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[vsnet-grb-info 39867] EP250911a: Optical detection with Kinder observations
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41801 SUBJECT: EP250911a: Optical detection with Kinder observations DATE: 25/09/11 17:32:52 GMT FROM: Janet Chen at National Central University <janetstars(a)gmail.com> C.-H. Lai, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), Y.-C. Shiau (FEMH), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, 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: We observed the field of the fast X-ray transient EP250911a (Liang et al., GCN 41788) using the 1m LOT and 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first LOT epoch of observations in r-band started at 13:17 UTC on the 11th of September 2025 (MJD 60929.554), 4.31 hr after the EP WXT trigger. Moreover, the first SLT epoch of observations in i-band started at 13:18 UTC on the 11th of September 2025 (MJD 60929.554), 4.33 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. In the stacked frames of r, i and z bands from LOT and i-band from SLT, we clearly detected the optical counterpart candidate proposed by Quirola-Vasquez et al. (GCN 41790) and confirmed by Angulo et al. (GCN 41793). Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured magnitude (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60929.554 | 4.31 | 300 * 6 | 21.30 +/- 0.05 | 0".74 | 1.29 SLT | i | 60929.554 | 4.33 | 300 * 24 | 20.44 +/- 0.11| 1".3 | 1.14 LOT | i | 60929.601 | 5.46 | 300 * 6 | 20.71 +/- 0.07 | 0".73 | 1.10 LOT | z | 60929.642 | 6.43 | 300 * 6 | 20.50 +/- 0.09 | 0".97| 1.02 The presented magnitudes are calibrated using the field stars from the SDSS catalog and are not corrected for the expected Galactic foreground reddening of E(B-V) = 0.05 mag in the direction of the transient (Schlafly & Finkbeiner 2011). View this GCN Circular online at https://gcn.nasa.gov/circulars/41801. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39866] GRB 250911A: Swift/UVOT Detection
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41800 SUBJECT: GRB 250911A: Swift/UVOT Detection DATE: 25/09/11 16:49:55 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) and N. J. Klingler (GSFC/UMBC/CRESSTII) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250911A 3.5 ks after the BAT trigger (Klingler et al., GCN Circ. 41781). A fading afterglow consistent the XRT position (Beardmore et al., GCN Circ. 41784) and COLIBRI afterglow (Angulo et al., GCN Circ. 41782) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 17:35:00.03 = 263.75011 (deg.) Dec (J2000) = -16:51:14.9 = -16.85413 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL white 3483 4252 344 19.95+/-0.16 v 4462 4662 196 >19.20 v 26629 26655 25 >17.87 b 3847 4047 196 19.40+/-0.22 b 10440 10955 503 20.58+/-0.33 u 3642 5214 330 19.18+/-0.19 u 21026 21075 48 >18.94 uvw1 4872 5072 196 >19.53 uvw1 20120 21019 885 >20.01 uvm2 4667 4867 196 >19.38 uvm2 16021 16815 782 >20.49 uvw2 4258 4458 196 >19.50 uvw2 25723 26622 885 >20.09 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.4185 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41800. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39865] GRB 250910A / EP250910a: Swift-XRT afterglow detection
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41799 SUBJECT: GRB 250910A / EP250910a: Swift-XRT afterglow detection DATE: 25/09/11 16:29:52 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250910A. We searched for X-ray sources in 1.2 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 1.2 ks, obtained between T0+29.5 ks and T0+30.7 ks. An uncatalogued X-ray source is detected and is believed to be the afterglow. Using 1195 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 39.80164, -39.69498 which is equivalent to: RA (J2000): 02h 39m 12.39s Dec(J2000): -39d 41' 41.9" with an uncertainty of 8.4 arcsec (radius, 90% confidence). This source is consistent with the position reported by EP/FXT (Liu et al., GCN 41776) and with the optical/NIR source detected with VLT (Schneider et al., GCN 41794) and SVOM/VT (Li et al., GCN 41798). We cannot determine at the present time whether the source is fading. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000083. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00036. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41799. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39864] GRB 250910A / EP250910a: SVOM/VT afterglow confirmation
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41798 SUBJECT: GRB 250910A / EP250910a: SVOM/VT afterglow confirmation DATE: 25/09/11 15:27:28 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H.L. Li (NAOC), Y.N. Ma, L.P. Xin, Y. L. Qiu, C. Wu, Z.H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), D. H. Zhao, J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Ma et al., GCN 41769) and EP-WXT (Liu et al., GCN 41776). The observation with good image quality began at 2025-09-10T04:00:48 UTC, ie. 44 min post trigger in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. A second ToO observation was performed to the field (Wu et al., GCN 41772; Li et al., GCN 41774; Garnichey et al., GCN 41777; Lipunov et al., GCN 41789), which stared at 2025-09-11T09:10:42 UTC, i.e., 1.24 day post trigger. The NIR counterpart reported by VLT (Schneider et al., GCN 41794) in the errorbox of SVOM/MXT (Götz et al., GCN 41770), EP-FXT (Liu et al., GCN 41776) and Swift/XRT (https://www.swift.ac.uk/SVOM/SVOM_FIELD00036/) was detected in our stacked images in both channels. It was fading from our earliest observation at about 22.7+/-0.3 mag to 23.6+/-0.4 mag (1.27 days) in VT_R band. 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/41798. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39863] GRB 250909B: Insight-HXMT detection
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41797 SUBJECT: GRB 250909B: Insight-HXMT detection DATE: 25/09/11 15:16:08 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 2025-09-09T17:49:10.200 (T0), Insight-HXMT/HE detected the burst GRB 250909B, which is also detected by AstroSat (M. Tembhurnikar et al., GCN#41775), SVOM/GRM (Wang et al., GCN#41796) and Konus-Wind. The Insight-HXMT/HE light curve mainly consists of multi-pulses with a T90 of 12.9 +1.2/-1.7 s. The 1s peak rate, measured from T0+0.250 s, is 5422 cnts/sec. The total counts from this burst is 23044 counts. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb250909B.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/41797. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39862] GRB 250909B: SVOM/GRM observation
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41796 SUBJECT: GRB 250909B: SVOM/GRM observation DATE: 25/09/11 15:07:29 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: Jean-Luc Atteia (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered on-ground by GRB 250909B at 2025-09-09T17:49:10.200 UTC (T0), which is also detected by AstroSat (M. Tembhurnikar et al., GCN#41775) and Konus-Wind. With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a multi-pulses with a T90 of 10.5 +3.5/-1.5 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/svgrb250909B.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/41796. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39861] LIGO/Virgo/KAGRA S250911ac: Updated Sky localization and Source Classification
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41795 SUBJECT: LIGO/Virgo/KAGRA S250911ac: Updated Sky localization and Source Classification DATE: 25/09/11 13:17:25 GMT FROM: natalie.williams(a)uni-potsdam.de 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 S250911ac (GCN Circular 41786). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250911ac %}Assuming the candidate is astrophysical, the updated classification of the GW signal, in order of descending probability, is BBH (96%), Terrestrial (4%), BNS (<1%), or NSBH (<1%). For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 952 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1124 +/- 285 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/41795. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39860] GRB 250910A / EP250910a : VLT/HAWK-I NIR counterpart discovery
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41794 SUBJECT: GRB 250910A / EP250910a : VLT/HAWK-I NIR counterpart discovery DATE: 25/09/11 12:51:17 GMT FROM: Benjamin Schneider at MIT <bschn(a)mit.edu> B. Schneider (LAM), A. Martin-Carrillo (UCD), M. Garnichey (LUX-Paris Obs.), V. D'Elia (ASI-SSDC), A. J. Levan (Radboud U.), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), A. L. Thakur (INAF-IAPS), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration: We observed again the field of GRB 250910A / EP250910a (Ma et al., GCN 41769; Götz et al. GCN 41770; Wu et al., GCN 41772; Li et al., GCN 41774; Liu et al., GCN 41776) using the ESO VLT UT4 (Yepun) equipped with the HAWK-I NIR camera. We obtained 15 min imaging in the J band, beginning on 2025-09-11 at 09:36:48 UT (1.26 days post-trigger). Carrying out image subtraction with HOTPANTS between our new and previous (Garnichey et al., GCN 41777) images, a clear transient source is detected at coordinates (0.5" error): RA(J2000) = 02:39:12.56 Dec(J2000) = -39:41:40.9 These coordinates are consistent with the EP/FXT X-ray location (Liu et al., GCN 41776) and Swift/XRT X-ray detection (https://www.swift.ac.uk/SVOM/SVOM_FIELD00036/) We measure for the transient a magnitude J = 22.73 +/- 0.08 (AB, at 5.2 hr post trigger), calibrated against nearby 2MASS stars and not corrected for Galactic extinction. We caution that strictly this is a lower limit to the source brightness, as it is possible that residual transient light was present in our second observation. The spatial association with the X-ray afterglow and its fading behavior indicate that this object is the NIR afterglow of GRB 250910A / EP250910a. We note that at this location, an extended optical source can be seen in the Legacy Survey (magnitudes g = 24.56, r = 23.80, i = 23.38, z = 23.66), likely being the host galaxy of this source. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Juan Carlos Olivares and Robert De Rosa. View this GCN Circular online at https://gcn.nasa.gov/circulars/41794. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39859] EP250911a: COLIBRÍ optical observations
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41793 SUBJECT: EP250911a: COLIBRÍ optical observations DATE: 25/09/11 12:10:52 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Camila Angulo (UNAM), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of the EP250711a (Liang et al., GCN Circ. 41788) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-11-09 09:29 to 10:33 UTC (from 0.52 to 1.58 hours after the trigger, and 13 minutes after the notice) and obtained 45 minutes of exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ pipeline and analysed 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 our both filters, we detected the optical counterpart reported by LCO (Quirola-Vasquez et al., GCN Circ. 41790), at preliminary magnitudes of: r = 20.57 +/- 0.02 z = 19.87 +/- 0.03 Further observations and analysis are ongoing. 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/41793. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39858] LIGO/Virgo/KAGRA S250908y: Updated Sky localization
by GCN Circulars 11 Sep '25

11 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41792 SUBJECT: LIGO/Virgo/KAGRA S250908y: Updated Sky localization DATE: 25/09/11 11:27:49 GMT FROM: natalie.williams(a)uni-potsdam.de The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1) and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250908y (GCN Circular 41750). 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/S250908y For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 9926 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1146 +/- 334 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/41792. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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