell) report on behalf of the ZTF collaboration: We acquired long-slit spectroscopy of AT 2025wkm (possibly associated with Einstein Probe X-ray flare EP250827b; Schroeder et al. GCN #41635) using GMOS-N at Gemini Observatory on 2025-08-31 12:15:31 UTC, under the target-of-opportunity program GN-2025B-Q-130 (PI Ho). The observation was carried out with a 1" slit, the R400 grating, and the GG455 filter. We took four 450s exposures, two at a central wavelength of 710 nm and two at 720 nm. The data was reduced using DRAGONS. The spectrum shows a blue continuum with narrow emission lines of H alpha, H beta, [O III], and [S II] at a consistent redshift of z=0.120. The implied peak measured absolute magnitude of AT 2025wkm is M_g = -19.5 mag, corrected for Milky Way extinction. The rapid rise (~3d) to such a high luminosity is unusual for extragalactic transients. We thank the Gemini Observatory staff for assistance in rapidly obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41639. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39704] LIGO/Virgo/KAGRA S250901cb: Identification of a GW compact binary merger candidate
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41638 SUBJECT: LIGO/Virgo/KAGRA S250901cb: Identification of a GW compact binary merger candidate DATE: 25/09/01 19:47:05 GMT FROM: shoki.iwaguchi(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250901cb during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-01 18:59:41.965 UTC (GPS time: 1440788399.965). The candidate was found by the Aframe [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines. S250901cb is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-14 Hz, or about one in 1e6 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250901cb The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time. * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 935 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis): icrs; ellipse(13h05m, -07d35m, 30.96d, 9.71d, 118.56d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1960 +/- 524 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [8] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41638. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39703] EP250831a: NOT optical observations and tentative afterglow candidate
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41637 SUBJECT: EP250831a: NOT optical observations and tentative afterglow candidate DATE: 25/09/01 19:46:02 GMT FROM: Gregory Corcoran at University College Dublin <gregory.corcoran(a)ucdconnect.ie> S.Y. Fu (HUST), J. Sanchez-Sierras (Radboud), J. An, L.B. He, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), G. Corcoran (UCD), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. A. Quirola-Vasquez (Radboud), A. Henderson (NOT) report: We observed the field of EP250831a (Yang et al., GCN 41619) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). We obtained 6 x 300 s r-band and 9 x 200 s z-band images. Within the EP-FXT error circle (Yang et al., GCN 41625), a new optical source is marginally visible in the stacked r-band image, at coordinates R.A. (J2000) = 02:51:55.49 Dec. (J2000) = -36:11:14.9 with an uncertainty of ~ 0.5 arcsec. We measure an AB magnitude r ~ 23.8 mag at an epoch of 18.23 hr after the EP trigger, calibrated against the Legacy Survey DR10 and not corrected for Galactic extinction. This source is not detected in the stacked z-band image, with a limiting magnitude of z > 22.0 mag (AB). There is a source in the Legacy Survey about 1.7” SE of our candidate, with a tabulated magnitude r = 23.83. We caution about the low S/N of our detection so we only report this object as a tentative optical counterpart to EP250831a. Further observations are planned in order to secure a template imaging and assess fading. View this GCN Circular online at https://gcn.nasa.gov/circulars/41637. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39702] EP250827b: FTW Observations of AT 2025wkm
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41636 SUBJECT: EP250827b: FTW Observations of AT 2025wkm DATE: 25/09/01 19:30:16 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), Mansi Kasliwal (Caltech), Tomas Ahumada (Caltech), and Gokul Srinivasaragavan (UMD) report: We observed the localization of the X-ray flare EP250827b and optical transient AT 2025wkm as reported by Schroeder et al. (GCN 41635), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands starting at 2025-08-31T02:01:59 UTC (3.82 days after the X-ray flare) for 24 x 180s. In the difference imaging with templates from PS1, we detect AT 2025wkm at r = 20.14 +/- 0.02 AB mag, The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction. Furthermore, we see no other candidates in the difference imaging to a limiting depth of r = 23.15, which supports the association of AT 2025wkn and EP250827b. Further analysis is underway. We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41636. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39701] EP250827b: Contemporaneous detection of EP X-ray flare EP250827b and ZTF transient ZTF25abmpngy/AT2025wkm
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41635 SUBJECT: EP250827b: Contemporaneous detection of EP X-ray flare EP250827b and ZTF transient ZTF25abmpngy/AT2025wkm DATE: 25/09/01 19:20:45 GMT FROM: Tomas Ahumada at Caltech <tfahumada94(a)gmail.com> G. Schroeder (Cornell), T. Ahumada (Caltech), A. Y. Q. Ho (Cornell), M. Kasliwal (Caltech), L.Yan (Caltech), G. Srinivasaragavan (UMD), X. J. Hall (CMU), M. Coughlin (UMN), D. Y. Li (NAO, CAS), J. Yang (ZZU), X. Tian (GXU), K.R., Ni, (CCNU), H. L., Peng (NJNU), H. Y, Liu, Y. J., Song, H. Sun, Y. Liu, C.C.Jin, W. Yuan (NAO, CAS) on behalf of the Zwicky Transient Facility and Einstein Probe (EP) Partnership We report the contemporaneous detection of the X-ray flare EP250827b associated with the optical transient ZTF25abmpngy/AT 2025wkm (RA = 02:26:20.64, Dec = +37:29:59.90). This event was highlighted in the cross-matching of ZTF alerts with EP alerts, an experiment first announced in Ahumada et al (GCN 39791). ZTF25abmpngy/AT 2025wkm was first detected on 2025-08-27T09:38:04 UT, 3.27 hrs after the X-ray flare, and with an r-band apparent magnitude of r = 20.33 +/- 0.11 mag. We note the transient sits on top of a potential host galaxy with no archival spectroscopic redshift. EP250827b was first detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at RA.= 36.581, DEC.= 37.499, with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic). The X-ray detection is 14.7 arcseconds away from the optical position. The X-ray transient started at 2025-08-27T06:22:27 (UT), and lasted for more than 1000 seconds as seen from the WXT light curve. The X-ray spectrum can be fitted by an absorbed power law model, with the absorption fixed at the Galactic value of 5.4x10^20 cm^-2 , and a photon index of 2.2 (-0.7, +0.8). The average unabsorbed 0.5-4keV flux is 2.6 (-0.9, +1.3) x 10^-11 erg/s/cm2. In the follow-up WXT coverage approximately 5 hours later at the position, the source was no longer detected, indicating a fast decay in the soft X-ray band. We performed a follow-up target of opportunity observation with EP-FXT. The observation began at 2025-09-01T02:35:49 with an exposure time of 2995 seconds, about 5 days after the burst detected by EP-WXT. Within the WXT error circle, no X-ray source was detected at the optical position, setting a flux upper limit at 90% confidence level around 5 x 10^-14 erg/s/cm2. Based on the current temporal and spatial consistency, EP250827b is likely associated with AT2025wkn. However, due to the significant systematic uncertainty in the WXT position and the non-detection in the follow-up FXT observation, this association cannot be fully confirmed. We are pursuing and encouraging follow-up observations to classify the source and validate the X-ray and optical association. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. View this GCN Circular online at https://gcn.nasa.gov/circulars/41635. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39700] EP250831a: EP-FXT follow-up observation
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41625 SUBJECT: EP250831a: EP-FXT follow-up observation DATE: 25/09/01 07:35:09 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. Yang (ZZU), X. Tian (GXU), D. Y. Li, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of EP250831a (GCN 41619) at 2025-08-31T15:30:41 (UTC), about 6 hours after the WXT detection, with an exposure time of 2.9 ks. The source was also followed by Swift-XRT (GCN 41624). FXT detected an uncatalogued source within the WXT error circle at R.A. = 42.9816, DEC = -36.1872 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with the absorption fixed at the Galactic value of 1.9x10^20 cm^-2, and a photon index of 2.3(+/-0.6). The derived unabsorbed 0.5-10 keV flux is approximately 8.15x10^-14 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. We noted that EP-FXT also detected the uncatalogued X-ray source outside the WXT error circle detected by Swift-XRT. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41625. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39699] GRB 250901A: SVOM detection of a burst
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41634 SUBJECT: GRB 250901A: SVOM detection of a burst DATE: 25/09/01 19:02:28 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> F. Piron (LUPM), C. Guépin (LUPM), S. Schanne (CEA), T. Maiolino (LUPM), C.-W. Wang (IHEP), M. Moita (CEA), report on behalf of the SVOM mission team: At 2025-09-01T18:14:22 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250901A (SVOM burst-id sb25090101). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network. The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 9 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.49 in the [8-120] keV energy band over a time window of 20.48 seconds starting at 2025-09-01T18:14:17. The localization of the best alert is R.A., Dec. 356.0950, -15.6712 degrees (J2000) with a 90% confidence level (C.L.) radius of 7.61 arcmin (including systematic error of 2 arcmin added in quadrature). This burst was also detected by SVOM/GRM with a significance of 10. The SVOM/GRM light curve showed a single peak structure with a T90 duration of about 17 seconds. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250901A.png SVOM slewed to the burst. SVOM/MXT began observing the field at 2025-09-01T18:17:20 UTC, 178 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 356.1835, -15.7952 degrees: R.A. (J2000) = 23h44m44.04s Dec. (J2000) = -15d47m42.73s with a 90% C.L. radius of 194 arcseconds (including systematic error). This location is 9.03 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received. VT began observing the field after the slew. The analysis of the data will be published in a future circular. We notice that the source SWIFT J2344.7-1573 is located 3.9 arcmin from the MXT position and 5.4 arcmin from the ECLAIRs position. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. The Burst Advocate (BA) on shift for this alert is Frédéric Piron: piron(a)in2p3.fr. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/41634. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39698] GRB 250831B: SVOM/GRM observation
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41633 SUBJECT: GRB 250831B: SVOM/GRM observation DATE: 25/09/01 18:09:16 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Olivier GODET (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by GRB 250831B (SVOM trigger reference: sb25083101) at 2025-08-31T06:09:22.000 UTC (T0). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 9.5 +4.0/-3.0 s in the 15-5000 keV band. ECLAIRs was not collecting data at the time of this burst. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250831B.png The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/41633. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39697] GRB 250830C: Insight-HXMT detection
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41632 SUBJECT: GRB 250830C: Insight-HXMT detection DATE: 25/09/01 18:07:08 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team: At 2025-08-30T14:51:55.100 (T0), Insight-HXMT/HE detected the burst GRB 250830C, which is also detected by GECAM-B (Wang et al., GCN #41630). The Insight-HXMT/HE light curve mainly consists of multi-pulses with a T90 of 12.6 +8.2/-3.6 s. The 1s peak rate, measured from T0+26.300 s, is 2735 cnts/sec. The total counts from this burst is 10641 counts. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb250830C.png All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org. View this GCN Circular online at https://gcn.nasa.gov/circulars/41632. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39696] EP250901a: Mephisto optical upper limits
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41631 SUBJECT: EP250901a: Mephisto optical upper limits DATE: 25/09/01 18:00:45 GMT FROM: liuxk(a)ynu.edu.cn Xiangkun Liu, Chenxu Liu, Guowang Du, Xufeng Zhu, Shuai Xie, Xiaowei Liu (all SWIFAR, YNU), and Chao Wu (EP team) report on behalf of the Mephisto Team: The field of EP250901a, detected by EP-WXT (ID: 00109201113, triggered at 2025-09-01T12:05:03 UTC) and subsequently followed up by EP-FXT (GCN 41627), was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgr band photometric observations were conducted starting from 13:30:37 2025-09-01 UT (~1.5 hr after the trigger) and 6 frames with 45s exposure times were taken. No new optical counterpart was detected (consistent with the upper limits reported by Global MASTER-Net, GCN 41629) in the stacked images of uvgr bands. The preliminary photometry and 3 sigma upper limits are listed below. Start_Time(UT) | Band | Exp(s) | LimMag (AB) --------------------|------|--------|------------ 2025-09-01T13:30:37 | u | 45*6 | >21.68 2025-09-01T13:30:37 | v | 45*6 | >21.77 2025-09-01T13:37:13 | g | 45*6 | >22.17 2025-09-01T13:37:13 | r | 45*6 | >22.31 ------------------------------------------------------------------------------------------------- Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21. ------------------------------------------------------------------------------------------------- View this GCN Circular online at https://gcn.nasa.gov/circulars/41631. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39695] GRB 250830C: GECAM-B detection
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41630 SUBJECT: GRB 250830C: GECAM-B detection DATE: 25/09/01 17:50:03 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team: GECAM-B was triggered in-flight by a bright long burst GRB 250830C at 2025-08-30T14:51:55.100 UTC (denoted as T0). According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of a precursor followed by multiple pulses main emission with a duration (T90) of 23.6 +12.0/-8.4 s. The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecamgrb250830C.png GECAM-B localized this burst to the following position (J2000): Ra: 240.1 deg Dec: 40.5 deg Err: 1.1 deg (1-sigma, statistical only) The time-averaged spectrum from T0-2 s to T0+40 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.06 +0.23/-0.24 and the cutoff energy, parameterized as Epeak, is 255 +121/-46 keV. The event fluence (10-1000 keV) in this time interval is (2.41 +/-0.36)E-05 erg/cm^2. The 'Amati' relation diagram of GRB 250830C is shown at: https://www.bursthub.cn//admin/static/grb250830C_amati.png Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS). View this GCN Circular online at https://gcn.nasa.gov/circulars/41630. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39694] EP250901a: Global MASTER-Net observations report
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41629 SUBJECT: EP250901a: Global MASTER-Net observations report DATE: 25/09/01 17:18:37 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the EP250901a ( EP Team et al., GCN 41627) errorbox 11710 sec after notice time and 17745 sec after trigger time at 2025-09-01 17:00:48 UT, with upper limit up to 17.5 mag. Observations started at twilight. The observations began at zenith distance = 53 deg. The sun altitude is -9.0 deg. The galactic latitude b = 16 deg., longitude l = 46 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2981475 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 17775 | 2025-09-01 17:00:48 | MASTER-SAAO | (18h 08m 55.12s , +18d 41m 44.3s) | C | 60 | 16.8 | 17776 | 2025-09-01 17:00:48 | MASTER-SAAO | (18h 10m 43.80s , +18d 26m 21.7s) | C | 60 | 15.2 | 18076 | 2025-09-01 17:05:48 | MASTER-SAAO | (18h 09m 00.19s , +18d 41m 53.8s) | C | 60 | 17.1 | 18076 | 2025-09-01 17:05:48 | MASTER-SAAO | (18h 10m 48.74s , +18d 26m 31.7s) | C | 60 | 16.3 | 18369 | 2025-09-01 17:10:42 | MASTER-SAAO | (18h 10m 47.53s , +18d 26m 45.9s) | C | 60 | 16.7 | 18369 | 2025-09-01 17:10:42 | MASTER-SAAO | (18h 08m 58.87s , +18d 42m 08.5s) | C | 60 | 17.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41629. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39693] GRB 250830C: AstroSat CZTI detection of a long burst
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41628 SUBJECT: GRB 250830C: AstroSat CZTI detection of a long burst DATE: 25/09/01 17:14:16 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> Harsha K H (IUCAA), A. Arya (IITB), M. Tembhurnikar (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration bright GRB 250830C which was also detected by Konus-Wind (IPN notices). We note that this burst is towards the tail end of solar activity as seen in GOES data. The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-08-30 14:52:21.50 UTC. The measured peak count rate associated with the burst is 248 (+42, -14) counts/s above the background in the combined data of two quadrants (out of four), with a total of 1394 (+282, -55) counts. The local mean background count rate was 135 (+5, -9) counts/s. Using cumulative rates, we measure a T90 of 9.5 (+4.8, -0.7) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-08-30 14:52:21.29 UTC. The measured peak count rate associated with the burst is 1516 (+85, -91) counts/s above the background in the combined data of all quadrants, with a total of 7100 (+323, -375) counts. The local mean background count rate was 1307 (+5, -4) counts/s. We measure a T90 of 9.2 (+1.5, -0.9) s from the cumulative Veto light curve. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/41628. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39692] EP250901a: Einstein Probe detection of an X-ray transient
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41627 SUBJECT: EP250901a: Einstein Probe detection of an X-ray transient DATE: 25/09/01 13:43:28 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. J. Song, T. Zhao, J. W. Hu, C. L. Guo, Z. X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250901a. The transient triggered EP-WXT (ID: 00109201113) at 2025-09-01T12:05:03 (UTC). The WXT position of the source is R.A. = 273.025 deg, DEC = 18.676 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The estimated WXT flux is around 4e-10 erg/s/cm^2 in 0.5-4.0 keV. A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an X-ray source was detected around 3 minutes later at R.A. = 273.0416 deg, DEC = 18.7062 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). The estimated FXT flux is around 2e-11 erg/s/cm^2 in 0.5-10.0 keV. We note that a historical ROSAT source (1RXS J181210.7+184228) positioned just 12 arcseconds from our FXT detection coordinates, with a flux about three orders of magnitude lower than the present WXT measurement. Further information will be updated when the telemetry data is received. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41627. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39691] LIGO/Virgo/KAGRA S250830m: Updated Sky localization
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41626 SUBJECT: LIGO/Virgo/KAGRA S250830m: Updated Sky localization DATE: 25/09/01 08:29:41 GMT FROM: alexandresebastien.goettel(a)LIGO.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250830m (GCN Circular 41601). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250830m For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 3194 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3731 +/- 1474 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/41626. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39690] EP250831a: EP-FXT follow-up observation
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41625 SUBJECT: EP250831a: EP-FXT follow-up observation DATE: 25/09/01 07:35:09 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. Yang (ZZU), X. Tian (GXU), D. Y. Li, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of EP250831a (GCN 41619) at 2025-08-31T15:30:41 (UTC), about 6 hours after the WXT detection, with an exposure time of 2.9 ks. The source was also followed by Swift-XRT (GCN 41624). FXT detected an uncatalogued source within the WXT error circle at R.A. = 42.9816, DEC = -36.1872 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with the absorption fixed at the Galactic value of 1.9x10^20 cm^-2, and a photon index of 1.4(-0.8, +0.8). The derived unabsorbed 0.5-10 keV flux is approximately 1.84x10^-13 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. We noted that EP-FXT also detected the uncatalogued X-ray source outside the WXT error circle detected by Swift-XRT. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41625. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39689] EP250831a: Swift-XRT observations
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41624 SUBJECT: EP250831a: Swift-XRT observations DATE: 25/09/01 04:39:35 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250831a, collecting 1.8 ks of Photon Counting (PC) mode data between T0+24 ks and T0+30 ks after the trigger. We have detected 1 source 4.6 arcmin from the WXT position, somewhat outside the 90% error region of 2.9 arcmin (GCN 41619). This has been automatically classified as an uncatalogued X-ray source. Uncatalogued X-ray sources -------------------------- Source 1 (SWIFT J025130.9-360728): ================================== RA (J2000.0): 42.8789 = 02 51 30.94 Dec (J2000.0): -36.1245 = -36 07 28.2 Error: 5.0 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 4.6 arcmin from the Einstein Probe/WXT position. Mean rate: (7.6 [+2.8, -2.3])e-3 ct s^-1 Mean flux: (1.97 [+0.73, -0.59])e-13 erg cm^-2 s^-1 Peak rate: (7.6 [+2.8, -2.3])e-3 ct s^-1 Peak flux: (1.97 [+0.73, -0.59])e-13 erg cm^-2 s^-1 ECF: 2.58e-11 erg cm^-2 ct^-1 assuming NH=3.05e+20 cm^-2, gamma=2.54 determined from a spectral fit. LSXPS UL: 1.7e-02 ct/sec, (0.3-10 keV) so the source is not above this 3-sigma upper limit. There is no evidence for fading. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00066. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41624. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39688] LIGO/Virgo/KAGRA S250831cc: Coverage and upper limits from MAXI/GSC observations
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41623 SUBJECT: LIGO/Virgo/KAGRA S250831cc: Coverage and upper limits from MAXI/GSC observations DATE: 25/09/01 03:03:21 GMT FROM: Hitoshi Negoro at Nihon University/MAXI team <negoro.hitoshi(a)nihon-u.ac.jp> H. Negoro, M. Nakajima, K. Takagi (Nihon U.), N. Kawai, T. Mihara, (RIKEN), S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU) report on behalf of the MAXI team: We examined MAXI/GSC all-sky X-ray images (2-20 keV) after compact binary merger candidate S250831cc at 2025-08-31 12:30:48.228 UTC. At the trigger time of S250831cc, the high-voltage of MAXI/GSC was on, but the FOV was out of the 90% credible region of the bayestar skymap. The first one-orbit (92 min) scan observation with GSC after the event covered 48% of the 90% credible region of the bayestar skymap from 13:12:46 to 13:24:01 UTC (T0+2518 to T0+3193 sec). No significant new source was found in the region in the one-orbit scan observation. A typical 1-sigma averaged upper limit obtained in one scan observation is 20 mCrab at 2-20 keV. If you require information about X-ray flux by MAXI/GSC at specific coordinates, please contact the submitter of this circular by email. View this GCN Circular online at https://gcn.nasa.gov/circulars/41623. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39687] IceCube Alert 250831.75: Global MASTER-Net observations report
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41622 SUBJECT: IceCube Alert 250831.75: Global MASTER-Net observations report DATE: 25/09/01 00:39:34 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the IceCube Alert 250831.75 (trigger No 5972385,07h 02m 33.60s , +05d 02m 24.0s, R=0.53) errorbox 20982 sec after notice time and 21045 sec after trigger time at 2025-08-31 23:51:31 UT, with upper limit up to 17.3 mag. The observations began at zenith distance = 83 deg. The sun altitude is -26.9 deg. The galactic latitude b = 5 deg., longitude l = 210 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2980982 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 21075 | 2025-08-31 23:51:31 | MASTER-Kislovodsk | (07h 00m 41.24s , +05d 07m 13.0s) | C | 60 | 15.9 | 21135 | 2025-08-31 23:51:31 | MASTER-Kislovodsk | (07h 00m 41.26s , +05d 07m 12.7s) | C | 180 | 16.4 | Coadd 21075 | 2025-08-31 23:51:31 | MASTER-Kislovodsk | (07h 00m 37.35s , +04d 44m 38.4s) | C | 60 | 15.9 | 21155 | 2025-08-31 23:52:51 | MASTER-Kislovodsk | (07h 00m 43.33s , +05d 08m 51.7s) | C | 60 | 15.8 | 21155 | 2025-08-31 23:52:51 | MASTER-Kislovodsk | (07h 00m 39.47s , +04d 46m 16.8s) | C | 60 | 15.8 | 21235 | 2025-08-31 23:54:11 | MASTER-Kislovodsk | (07h 00m 38.32s , +05d 08m 20.3s) | C | 60 | 15.7 | 21235 | 2025-08-31 23:54:11 | MASTER-Kislovodsk | (07h 00m 34.57s , +04d 45m 45.1s) | C | 60 | 15.9 | 21315 | 2025-08-31 23:55:31 | MASTER-Kislovodsk | (07h 00m 37.37s , +05d 09m 24.8s) | C | 60 | 15.9 | 21375 | 2025-08-31 23:55:31 | MASTER-Kislovodsk | (07h 00m 37.37s , +05d 09m 24.7s) | C | 180 | 16.4 | Coadd 21315 | 2025-08-31 23:55:31 | MASTER-Kislovodsk | (07h 00m 33.70s , +04d 46m 50.1s) | C | 60 | 16.0 | 21395 | 2025-08-31 23:56:50 | MASTER-Kislovodsk | (07h 00m 44.34s , +05d 08m 27.2s) | C | 60 | 15.9 | 21395 | 2025-08-31 23:56:50 | MASTER-Kislovodsk | (07h 00m 40.78s , +04d 45m 52.4s) | C | 60 | 15.9 | 21474 | 2025-08-31 23:58:10 | MASTER-Kislovodsk | (07h 00m 39.37s , +05d 07m 32.4s) | C | 60 | 16.0 | 21475 | 2025-08-31 23:58:10 | MASTER-Kislovodsk | (07h 00m 35.89s , +04d 44m 57.5s) | C | 60 | 16.0 | 22440 | 2025-09-01 00:13:15 | MASTER-Kislovodsk | (07h 00m 37.64s , +05d 10m 27.0s) | C | 180 | 17.1 | 22440 | 2025-09-01 00:13:15 | MASTER-Kislovodsk | (07h 00m 39.06s , +04d 47m 32.5s) | C | 180 | 17.3 | 23039 | 2025-09-01 00:23:14 | MASTER-Kislovodsk | (07h 00m 37.10s , +04d 48m 25.8s) | C | 180 | 17.0 | 23039 | 2025-09-01 00:23:14 | MASTER-Kislovodsk | (07h 00m 38.51s , +05d 11m 07.6s) | C | 180 | 16.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41622. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39686] EP250831a: Global MASTER-Net observations report
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41621 SUBJECT: EP250831a: Global MASTER-Net observations report DATE: 25/08/31 21:09:50 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the EP250831a ( EP Team et al., GCN 41619) errorbox 21491 sec after notice time and 40170 sec after trigger time at 2025-08-31 20:58:42 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 70 deg. The sun altitude is -56.9 deg. The galactic latitude b = -63 deg., longitude l = 239 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2980777 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 40201 | 2025-08-31 20:58:42 | MASTER-SAAO | (02h 50m 10.03s , -36d 31m 25.6s) | C | 60 | 18.8 | 40201 | 2025-08-31 20:58:42 | MASTER-SAAO | (02h 48m 03.66s , -36d 15m 35.6s) | C | 60 | 18.8 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41621. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39685] IceCube-250831A - IceCube observation of a high-energy neutrino candidate track-like event
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41620 SUBJECT: IceCube-250831A - IceCube observation of a high-energy neutrino candidate track-like event DATE: 25/08/31 20:44:36 GMT FROM: Erik Blaufuss at University of Maryland, College Park <blaufuss(a)umd.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: On 2025-08-31 18:00:45.82 UT IceCube detected a track-like event with a moderate probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_Bronze alert stream. The average astrophysical neutrino purity for Bronze alerts is 30%. This alert has an estimated false alarm rate of 2.14 events per year due to atmospheric backgrounds. The IceCube detector was in a normal operating state at the time of detection. After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/141300_5972385.amon) more sophisticated reconstruction algorithms have been applied offline, with the direction refined to: Date: 2025-08-31 Time: 18:00:45.82 UT RA: 105.64 (+0.51, -0.48 deg 90% PSF containment) J2000 Dec: 5.04 (+0.61, -0.50 deg 90% PSF containment) J2000 We encourage follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino. No known gamma-ray sources listed in the Fermi 4FGL-DR4 or 3FHL catalogs are located within the 90% uncertainty region of the event. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu View this GCN Circular online at https://gcn.nasa.gov/circulars/41620. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39684] EP250831a: EP-WXT detection of a fast X-ray transient
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41619 SUBJECT: EP250831a: EP-WXT detection of a fast X-ray transient DATE: 25/08/31 15:00:03 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J.Yang (ZZU), X. Tian (GXU), D. Y. Li, Y. Liu (NAO, CAS) report on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250831a (trigger ID: 11916651030). The WXT position of the source is R.A. = 42.951 deg, DEC = -36.175 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic). The transient started at 2025-08-31T09:49:12 (UTC) and lasted for about 300 seconds seen from the WXT light curve. The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with Galactic hydrogen column density fixed to 1.9x10^20 cm^-2 and a photon index of 1.07(-0.49, +0.52). The average unabsorbed 0.5-4 keV flux is approximately 1.4 x 10^-10 erg/s/cm^2. The peak flux is estimated to be 6.8 x 10^-10 erg/s/cm^2. Follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP have been scheduled, and further information will be updated when the telemetry data received. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41619. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39683] GRB 250831A (SVOM trigger sb25083102) is likely an outburst from star G 272-61
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41618 SUBJECT: GRB 250831A (SVOM trigger sb25083102) is likely an outburst from star G 272-61 DATE: 25/08/31 13:56:49 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> W.J. Xie, L. Lan, L.P. Xin (NAOC), Francesco Magnani (CPPM), D. B. Malesani (DAWN/NBI and Radboud) report on behalf of the SVOM mission team: At 2025-08-31T09:29:33 UTC SVOM issued an alert with burst-id sb25083102, which was previously referred to as GRB 250831A (Xie et al., GCN 41614). We found that there are several X-ray sources within the error box of ECLAIRs, including the high proper motion star G 272-61 (distance: 2.7 pc). This source was also noticed by Wu et al., (GCN 41615) in VT and Magnani et al. (GCN 41616) in COLIBRÍ (FM-GFT) images. Considering the proper motion of the source and the distance from the MXT localization (Xie et al., GCN 41614) and the XRT position (Evans et al., GCN 41617), we proposed that this SVOM trigger originated from an X-ray outburst of this star, and not an extragalactic GRB. We apologize for any inconvenience this may have caused. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. The Burst Advocate (BA) on shift for this alert is: Wenjin XIE (xiewj(a)bao.ac.cn) Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/41618. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39682] GRB 250831A: Swift-XRT counterpart detection
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41617 SUBJECT: GRB 250831A: Swift-XRT counterpart detection DATE: 25/08/31 13:40:52 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source GRB 250831A (trigger sb25083102), collecting 1.5 ks of Photon Counting (PC) mode data between T0+3.0 ks and T0+4.7 ks after the trigger. A likely counterpart has been found. The details of this source are: Source 1 (SWIFT J013907.0-175644): ================================== RA (J2000.0): 24.7795 = 01 39 07.08 Dec (J2000.0): -17.9457 = -17 56 44.5 Error: 3.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 3.4 arcmin from the SVOM/ECLAIRs position. Mean rate: 5.60 +/- 0.16 ct s^-1 Mean flux: (1.571 +/- 0.046)e-10 erg cm^-2 s^-1 Peak rate: 9.9 +/- 2.2 ct s^-1 Peak flux: (2.78 +/- 0.61)e-10 erg cm^-2 s^-1 ECF: 2.81e-11 erg cm^-2 ct^-1 assuming NH=1.22e+21 cm^-2, gamma=2.40 determined from a spectral fit. LSXPS UL: 6.6e-03 ct/sec, (0.3-10 keV) so the source is 4.6-sigma above this 3-sigma upper limit. There is no evidence for fading. GCN Circs. 41615 and 41616 noted that the high-proper-motion M-dwarf binary star system G 272-61 was within the SVOM field of view; with a proper motion of ~3.3 arcsec yr^-1, its position appears consistent with this XRT source. We have detected a total of 3 sources. These have been automatically classified as follows: * 1 likely counterpart * 0 candidate counterparts * 1 uncatalogued X-ray source * 1 known X-ray source Uncatalogued X-ray sources -------------------------- Source 2 (SWIFT J013909.3-175732): ================================== RA (J2000.0): 24.7888 = 01 39 09.31 Dec (J2000.0): -17.9590 = -17 57 32.4 Error: 5.4 (arcsec, radius, 90% confidence). Detect flag: POOR Distance: 3.2 arcmin from the SVOM/ECLAIRs position. Mean rate: 0.242 +/- 0.015 ct s^-1 Mean flux: (6.73 +/- 0.42)e-12 erg cm^-2 s^-1 Peak rate: 0.392 +/- 0.082 ct s^-1 Peak flux: (1.09 +/- 0.23)e-11 erg cm^-2 s^-1 ECF: 2.78e-11 erg cm^-2 ct^-1 assuming NH=8.33e+20 cm^-2, gamma=2.42 determined from a spectral fit. LSXPS UL: 5.7e-04 ct/sec, (0.3-10 keV) so the source is 4.8-sigma above this 3-sigma upper limit. There is no evidence for fading. Known X-ray sources ------------------- Source 3 (SWIFT J013932.8-175532): ================================== RA (J2000.0): 24.8869 = 01 39 32.86 Dec (J2000.0): -17.9257 = -17 55 32.5 Error: 8.1 (arcsec, radius, 90% confidence). Detect flag: POOR Distance: 2.9 arcmin from the SVOM/ECLAIRs position. Mean rate: (-1.0 +/- -1.0)e0 ct s^-1 Mean flux: (-2.2 +/- -2.2)e-24 erg cm^-2 s^-1 Peak rate: (3.0 [+2.2, -1.6])e-3 ct s^-1 Peak flux: (6.6 [+4.9, -3.7])e-27 erg cm^-2 s^-1 ECF: 2.24e-24 erg cm^-2 ct^-1 assuming NH=3.77e+20 cm^-2, gamma=98.61 determined from a spectral fit. This matches a catalogued X-ray source LSXPS J013933.3-175540 in the LSXPS catalogue. Details: Separation: 10.3" from the XRT source Cat Rate: 9.9e-04 +/- 2.4e-04 ct s^-1 Cat Flux: 2.2e-27 +/- 5.5e-28 erg cm^-2 s^-1 (0.3-10 keV) so the source is 1.0-sigma above the catalogued flux. There is no evidence for fading. A SIMBAD object `SDSS J013934.38-175541.9' is 23.8" away. There are 2 2MASS objects within the source's 3-sigma error radius. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00032. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41617. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39681] GRB 250831A: SVOM/COLIBRÍ (FM-GFT) optical upper limit
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41616 SUBJECT: GRB 250831A: SVOM/COLIBRÍ (FM-GFT) optical upper limit DATE: 25/08/31 12:59:04 GMT FROM: Francesco at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work(a)gmail.com> Francesco Magnani (CPPM), Alan M. Watson (UNAM), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Antonio de Ugarte Postigo (LAM), W.J. Xie, L. Lan (NAOC): We imaged the field of the SVOM GRB 250831A (W.J. Xie et al., GCN Circ. 41614) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-08-31T09:37:56 to 2025-08-31T11:43:45 UTC (from 8.4 to 134.2 minutes after the trigger) and obtained 88 minutes of exposure in the i and z filters. The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In the stacked image, we do not detect any new source at the MXT source position (W.J. Xie et al., GCN Circ. 41614) down to the following 5-sigma limit: i > 23.5 We notice the presence of a bright star, G 272-61, within the MXT field of view (position in agreement with a proper motion of approximately 3.3 arcsec / year). We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/41616. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39680] GRB 250831A: SVOM/VT upper limit with VHF data
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41615 SUBJECT: GRB 250831A: SVOM/VT upper limit with VHF data DATE: 25/08/31 11:12:25 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> C. Wu, L.P. Xin, Y. L. Qiu, H.L. Li, Z.H. Yao, Y.N. Ma, X.H. Han, J. Wang, W.J. Xie, Y. Xu, H.B. Cai, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM team. SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Xie et al., GCN 41614). SVOM/VT began observing the field automatically at 2025-08-31T09:35:46.00 UTC, 523 seconds after the T0, with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With downlinked VHF data, no any uncatalogued sources were found in stacked images within the errorbox of SVOM/MXT (Xie et al., GCN 41614) compared to Legacy survey. The 3 sigma limit in AB magnitude was derived as follows: --------------------------------------------------------- (T-T0)_mid exptime Band upperlim (3sigma) 523 sec 300 sec VT_R 21.5 mag We noticed that there is a very bright cataloged M type binary (G 272-61) near the MXT position, which is saturated in VT images. Its position is RA=24.75605 and D.E.=-17.95056 in J2000. More follow-ups are encouraged to investigate the nature of the burst. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41615. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39679] GRB 250831A: SVOM detection of a soft burst
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41614 SUBJECT: GRB 250831A: SVOM detection of a soft burst DATE: 25/08/31 10:30:46 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> W.J. Xie, L. Lan (NAOC), S. Guillot, O. Godet (IRAP), M. Moita (CEA), L. Zhang (IHEP) report on behalf of the SVOM mission team: At 2025-08-31T09:29:33 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250831A (SVOM burst-id sb25083102). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network. The burst was only detected by the Image Trigger (IMT), which produced a sequence of 3 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 9.21 in the [5-20] keV energy band over a time window of 163.84 seconds starting at 2025-08-31T09:28:52. The localization of the best alert is R.A., Dec. 24.8358, -17.9289 degrees (J2000) with a 90% confidence level (C.L.) radius of 8.60 arcmin (including systematic error of 2 arcmin added in quadrature). SVOM slewed to the burst. SVOM/MXT began observing the field at 2025-08-31T09:33:53 UTC, 260 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 24.7771, -17.9379 degrees: R.A. (J2000) = 1h39m07.44s Dec. (J2000) = -17d56m31.2s with a 90% C.L. radius of 28.7 arcseconds (systematic and statistical). This location is 3.39 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received. VT began observing the field after the slew. The analysis of the data will be published in a future circular. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. The Burst Advocate (BA) on shift for this alert is : Wenjin XIE (xiewj(a)bao.ac.cn) Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/41614. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39678] GRB 250830B: Swift-XRT observations
by GCN Circulars 31 Aug '25

31 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41613 SUBJECT: GRB 250830B: Swift-XRT observations DATE: 25/08/31 04:57:05 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the MAXI-detected burst GRB 250830B, collecting 2.4 ks of Photon Counting (PC) mode data between T0+11.0 ks and T0+50.8 ks. This event also triggered Einstein Probe around 25 s after MAXI, and was separately designated EP250830a. One uncatalogued X-ray source has been detected within the estimated 3-sigma EP error region (296 arcsec), also consistent with the MAXI error region; it is below the RASS limit and shows no definitive signs of fading. Therefore, at the present time we cannot confirm this as the afterglow. Details of this source are given below: Source 1: RA (J2000.0): 105.7399 = 07h 02m 57.59s Dec (J2000.0): -64.8525 = -64d 51' 09.1" Error: 3.7 arcsec (radius, 90% conf. [Enhanced position]) Count-rate: 0.0323 +/- 0.0046 ct s^-1 Distance: 134 arcsec from EP position. Flux: (1.74 +/- 0.25)e-12 erg cm^-2 s^-1 (observed, 0.3-10 keV) A further uncatalogued X-ray source was also detected, however this was too far from the EP position to be the afterglow. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021858. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41613. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39677] EP250830a: Global MASTER-Net observations report
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41612 SUBJECT: EP250830a: Global MASTER-Net observations report DATE: 25/08/30 14:27:21 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250830a ( EP Team et al., GCN 41611) errorbox 9623 sec after trigger time at 2025-08-30 06:37:32 UT, with upper limit up to 20.1 mag. The observations began at zenith distance = 65 deg. The sun altitude is -54.1 deg. The galactic latitude b = -23 deg., longitude l = 275 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2980099 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 9644 | 2025-08-30 06:37:32 | MASTER-OAFA | (07h 03m 18.97s , -64d 47m 00.0s) | C | 40 | 19.3 | 9684 | 2025-08-30 06:37:32 | MASTER-OAFA | (07h 03m 18.97s , -64d 46m 60.0s) | C | 120 | 20.1 | Coadd 9690 | 2025-08-30 06:38:18 | MASTER-OAFA | (07h 03m 18.83s , -64d 46m 58.5s) | C | 40 | 19.3 | 9735 | 2025-08-30 06:39:04 | MASTER-OAFA | (07h 03m 18.65s , -64d 46m 56.6s) | C | 40 | 19.3 | 9781 | 2025-08-30 06:39:50 | MASTER-OAFA | (07h 03m 18.51s , -64d 46m 55.0s) | C | 40 | 19.3 | 9821 | 2025-08-30 06:39:50 | MASTER-OAFA | (07h 03m 18.52s , -64d 46m 54.9s) | C | 120 | 20.1 | Coadd 9827 | 2025-08-30 06:40:35 | MASTER-OAFA | (07h 03m 18.38s , -64d 46m 53.5s) | C | 40 | 19.3 | 9872 | 2025-08-30 06:41:21 | MASTER-OAFA | (07h 03m 18.20s , -64d 46m 52.4s) | C | 40 | 19.3 | 9918 | 2025-08-30 06:42:07 | MASTER-OAFA | (07h 03m 18.10s , -64d 46m 51.1s) | C | 40 | 19.4 | 9964 | 2025-08-30 06:42:52 | MASTER-OAFA | (07h 03m 17.96s , -64d 46m 49.8s) | C | 40 | 19.4 | 10010 | 2025-08-30 06:43:38 | MASTER-OAFA | (07h 03m 17.75s , -64d 46m 48.2s) | C | 40 | 19.3 | 10056 | 2025-08-30 06:44:24 | MASTER-OAFA | (07h 03m 17.57s , -64d 46m 46.8s) | C | 40 | 19.3 | 10101 | 2025-08-30 06:45:10 | MASTER-OAFA | (07h 03m 17.44s , -64d 46m 45.6s) | C | 40 | 16.0 | 10147 | 2025-08-30 06:45:56 | MASTER-OAFA | (07h 03m 17.20s , -64d 46m 44.0s) | C | 40 | 19.1 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41612. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39676] EP250830a/GRB 250830B: Einstein Probe detection of an X-ray transient and Swift/XRT follow-up
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41611 SUBJECT: EP250830a/GRB 250830B: Einstein Probe detection of an X-ray transient and Swift/XRT follow-up DATE: 25/08/30 14:16:50 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> R.D. Liang (NAOC), K. R. Ni (CCNU), X. Tian(GXU), W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250830a. The transient triggered EP-WXT (ID: 01709200995) at 2025-08-30T03:57:09 (UTC), whcih is about 25 s after the trigger of GRB 250830B by MAXI (GCN 41602). The WXT position of the source is R.A. = 105.723 deg, DEC = -64.816 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic), spatially consistent with GRB 250830B. The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with Galactic hydrogen column density fixed to 1.1 x 10^21 cm^-2 and a photon index of 1.6 (+/- 0.2), and a peak flux of approximately 1 x 10^-9 erg/s/cm^2. We proposed a follow-up observation with the Swift XRT that was performed at 2025-08-30T07:00:13, approximately 3.05 h after the trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 105.73708 deg, DEC = -64.85286 deg (J2000) with an uncertainty of 5 arcsec in radius (90% C.L. statistical and systematic). The integrated 0.3-10 keV spectrum can be fitted with an absorbed powerlaw model with fixed identity Galactic hydrogen column density and a photon index of 0.9 (+/-0.5), and an averaged flux of 2.5 x 10^-12 erg/s/cm^2. We gratefully acknowledge the support of the Swift team. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41611. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39675] LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41610 SUBJECT: LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations DATE: 25/08/30 14:05:37 GMT FROM: Satoshi Sugita at Aoyama Gakuin U. <sugita(a)phys.aoyama.ac.jp> T. Mihara, N. Kawai (RIKEN), M. Nakajima, H. Negoro, K. Takagi (Nihon U.), S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU) report on behalf of the MAXI team: We examined MAXI/GSC all-sky X-ray images (2-20 keV) after compact binary merger candidate S250830bp at 2025-08-30 10:24:18.852 UTC (GCN 41606, 41607). At the trigger time of S250830bp, the high-voltage of MAXI/GSC was on, but the FOV was out of the 90% credible region of the Bilby skymap. The credible region was observed about 3 minutes before the GW trigger time with GSC, and the first one-orbit (92 min) scan observation after the event covered 100% of the 90% credible region from 11:52:34 to 11:53:19 UTC (T0+5296 to T0+5341 sec). No significant new source was found in the region in those observations. A typical 1-sigma averaged upper limit obtained in one scan observation is 20 mCrab at 2-20 keV. If you require information about X-ray flux by MAXI/GSC at specific coordinates, please contact the submitter of this circular by email. View this GCN Circular online at https://gcn.nasa.gov/circulars/41610. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39674] LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41609 SUBJECT: LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume DATE: 25/08/30 14:01:52 GMT FROM: Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti(a)ligo.org> M. L. Brozzetti (UniPG/INFN), G. Dálya (L2IT/EotvosU), G. Greco (INFN), M. Bawaj (UniPG/INFN), T. Matcovich (UniPG/INFN), S. Cutini (INFN) , R. De Pietri (UniPR/INFN), Marica Branchesi (GSSI) On behalf of the GLADEnet Team. We analyzed the completeness of the GLADE+ [1] catalog within the 90% credible localization volume of the S250830bp event from the 4-Update alert from the GCN Circular 41606. The completeness value is 6.6e-1 in the B-band using the last released skymap : [Bilby.multiorder.fits,0](https://gracedb.ligo.org/api/superevents/S250830bp…, which means that the catalogue contains 66% of the total light in the B-band expected from galaxies in the localization volume. A total of 517 galaxies are identified within the 90% gravitational volume. The complete list of galaxies can be downloaded from the GLADEnet webpage [2] : [https://virgo.pg.infn.it/gladenet/catalogs/](https://virgo.pg.infn.it/glade… GLADEnet allows for the interactive visualization of the 90% localization area and its intersection with regions of high extinction as defined in GLADE+. Furthermore, the first 1000 galaxies can be explored interactively, enabling users to filter galaxies based on their 3D probability density or their absolute B magnitude. The ligo.skymap cross-match method [3,4] is used to obtain the list of galaxies. References: [1]GLADE+: An Extended Galaxy Catalogue for Multimessenger Searches with Advanced Gravitational-wave Detectors G. Dálya et al. MNRAS, 514,1, pp.1403-1411, 2022 [2] GLADEnet: A progressive web app for multi-messenger cosmology and electromagnetic follow-ups of gravitational-wave sources M. L. Brozzetti, G. Dálya, G. Greco, M. Bawaj, T. Matcovich, M. Branchesi, T. Boch, M. Baumann, S. Cutini, R. De Pietri et al. (4 more) A&A, 684, A44 (2024) [3] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Lett., 829, L15. doi:10.3847/2041-8205/829/1/L15 [4] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Supp., 226, 10. doi:10.3847/0067-0049/226/1/10 View this GCN Circular online at https://gcn.nasa.gov/circulars/41609. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39673] LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41608 SUBJECT: LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume DATE: 25/08/30 13:49:57 GMT FROM: David Cook at Caltech/IPAC-NED <dcook(a)ipac.caltech.edu> David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC) On behalf of the NASA/IPAC Extragalactic Database (NED) Team. We spatially cross-matched the LVK S250830bp-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 72 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links: Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4 Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4/20 The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume. | objname| ra| dec|objtype| DistMpc|DistMpc_unc| m_NUV| m_NUV_unc| m_Ks| m_Ks_unc| m_W1| m_W1_unc| P_3D|P_3D_LumW1| |-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------| |WISEA J215127.50-771123.7| 327.86462| -77.18992| G| 452.47| 0.65| null| null| 12.307| 0.126| 10.624| 0.006|1.78e-05| 2.49e-06| |WISEA J214848.24-774454.9| 327.20101| -77.74860| G| 463.54| null| null| null| 13.779| 0.215| 12.525| 0.010|5.02e-05| 1.28e-06| |WISEA J214657.45-773648.1| 326.73941| -77.61338| G| 462.69| null| null| null| 13.264| 0.192| 12.822| 0.014|5.09e-05| 9.83e-07| |WISEA J215919.74-775235.8| 329.83229| -77.87662| G| 451.54| 0.65| null| null| 12.506| 0.114| 12.064| 0.008|1.84e-05| 6.79e-07| |WISEA J213625.95-773427.9| 324.10814| -77.57443| G| 382.59| null| null| null| 13.364| 0.175| 12.873| 0.009|5.20e-05| 6.48e-07| |WISEA J214445.54-771938.0| 326.18978| -77.32723| G| 452.13| null| 21.430| 0.312| 13.324| 0.160| 13.240| 0.012|4.93e-05| 6.16e-07| |WISEA J213608.67-772811.0| 324.03613| -77.46974| G| 356.09| null| null| null| 13.524| 0.185| 12.576| 0.014|4.08e-05| 5.79e-07| |WISEA J215658.48-774037.0| 329.24368| -77.67696| G| 403.25| null| null| null| 13.090| 0.145| 12.789| 0.009|3.68e-05| 5.55e-07| |WISEA J213700.75-773426.0| 324.25313| -77.57391| G| 325.21| 0.65| 20.310| 0.217| 12.016| 0.075| 11.838| 0.006|2.20e-05| 5.14e-07| |WISEA J213454.05-772745.8| 323.72522| -77.46275| G| 372.81| null| null| null| 13.059| 0.143| 13.016| 0.016|4.75e-05| 4.92e-07| |WISEA J215146.52-780557.3| 327.94386| -78.09927| G| 368.25| 0.65| null| null| 12.882| 0.135| 12.228| 0.010|2.20e-05| 4.63e-07| |WISEA J213733.97-780150.6| 324.39156| -78.03074| G| 345.10| null| 20.443| 0.108| 13.417| 0.204| 11.831| 0.007|1.74e-05| 4.63e-07| |WISEA J212943.06-775542.2| 322.42945| -77.92839| G| 372.02| 0.65| null| null| 12.273| 0.114| 12.187| 0.009|1.85e-05| 4.12e-07| |WISEA J213608.13-772733.1| 324.03389| -77.45922| G| 319.73| 0.65| 21.351| 0.351| 12.228| 0.076| 11.985| 0.007|2.01e-05| 3.96e-07| |WISEA J212913.11-773431.7| 322.30466| -77.57550| G| 378.32| 0.65| 20.656| 0.234| 13.354| 0.165| 12.886| 0.013|2.92e-05| 3.50e-07| |WISEA J213641.68-775334.3| 324.17371| -77.89287| G| 411.08| null| 21.445| 0.179| 13.521| 0.179| 13.546| 0.029|4.09e-05| 3.18e-07| |WISEA J213632.95-765253.9| 324.13731| -76.88165| G| 460.55| 0.65| null| null| 12.341| 0.134| 12.450| 0.009|1.07e-05| 2.86e-07| |WISEA J213724.56-773623.7| 324.35235| -77.60660| G| 536.58| null| null| null| 13.729| 0.212| 13.039| 0.022|1.35e-05| 2.84e-07| |WISEA J215903.81-773020.5| 329.76589| -77.50571| G| 426.05| null| null| null| 13.020| 0.144| 12.826| 0.010|1.74e-05| 2.83e-07| |WISEA J212501.23-771251.5| 321.25514| -77.21431| G| 436.36| null| null| null| 13.307| 0.143| 12.903| 0.015|1.69e-05| 2.66e-07| Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250830bp sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). View this GCN Circular online at https://gcn.nasa.gov/circulars/41608. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39672] LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41607 SUBJECT: LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification DATE: 25/08/30 12:55:43 GMT FROM: Colm Talbot at University of Chicago <talbotcolm(a)gmail.com> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250830bp (GCN Circular 41606). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250830bp Assuming the candidate is astrophysical, the updated classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%). Based on posterior support from parameter estimation [1], under the assumption that the candidate S250830bp is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%. For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 4 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis): icrs; ellipse(21h43m, -77d36m, 1.36d, 0.88d, 15.49d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 427 +/- 69 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe View this GCN Circular online at https://gcn.nasa.gov/circulars/41607. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39671] LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41606 SUBJECT: LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate DATE: 25/08/30 10:55:06 GMT FROM: Pedro Jeronimo Santos da Silva <pedro.jeronimo(a)unesp.br> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250830bp during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-30 10:24:18.852 UTC (GPS time: 1440584676.852). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], MLy [6], PyCBC Live [7], and SPIIR [8] analysis pipelines. S250830bp is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250830bp The classification of the GW signal, in order of descending probability, is BBH (84%), NSBH (16%), Terrestrial (<1%), or BNS (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [9] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [9] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 16%. The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [10], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time. * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [11], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 11 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 429 +/- 100 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [6] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034 [7] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [8] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [9] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [10] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982 [11] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41606. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39670] EP-WXT trigger 01709200994: Liverpool Telescope upper limits
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41605 SUBJECT: EP-WXT trigger 01709200994: Liverpool Telescope upper limits DATE: 25/08/30 10:11:26 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris and N. Habeeb (U of Leicester) report on behalf of a larger collaboration: We observed the field of the EP-WXT trigger 01709200994 with the 2m Liverpool Telescope using the IO:O instrument. We obtained 6x150s exposures in each of the SDSS r’ and SDSS g’ filters starting at 2025-08-29 23:55:21 UT, approximately 3.0 hours after the X-ray detection. We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. We detect no new sources in either the subtractions or stacked images. This is consistent with 01709200994 being a stellar flare (Ni et al., GCN 41599; Perez-Garcia et al., GCN 41603). We derive 3-sigma upper limits of r’ > 22.6 and g’ > 22.4 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41605. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39669] GRB 250830B: Swift ToO observations
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41604 SUBJECT: GRB 250830B: Swift ToO observations DATE: 25/08/30 07:01:14 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P. A. Evans (U. Leicester) reports on behalf of the Swift team: Swift has initiated a ToO observation of the MAXI-detected event GRB 250830B. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021858 Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the MAXI event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration. Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8). This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41604. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39668] EP-WXT trigger 01709200994: confirmation of a stellar flare by BOOTES-6
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41603 SUBJECT: EP-WXT trigger 01709200994: confirmation of a stellar flare by BOOTES-6 DATE: 25/08/30 05:46:00 GMT FROM: Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct(a)iaa.es> I. Perez-Garcia, S.-Y. Wu, E. Fernandez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martin-Carrillo and L. Hanlon (UCD, Ireland), M. Gritsevich (Univ. of Helsinki) and C. J. Perez del Pulgar (UMA, Malaga), on behalf of a larger collaboration, report: Following the detection of EP-WXT trigger 01709200994 (Ni et al. GCNC 41599), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) responded to the alert on Aug 29, 21:51 UT (i.e. 54 min after detection and 2 min after notification). Within the reported EP-FXT error circle we find the star LP 524-48 decreasing in brightness from 14.2 to 14.6 mag in clear filter (using Gaia DR3 Gmag magnitude as reference) during a 1 hour time interval, confirming this event as due to a stellar flare, as already suggested by the EP team. We thank the staff at Boyden Observatory for their excellent support. View this GCN Circular online at https://gcn.nasa.gov/circulars/41603. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39667] GRB 250830B: MAXI/GSC detection
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41602 SUBJECT: GRB 250830B: MAXI/GSC detection DATE: 25/08/30 05:37:22 GMT FROM: Hitoshi Negoro at Nihon University/MAXI team <negoro.hitoshi(a)nihon-u.ac.jp> T. Usuki (Ehime U.), H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U),report on behalf of the MAXI team: The MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source at 03:56:44 UT on August 30, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (105.209 deg, -64.628 deg) = (07 00 50, -64 37 40) (J2000) with a 90% C.L. statistical error of 0.33 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 225 +- 36 mCrab (4.0-10.0keV, 1 sigma error). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (R.A., Dec) = (105.751, -63.592) deg = (07 03 00, -63 35 31) (J2000) (R.A., Dec) = (104.300, -63.666) deg = (06 57 11, -63 39 57) (J2000) (R.A., Dec) = (104.713, -65.344) deg = (06 58 51, -65 20 38) (J2000) (R.A., Dec) = (106.255, -65.266) deg = (07 05 01, -65 15 57) (J2000) There was no significant excess flux in the previous transit at 02:23 UT and in the next transit at 05:30 UT with an upper limit of 20 mCrab for each. We requested Swift/XRT followup observation. View this GCN Circular online at https://gcn.nasa.gov/circulars/41602. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39666] LIGO/Virgo/KAGRA S250830m: Identification of a GW compact binary merger candidate
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41601 SUBJECT: LIGO/Virgo/KAGRA S250830m: Identification of a GW compact binary merger candidate DATE: 25/08/30 04:40:26 GMT FROM: Jyotirmaya Mohanta at University of Tsukuba <s2430161(a)u.tsukuba.ac.jp> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250830m during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-08-30 03:27:09.028 UTC (GPS time: 1440559647.028). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], PyCBC Live [6], and SPIIR [7] analysis pipelines. S250830m is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.2e-08 Hz, or about one in 2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250830m The classification of the GW signal, in order of descending probability, is BBH (98%), Terrestrial (2%), BNS (<1%), or NSBH (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [9], distributed via GCN and SCiMMA notices about 22 seconds after the candidate event time. * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [10], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 3130 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3705 +/- 1112 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [9] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982 [10] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41601. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39665] GRB 250830A / MAXI J0853+337: MAXI/GSC detection
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41600 SUBJECT: GRB 250830A / MAXI J0853+337: MAXI/GSC detection DATE: 25/08/30 03:56:44 GMT FROM: Motoko Serino at Aoyama Gakuin U. <serino(a)phys.aoyama.ac.jp> T. Usuki (Ehime U.), M. Nakajima, H. Negoro (Nihon U.), M. Serino (AGU), K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U), report on behalf of the MAXI team: The MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source at 00:53:04 UT on August 30, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (133.397 deg, 33.778 deg) = (08 53 35, +33 46 40) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.26 deg and 0.26 deg, respectively. The roll angle of the long axis from the north direction is 34.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 182 +- 40 mCrab (4.0-10.0keV, 1 sigma error). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (R.A., Dec) = (132.876, 34.855) deg = (08 51 30, +34 51 17) (J2000) (R.A., Dec) = (132.372, 34.578) deg = (08 49 29, +34 34 40) (J2000) (R.A., Dec) = (133.708, 32.906) deg = (08 54 49, +32 54 21) (J2000) (R.A., Dec) = (134.206, 33.177) deg = (08 56 49, +33 10 37) (J2000) There was no significant excess flux in the previous transit at August 29 23:20 UT and in the next transit at August 30 02:26 UT with an upper limit of 20 mCrab for each. We tantatively name the source MAXI J0853+337, although we called it MAXI J0853+338 in MAXI new-transient ML. View this GCN Circular online at https://gcn.nasa.gov/circulars/41600. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39664] The EP-WXT trigger 01709200994 is likely a flaring star
by GCN Circulars 30 Aug '25

30 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41599 SUBJECT: The EP-WXT trigger 01709200994 is likely a flaring star DATE: 25/08/30 03:22:25 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> K.R. Ni (CCNU), X. Tian (GXU), Y. J. Song (NAO,CAS) and R. D. Liang, W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709200994 at the time of 2025-08-29 20:57:26, is likely a stellar flare associated with LP 524-48. The estimated flux of the flare is around 6.92e-12 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 4.52e29 erg/s. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41599. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39663] GRB 250828B: Swift-BAT refined analysis
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41598 SUBJECT: GRB 250828B: Swift-BAT refined analysis DATE: 25/08/29 23:16:38 GMT FROM: Amy <yarleen(a)gmail.com> M. J. Moss (GSFC), S. D. Barthelmy (GSFC), R. Gupta (GSFC), P. A. Evans (U Leicester), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team): Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250828B (trigger #1345836) (Evans et al., GCN Circ. 41581). The BAT ground-calculated position is RA, Dec = 124.867, -17.352 deg which is RA(J2000) = 08h 19m 28.2s Dec(J2000) = -17d 21' 07.8" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%. The mask-weighted light curve shows a complex structure. The main pulse starts and peaks at ~T0, and ends at ~T+5 s. In addition, there are some weak emission that starts prior to the main pulse at ~T-60 s and persists after the main pulse until ~T+60 s. T90 (15-350 keV) is 94.09 +- 32.03 sec (estimated error including systematics). The time-averaged spectrum from T-56.23 to T+76.66 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.13 +- 0.19. The fluence in the 15-150 keV band is 1.5 +- 0.2 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T+0.12 sec in the 15-150 keV band is 1.7 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1345836 View this GCN Circular online at https://gcn.nasa.gov/circulars/41598. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39662] EP250821a: SOAR observations
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41597 SUBJECT: EP250821a: SOAR observations DATE: 25/08/29 19:29:04 GMT FROM: James Freeburn at University of North Carolina at Chapel Hill <jamesfreeburn54(a)gmail.com> J. Freeburn (UNC), G. Srinivasaragavan (UMD), I. Andreoni (UNC), Anna Ho (Cornell) We observed the location of the X-ray transient, EP250821a (Hu et al., GCN 41459) with the Goodman High Throughput Spectrograph mounted on the SOAR telescope in imaging mode (PI: Andreoni). We took two 300s exposures each in g, r and i-band between 2025-08-29T01:15:29 and 2025-08-29T01:43:27 UTC corresponding to ~8 days after the initial trigger. With image subtraction using Legacy Survey images (Dey et al., 2019), we do not detect the optical counterpart associated with EP250821a (Dichiara et al., GCN 41469; Li et al., GCN 41470; Yao et al., GCN 41485; Oates et al., GCN 41491) and place the following 5-sigma AB magnitude upper limits: g > 22.9 r > 22.9 i > 22.8 View this GCN Circular online at https://gcn.nasa.gov/circulars/41597. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39661] EP250827a: very early ATLAS detections, starting 43 sec after the EP trigger
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41596 SUBJECT: EP250827a: very early ATLAS detections, starting 43 sec after the EP trigger DATE: 25/08/29 19:11:08 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), A. López-Oramas, and D.S. Aguado (IAC and ULL) We report on very early ATLAS detections of the optical afterglow of EP250827a, a fast X-ray transient discovered by Einstein Probe WXT (ID: 01709200787) on 2025-08-27 07:40:43 UTC (Hua et al., GCN 41553; and Ni et al., GCN 41573), with UV-optical detections reported in Levan et al. (GCN 41554), Li et al. (GCN 41555), Levan et al. (GCN 41557), Lipunov et al. (GCN 41558), Gritsevich et al. (GCN 41559), An et al. (GCN 41560), Ghosh et al. (GCN 41564), O’Neill et al. (GCN 41567), Pérez-Fournon et al. (GCN 41568), Passaleva et al. (GCN 41570), Eyles-Ferris et al. (GCN 41576), Santos et al. (GCN 41580), and Siegel et al. (GCN 41585), spectroscopic redshift of z = 1.613-1.61 (Levan et al., GCN 41557; and Passaleva et al., GCN 41570) and Swift-XRT follow-up observations (Evans et al., GCN 41563). Using the ATLAS forced-photometry server (Shingles et al. 2021, Transient Name Server AstroNote 2021-7), we found that there were four ATLAS observations that covered the EP250827a position on 2025-08-27, two of them before the EP trigger and two after it. As expected, the source was not detected in the ATLAS pre-trigger observations but it is well detected in the two post-trigger observations, the first of them at a start time of only about 43 sec (about 16 sec in the source rest frame) after the EP trigger. There is a flux rise between the two observations. The source is well detected in both the original (reduced) and difference (reduced - reference) ATLAS images. For the forced photometry measurements, we used the coordinates of the optical counterpart given by Levan et al. (GCN 41554) from the analysis of Gemini-South GMOS observations. In the following, we give the ATLAS limiting magnitudes and forced-photometry magnitudes (using difference images), not corrected for Galactic extinction. MJD | 5-sigma limit | filter | ATLAS observation | 60914.306552 | 20.14 | orange | 04a60914o0247o | 60914.313502 | 20.18 | orange | 04a60914o0252o | MJD | mag | error | filter | ATLAS observation | 60914.320437 | 18.707 | 0.070 | orange | 04a60914o0257o | 60914.327372 | 18.568 | 0.065 | orange | 04a60914o0262o | The reported ATLAS observations MJD times are the start times of the individual 30-sec exposures. The first detection is likely the earliest one after the trigger of an optical afterglow of EP-discovered fast X-ray transients. View this GCN Circular online at https://gcn.nasa.gov/circulars/41596. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39660] LIGO/Virgo/KAGRA S250725j: Observations with the NSF-DOE Vera C. Rubin Observatory
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41595 SUBJECT: LIGO/Virgo/KAGRA S250725j: Observations with the NSF-DOE Vera C. Rubin Observatory DATE: 25/08/29 19:02:14 GMT FROM: Sean MacBride at University of Zurich <sean.p.macbride(a)gmail.com> Sean MacBride (University of Zurich), Shreya Anand (Stanford University), Erin Howard, Ian Sullivan, David Wang, Lynne Jones, Peter Yoachim, Eric Bellm (University of Washington), Bob Armstrong (Lawrence Livermore National Laboratory), Michael Wood-Vasey (University of Pittsburgh), Alex Drlica-Wagner, Ken Herner (Fermi National Accelerator Laboratory), Gautham Narayan (University of Illinois), Tatiana Acero Cuellar, Federica Bianco (University of Delaware), Bruno Sánchez (Aix Marseille Université), John Banovetz (Lawrence Berkeley National Laboratory), Kris Mortensen, Paulina Venegas, Yijung Kang, Yousuke Utsumi, and Tiago Ribeiro (NSF-DOE Vera C Rubin Observatory), reporting on behalf of NSF-DOE Vera C Rubin Observatory: We observed the localization region of the binary black hole merger S250725j, reported by LIGO/Virgo/KAGRA (GCN 41154, GCN 41168) with the 9.6 square degree field of view LSST Camera mounted on the 8.4-m Simonyi Survey Telescope at the Vera C. Rubin Observatory. This is the first Target-of-Opportunity observation by Rubin Observatory. Rubin Observatory is conducting a Science Validation (SV) survey as part of its commissioning activities [1]. During this period, data quality and acquisition rate are variable. Not all data products that will be available during the Legacy Survey of Space and Time (LSST) operations are currently available [2]. Observations were conducted as a part of commissioning the Rubin Target-of-Opportunity (ToO) system. The observing strategy for this gravitational wave event follows the community recommendation for a binary black-hole merger candidate of this distance and sky condition [3]. The observing strategy calls for multiple epochs of observation, but weather conditions have prevented further observations. The localization region of S250725j is not inside the SV survey footprint, so no prior Rubin images of this region were available. We obtained two visits in each of the u, r, and i bands with 30s exposures. Observations were taken on the night of July 29, 2025, beginning at 02:28:07.875 UTC and concluding at 03:20:18.800 UTC. We observed with nine 9.6 square degree pointings centered on the following ICRS coordinates: | RA | DEC | |---------|---------| | 230.604 | -38.402 | | 232.198 | -35.510 | | 232.568 | -41.024 | | 233.614 | -32.596 | | 234.198 | -38.132 | | 235.633 | -35.209 | | 236.925 | -32.268 | | 237.769 | -37.776 | We covered 94% of the total localization area in all three bands, reaching a median depth of 24.1 mag in r band, 23.8 mag in the i band, and 23.6 in u band. Due to the lack of prior Rubin observations of the region, we performed image differencing against DECam templates from DES DR2 [4]. To support the usage of DECam templates, we devised a custom image processing task to create DECam templates from DES templates for use with LSSTCam observations. Apart from the DECam template task, we utilize the standard LSST data release production pipeline to obtain difference images and photometry on difference image sources [5]. With only a single epoch of observations at Rubin observatory, we are unable to characterize viable candidates from Rubin observations alone. Here, we report on candidates reported in GCN 41206 and GCN 41177. We detect four candidates reported by the DESGW collaboration (GCN 41206), and two candidates reported by the GW-MMADS collaboration (GCN 41177, two candidates recovered by both collaborations). AT2025sib, AT2025srk, and AT2025srm are detected in both r and i band difference images. At the position of AT2025srm and AT2025sib there is a galaxy in the DES template images, and a point source present in the difference images. AT2025srk shows a point source present in the LSSTCam images, but not the DES templates. We caution that AT2025shz may be a variable star, as there is a point source at the location in the DES templates. | AT name | Observation Date | RA | Dec | r | unc_r | i | unc_i | |-----------|------------------|-----------|------------|-------|-------|-------|-------| | AT2025sib | 2025-07-29 | 235.548892 | -35.002876 | 20.95 | 0.03 | 20.76 | 0.03 | | AT2025srk | 2025-07-29 | 234.755524 | -36.425993 | 21.17 | 0.04 | 21.31 | 0.06 | | AT2025srm | 2025-07-29 | 232.987656 | -36.816564 | 21.90 | 0.05 | 22.99 | 0.21 | | AT2025shz | 2025-07-29 | 231.072434 | -37.622634 | 22.25 | 0.07 | 21.97 | 0.11 | Analysis of these candidates is ongoing. This material is based upon work supported in part by the National Science Foundation through Cooperative Agreements AST-1258333 and AST-2241526 and Cooperative Support Agreements AST-1202910 and 2211468 managed by the Association of Universities for Research in Astronomy (AURA), and the Department of Energy under Contract No. DE-AC02-76SF00515 with the SLAC National Accelerator Laboratory managed by Stanford University. Additional Rubin Observatory funding comes from private donations, grants to universities, and in-kind support from LSST-DA Institutional Members. [1] https://sitcomtn-005.lsst.io, §6 [2] https://lse-163.lsst.io [3] www.inspirehep.net/literature/2846480 [4] www.inspirehep.net/literature/1841103 [5] https://pstn-019.lsst.io View this GCN Circular online at https://gcn.nasa.gov/circulars/41595. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39659] LIGO/Virgo/KAGRA S250818k: MeerKAT detection of an increase in radio flux from AT2025ulz
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41594 SUBJECT: LIGO/Virgo/KAGRA S250818k: MeerKAT detection of an increase in radio flux from AT2025ulz DATE: 25/08/29 18:07:17 GMT FROM: Gabriele Bruni at INAF <gabriele.bruni(a)inaf.it> G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report: We re-observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 28, 14:20 UT (at 10 days post-trigger) for a total of 4 hours (3.23 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 5.3 uJy/beam. We detect a clear flux increase in the radio emission with respect to the first epoch at 3.6 days post trigger (GCN 41500), with an integrated flux density of 157 +/- 9 uJy. The increment with respect to the first epoch is thus ~80 uJy. Further MeerKAT observations are planned. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. —------------------------ [1] INAF-IAPS View this GCN Circular online at https://gcn.nasa.gov/circulars/41594. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39658] GRB 250808B: SVOM detection of a burst through offline search
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41593 SUBJECT: GRB 250808B: SVOM detection of a burst through offline search DATE: 25/08/29 14:19:30 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> M.Brunet, O. Godet (IRAP), report on behalf of the SVOM team: The SVOM/ECLAIRs telescope detected a transient source, labelled GRB 250808B, starting at 2025-08-08T08:32:21.85 UTC (T0), through an offline search with the event-by-event data downloaded through the X-band ground station. The burst was detected within several energy ranges in a 5.12 s timescale. The best detection is obtained by the image trigger with a signal-to-noise ratio of 8.2 within 4-20 keV over a time window of 5.12 seconds starting at T0. The lightcurve shows a double peak structure lasting around 5 s. The localization of the source is RA, Dec = 358.829, 15.300 degrees: RA (J2000) = 23h55m18.96s Dec (J2000) = 15d18m00.00s with a 90% C.L. radius of 11.2 arcmin (including systematic error of 6 arcmin added in quadrature). We note that no bright catalogued X-ray sources are found in the error box. The time-averaged spectrum from T0 to T0+5s in the energy range 4-20 keV is best fitted by a power law. The powerlaw index is -2.37 -0.57/+0.64. With this model, the total fluence in 4-120 keV is (6.71 +0.68/-3.93)e-8 erg/cm^2. All quoted errors are at the 68% confidence level. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES, and IRAP. The SVOM point of contact for this event is : Marius Brunet : marius.brunet at irap.omp.eu Please contact him by email if you require additional information regarding the SVOM follow-up of this burst. View this GCN Circular online at https://gcn.nasa.gov/circulars/41593. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39657] EP250828a: refined analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41592 SUBJECT: EP250828a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/08/29 13:13:17 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> K. R. Ni (CCNU), H. L. Peng (NNU), H. Y. Liu, Y. J. Song and H. Sun (NAO,CAS) on behalf of the Einstein Probe (EP) team: We report on the refined analysis of EP-WXT and FXT observations of the fast X-ray transient EP250828a (Liu et al., GCN 41574), with the optical couterparts observed by Seimei (Taguchi et al., GCN 41582), SVOM/VT (Yao et al., GCN 41583), NOT (Fu et al., GCN 41584), Kinder (Aryan et al., GCN 41591). The WXT data shows that the event started at T0=2025-08-28T07:27:16 (UTC) and lasted for around 1500s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.75 x 10^20 cm^-2 and a photon index of 1.48 (-0.59/+0.63). The derived average unabsorbed 0.5-4 keV flux is 1.25(-0.39/+0.50) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The EP-FXT observed the field of EP250828a about 1.4 hrs after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 327.8932, DEC = 20.5429 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.75 x 10^20 cm^-2 and a photon index of 2.04(-0.41/+0.43). The derived average unabsorbed 0.5-10 keV flux is 4.43 (-1.29/+1.90) x 10^(-13)erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41592. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39656] EP250828a: Optical counterpart detection with Kinder observations
by GCN Circulars 29 Aug '25

29 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41591 SUBJECT: EP250828a: Optical counterpart detection with Kinder observations DATE: 25/08/29 10:12:24 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941(a)gmail.com> A. Aryan, C.-S. Lin (both NCU), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), T.-W. Chen (NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report: Followed by our r-band observations using the 1m LOT (Aryan et al., GCN 41579), we continued to observe the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) in i-band using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first SLT epoch of observations in i-band started at 14:17 UTC on the 28th of August 2025 (MJD 60915.595), 6.74 hr after the EP-WXT trigger. We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 (Chambers et al., 2016 arXiv:1612.05560) image using the 'sfft' (Hu et al., 2022, ApJ, 936,157) algorithm. In the stacked as well as in the difference image, we weakly detected the optical counterpart candidate proposed by Taguchi et al., (GCN 41582). The candidate counterpart was also confirmed by Yao et al. (GCN 41583) and Fu et al. (GCN 41584). In our i-band difference image, the optical counterpart was detected with an SNR of 2.5. The details of the observations and the measured photometry (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | i | 60915.595 | 6.74 | 300 * 24 | 22.46 +/- 0.27 | 1".48 | 1.01 The presented magnitude is calibrated using the field stars from the Pan-STARRS1 catalog and is not corrected for the expected Galactic foreground extinction of A_i = 0.21 mag in the direction of the transient (Schlafly & Finkbeiner 2011). View this GCN Circular online at https://gcn.nasa.gov/circulars/41591. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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