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[vsnet-grb-info 40165] GRB 251003A: COLIBRÍ photometric redshift
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42095 SUBJECT: GRB 251003A: COLIBRÍ photometric redshift DATE: 25/10/03 15:49:16 GMT FROM: Benjamin Schneider at MIT <bschn(a)mit.edu> Benjamin Schneider (LAM), Ny Avo Rakotondrainibe (LAM), Enrique Moreno Méndez (UNAM), Antonio de Ugarte Postigo (LAM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), 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), Francesco Magnani (CPPM), Margarita Pereyra (UNAM) and Fredd Sánchez Álvarez (UNAM): COLIBRÍ performed additional observations of the Fermi and Swift GRB 251003A (Fermi GBM team, GCN Circ. 42069; Beardmore et al., GCN Circ. 42070) starting from 2025-10-03 04:04:59 (2.1 hr after the trigger) and obtained images in the g, i, y filters. The photometry of the afterglow (Aceituno et al. GCN Circ. 42072; Moreno Méndez et al. GCN Circ. 42073; Strausbaugh et al. GCN Circ. 42074; Freeberg et al. GCN Circ. 42085; Worssam et al., GCN Circ. 42094) was performed using STDWeb/STDPipe (Karpov 2025), was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. After combining data obtained from Moreno Méndez et al., GCN Circ. 42073 and correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.36 mag (Schlafly & Finkbeiner 2011), we fit a power-law model to the grizy-bands using the SMC extinction curve and we derive a photometric redshift of z = 4.38+0.07-0.04 (1-sigma c.l., statistical errors only). Our result is consistent with the GTC/OSIRIS+ redshift of z = 4.41 (Sanchez-Ramirez et al., GCN Circular 42086). 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/42095. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40164] GRB 251003A: GOTO detection of the optical counterpart
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42094 SUBJECT: GRB 251003A: GOTO detection of the optical counterpart DATE: 25/10/03 14:27:54 GMT FROM: I. Worssam at University of Birmingham <ixw352(a)student.bham.ac.uk> I. Worssam, B. P. Gompertz, D. O’Neill, M. E. Wortley, G. Ramsay, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar,, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration: We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 251003A (Fermi GBM team, GCN 42069; Beardmore et al. GCN 42070). Targeted observations were performed at 2025-10-03 02:03:05 (4.57mins post trigger) and 2025-10-03 03:11:32 (1.22h post trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). We detect the optical counterpart (Aceituno et al., GCN 42072; Méndez et al., GCN 42073; Strausbaugh et al., GCN 42074; Freeberg et al., GCN 42085; Sanchez-Ramirez et al., GCN 42086) with measured AB magnitudes of L = 19.94 ± 0.24 and L = 20.83 ± 0.28 for the two epochs respectively. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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/42094. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40163] GRB 251002A: Fermi GBM Observation
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42093 SUBJECT: GRB 251002A: Fermi GBM Observation DATE: 25/10/03 14:21:41 GMT FROM: Jacob Smith at Fermi-GBM Team <jrs0118(a)uah.edu> Jacob Smith (UAH), O. Mukherjee (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 20:14:51.01 UT on 02 October 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251002A (trigger 781128896/251002844). which was also detected by SVOM/ECLAIRs (A. Saccardi et al. 2025, GCN 42060). The Fermi GBM on-ground location is consistent with the SVOM/ECLAIRs position. VLT/X-shooter detected a spectroscopic redshift of z = 2.178 (A. Saccardi et al. 2025, GCN 42076). The angle from the Fermi LAT boresight is 53 degrees. The GBM light curve multiple emission episodes with a duration (T90) of about 26 s (50-300 keV). The time-averaged spectrum from T0-6.9 to T0+14.6 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.88 +/- 0.07 and the cutoff energy, parameterized as Epeak, is 180 +/- 10 keV. The event fluence (10-1000 keV) in this time interval is (4.6 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+7.9 s in the 10-1000 keV band is 7.1 +/- 0.2 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 140 +/- 20 keV, alpha = -0.7 +/- 0.1 and beta = -2.2 +/- 0.1. 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/42093. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40162] LIGO/Virgo/KAGRA S251003at: Retraction of GW compact binary merger candidate
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42092 SUBJECT: LIGO/Virgo/KAGRA S251003at: Retraction of GW compact binary merger candidate DATE: 25/10/03 13:58:15 GMT FROM: Dounia Nanadoumgar Lacroze at IFAE Barcelona <dnala(a)ifae.es> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: The trigger S251003at is no longer considered to be a candidate of interest. This candidate was initially identified by one or more early-warning analyses by matching partial signal templates to the data. Analysis of additional data up to the putative merger time, with full signal templates, did not make a significant detection, indicating that the initial candidate was likely due to transient noise. View this GCN Circular online at https://gcn.nasa.gov/circulars/42092. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40161] GRB 251002A: GOTO detection of optical counterpart
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42091 SUBJECT: GRB 251002A: GOTO detection of optical counterpart DATE: 25/10/03 13:45:47 GMT FROM: MEW020(a)student.bham.ac.uk M. E. Wortley, B. P. Gompertz, D. O’Neill, G. Ramsay, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar,, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration: We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) that serendipitously covered the field of GRB 251002A (Saccardi et al., GCN 42060) during survey operations. We detect the optical counterpart (Palmerio et al., GCN 42061; Turpin et al., GCN 42062; Jelinek et al., GCN 42063; Julia-Maroto et al., GCN 42064; Moskvitin et al., GCN 42065; Perez-Garcia et al., GCN 42606; Senik et al., GCN 42067; Saccardi et al., GCN 42076; Mandarakas et al., GCN 42077; Cao et al., GCN 42078; Schneider et al., GCN 42080; Pankov et al., GCN 42081; Leonini et al., GCN 42082; Kennea et al., GCN 42084; Odeh et al., GCN 42087) in GOTO L-band (400-700 nm) imaging taken at 2025-10-03 01:03:06 UT (4.80h post-trigger) consisting of 4x45s exposures. The measured AB magnitude is L = 19.87 ± 0.31. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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/42091. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40160] GRB 251001C: SVOM/GRM observation
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42090 SUBJECT: GRB 251001C: SVOM/GRM observation DATE: 25/10/03 13:40:21 GMT FROM: yzh807926(a)163.com SVOM/GRM team: Zheng-Hang Yu, Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Frédéric Piron (LUPM), Jean-Luc Atteia (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a burst GRB 251001C (SVOM trigger reference: sb25100106) at 2025-10-01T15:07:27.000 UTC (T0), which is also detected by Glowbug gamma-ray telescope (C.C. Cheung et al. 2025, GCN#42049). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists mainly of a single pulse with a T90 of 11.7 +4.5/-4.1 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb251001C.png In addition, the position of this burst, as determined by Glowbug gamma-ray telescope (RA= 82.7, DEC= 21.9, GCN#42049), is located at about 59.47 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-3 to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.24 +0.19/-0.22 and the cutoff energy, parameterized as Epeak, is 301 +314/-97 keV. The event fluence (10-1000 keV) in this time interval is (3.46 +0.53/-0.50)E-06 erg/cm^2. The localization of GRB 251001C in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb251001C_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: Zheng-Hang Yu(IHEP)(zhyu(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/42090. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40159] EP250821a: iTelescope T-59 possible detection of the optical counterpart
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42089 SUBJECT: EP250821a: iTelescope T-59 possible detection of the optical counterpart DATE: 25/10/03 12:59:37 GMT FROM: Takanori Sakamoto at AGU <tsakamoto(a)phys.aoyama.ac.jp> H. Ban, T. Katou, Y. Nagata, K. Hotta, Y. Kawakubo, M. Serino, T. Sakamoto (AGU) We observed the field of EP250821a detected by the Einstein Probe (EP) (Hu et al., GCN Circ. 41459) with the iTelescope.Net (http://www.itelescope.net) T59 (Planewave 20" CDK) telescope located at the Siding Spring Observatory, Australia. We took nine images of 120-second exposures in the Red filter starting from August 21 at 10:25:34 (UT), about 3 hours after the trigger, and stopped at 12:48:14 (UT). Please note that there were no observations between 10:33 and 11:30, and between 11:39 and 12:41 due to our scheduling error. We have a weak detection of the optical counterpart (Dichiara et al., GCN Circ. 41469; Li et al., GCN Circ. 41470; Yao et al., GCN Circ. 41485) in the stacked image of the first three exposures from 10:25:34 to 10:32:47 UT. The estimated magnitude is ~18.5. No optical counterpart was detected in the following six images collected between 11:30:00 and 12:48:14 (UT). The 5-sigma upper limit of the stacked image of the last six images is 17.5 mag. View this GCN Circular online at https://gcn.nasa.gov/circulars/42089. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40158] GRB 251003A: Enhanced Swift-XRT position
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42088 SUBJECT: GRB 251003A: Enhanced Swift-XRT position DATE: 25/10/03 11:31:53 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) report on behalf of the Swift-XRT team. Using 1392 s of XRT Photon Counting mode data and 4 UVOT images for GRB 251003A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 18.52924, +56.49080 which is equivalent to: RA (J2000): 01h 14m 07.02s Dec (J2000): +56d 29' 26.9" with an uncertainty of 2.5 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular was automatically generated, and is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42088. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40157] GRB 251002A: AKO Early Optical Afterglow Detection
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42087 SUBJECT: GRB 251002A: AKO Early Optical Afterglow Detection DATE: 25/10/03 11:28:22 GMT FROM: Mohammad Odeh at Al Khatim Observatory M44 <mshodeh(a)gmail.com> Mohammad Odeh (Al-Khatim Observatory, AKO, operated by the International Astronomical Center in Abu Dhabi, UAE), and Shaikha Alshamsi, and Nidhal Guessoum (American University of Sharjah, UAE), report: We observed the field of GRB 251002A detected by SVOM/ECLAIRs (Saccardi et al., GCN 42060), using our 0.36m f/7.7 robotic telescope. The observation session began on 02 October 2025 at 21:08 UT, with a midpoint at 21:44 UT, approximately 1.49 hours after the trigger. We obtained 20 exposures of 180 seconds each using the Ic filter. The optical afterglow was detected at: R.A. (J2000): 00:55:38.0 Dec. (J2000): -05:34:01.9 Our detection is consistent with the results of (Palmerio et al., GCN 42061), (Turpin et al., GCN 42062), (Jelinek et al., GCN 42063), (Perez-Fournon et al., GCN 42064), (Moskvitin et al., GCN 42065), (Perez-Garcia et al., GCN 42066), (Lipunov et al., GCN 42067), (Saccardi et al., GCN 42076), (Mandrakas et al., GCN 42077), (Cao et al., GCN 42078), (Schneider et al., GCN 42080), (Pankov et al., GCN 42081) and (Leonini et al., GCN 42082). The following observation was calculated using the Atlas catalogue as a reference: ---------------------------------------------------------------------------- ObsTime (mid), Exposure (sec), Filter, Mag ---------------------------------------------------------------------------- 2025-10-02T21:44:20Z, 20x180s (stacked), Ic, 18.57 +/- 0.27 ---------------------------------------------------------------------------- The magnitude is not corrected for galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/42087. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40156] GRB 251003A: GTC/OSIRIS+ spectroscopic redshift z = 4.412
by GCN Circulars 03 Oct '25

03 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42086 SUBJECT: GRB 251003A: GTC/OSIRIS+ spectroscopic redshift z = 4.412 DATE: 25/10/03 10:34:36 GMT FROM: Ruben Sanchez-Ramirez at IAA-CSIC <ruben(a)iaa.es> R. Sanchez-Ramirez, A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), Y.-D. Hu (GXI), R. Scarpa (GTC, IAC), D. Gonzalez (GTC), A. Cabrera-Lavers (GTC, IAC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), J. Becerra-Gonzalez (IAC), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report: Following the detection of GRB 251003A by Fermi (Fermi Team, GCN 42069) and Swift (Beardmore et al. GCN 42070, Evans et al. GCN 42071), we observed the optical afterglow (Aceituno et al., GCN 42072, Moreno Mendez et al., GCN 42073, Strausbaugh et al., GCN 42074) with the 10.4m GTC telescope, at the Spanish Roque de los Muchachos Observatory, on the island of La Palma, equipped with the OSIRIS+ instrument. The spectroscopic observations consisted of 2x900s exposures using grism R1000B, and 1x900s using R2500I, covering the spectral range between 3,600 and 10,000 A. The observations started on Oct 3, 03:12 UT (i.e. 1.22 h after the burst trigger). From a preliminary reduction, we find a strong DLA at ~6500A with its corresponding Lyman break at ~5000A. Several metal absorption lines are detected as well, which we interpret as coming from SiII, SiII*, CII, SiIV, CIV, FeII, FeII*, NiII, AlII, AlIII, all at z=4.412. The detection of fine structure lines links this system to the GRB. View this GCN Circular online at https://gcn.nasa.gov/circulars/42086. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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