August 2025 - vsnet-grb-info - ooruri.kusastro.kyoto-u.ac.jp

[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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