GRB 251003A
GCN Circular 42170
Subject
GRB 251003A: Fermi GBM Observation
Date
2025-10-08T18:10:09Z (4 days ago)
From
Jacob Smith at Fermi-GBM Team <jrs0118@uah.edu>
Via
Web form
Jacob Smith (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 01:58:31.96 UT on 03 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251003A (trigger 781149516/251003082),
which was also detected by Swift BAT (A. P. Beardmore et al. 2025, GCN 42070).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
GTC/OSIRIS+ detected a spectroscopic redshift z = 4.412 (R. Sanchez-Ramirez et al. 2025, GCN 42086).
The angle from the Fermi LAT boresight is 56 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 10 s (50-300 keV). The time-averaged spectrum
from T0-4.1 to T0+6.1 s is best fit by
a simple power law function with index -1.7 +/- 0.1.
A Comptonized function fits equally well with a Epeak of 118 +/- 35 and alpha of -1.3 +/- 0.2.
The event fluence (10-1000 keV) in this time interval is
(1.1 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-1.5 s in the 10-1000 keV band is 1.6 +/- 0.2 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/"
GCN Circular 42126
Subject
GRB 251003A: LAST optical observations
Date
2025-10-06T17:54:49Z (6 days ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form
R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) on behalf of the LAST Collaboration
We report observations of GRB 251003A, detected by Fermi (Fermi team, GCNC 42069) and Swift (Beardmore et al., GCNC 42070), with the Large Array Survey Telescope (LAST; Ofek et al. 2023 PASP 135, 5001; Ben-Ami et al. 2023 PASP 135, 5002).
We observe the field of GRB 251003A using 4 divergent telescopes, each with a FoV of 7.4 deg^2 and no filter (clear - similar to the GAIA Bp band) over several epochs. In each epoch, we coadd 20 images with each of 20s exposure. We detect a faint source coincident with the optical counterpart reported by 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; Worssam et al., GCN 42094; Schneider et al., GCN 42095; Mo et al., GCN 42106.
The earliest detection with a 20x20s exposure image is confirmed at 2025-10-03 02:03:10 UTC (T-T0=280s) at an AB magnitude of 19.60 +/- 0.08.
LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).
GCN Circular 42106
Subject
GRB 251003A: J-band detection with WINTER
Date
2025-10-04T04:04:00Z (9 days ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (Caltech), Viraj Karambelkar (Columbia/CCA), Benjamin Schneider (LAM), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 251003A (Fermi GBM Team, GCN 42069; Beardmore et al., GCN 42070; Evans et al., GCN 42071; Evans et al., GCN 42088; Burrows et al., GCN 42096; Laha et al., GCN 42103) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began at 2025-10-03T02:08:08 UTC in the J band (~9.6 minutes after the GRB trigger), consisting of 15 x 120 s exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We detect a clear source at the optical counterpart location (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; Worssam et al., GCN 42094; Schneider et al., GCN 42095; Shilling et al., GCN 42100), with magnitude J ~ 18.4 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 42103
Subject
GRB 251003A: Swift-BAT refined analysis
Date
2025-10-04T00:52:50Z (9 days ago)
From
Mike Moss at NASA GSFC <mikejmoss3@gmail.com>
Via
Web form
S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), R. Gupta (GSFC),
H. A. Krimm (NSF), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), M. J. Moss (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 251003A (trigger #1400964)
(Beardmore, et al., GCN Circ. 42070). The BAT ground-calculated position is
RA, Dec = 18.495, 56.483 deg which is
RA(J2000) = 01h 13m 58.7s
Dec(J2000) = +56d 28' 57.1"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 97%.
The mask-weighted light curve displays a single fast-rise exponential decay pulse
lasting between ~T0-2 sec and ~T0+10 seconds. T90 (15-350 keV) is 10.78 +- 2.35 sec
(estimated error including systematics).
The time-averaged spectrum from T-2.41 to T+9.80 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.64 +- 0.15. The fluence in the 15-150 keV band is 4.1 +- 0.4 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+0.92 sec in the 15-150 keV band
is 1.0 +- 0.1 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/1400964
GCN Circular 42100
Subject
GRB 251003A: Swift/UVOT Upper Limits
Date
2025-10-03T18:33:33Z (9 days ago)
From
s.shilling@lancaster.ac.uk
Via
Web form
S. P. R. Shilling (Lancaster U.), M. De Pasquale (Univ. Messina) and
A. P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251003A
79 s after the BAT trigger (Beardmore et al., GCN Circ. 42070).
No optical afterglow consistent with the enhanced XRT position (Evans
et al., GCN Circ. 42088) or those reported by other instruments (Aceituno et al.,
GCN Circ. 42072; Moreno Méndez et al., GCN Circ. 42073; Strausbaugh et al., GCN
Circ. 42074; Worssam et al., GCN Circ. 42094), is detected in the initial UVOT
exposures.
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
white 79 229 147 >20.70
u 291 474 180 >19.78
v 33828 34558 711 >20.01
b 43530 43837 298 >20.30
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.408 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 42096
Subject
GRB 251003A: Swift-XRT refined Analysis
Date
2025-10-03T16:16:36Z (9 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M.
Capalbi (INAF-OAR), S. Lanava (PSU), S. Dichiara (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:
We have analysed 6.8 ks of XRT data for GRB 251003A, from 59 s to 34.6
ks after the trigger. The data comprise 126 s in Windowed Timing (WT)
mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode.
The late-time light curve (from T0+4.6 ks) can be modelled with a
power-law decay with a decay index of alpha=0.6 (+0.4, -0.3).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.67 (+0.11, -0.10). The
best-fitting absorption column is 1.4 (+3.0, -1.4) x 10^22 cm^-2, at a
redshift of 4.412, in addition to the Galactic value of 4.4 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 2.8 x
10^-11 (7.5 x 10^-11) erg cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Galactic foreground: 4.4 x 10^21 cm^-2
Intrinsic column: 1.4 (+3.0, -1.4) x 10^22 cm^-2 at z=4.412
Photon index: 2.67 (+0.11, -0.10)
If the light curve continues to decay with a power-law decay index of
0.6, the count rate at T+24 hours will be 8.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.3 x
10^-13 (6.1 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01400964.
This circular is an official product of the Swift-XRT team.
GCN Circular 42095
Subject
GRB 251003A: COLIBRÍ photometric redshift
Date
2025-10-03T15:49:16Z (9 days ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
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.
GCN Circular 42094
Subject
GRB 251003A: GOTO detection of the optical counterpart
Date
2025-10-03T14:27:54Z (9 days ago)
From
I. Worssam at University of Birmingham <ixw352@student.bham.ac.uk>
Via
Web form
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).
GCN Circular 42088
Subject
GRB 251003A: Enhanced Swift-XRT position
Date
2025-10-03T11:31:53Z (9 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
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.
GCN Circular 42086
Subject
GRB 251003A: GTC/OSIRIS+ spectroscopic redshift z = 4.412
Date
2025-10-03T10:34:36Z (10 days ago)
From
Ruben Sanchez-Ramirez at IAA-CSIC <ruben@iaa.es>
Via
Web form
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.
GCN Circular 42085
Subject
GRB 251003A: Kilonova-Catcher optical afterglow detection
Date
2025-10-03T10:33:41Z (10 days ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 251003A detected by Fermi/GBM (Fermi GBM team, GCN 42069) and Swift/BAT (Beardmore et al., GCN 42070) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the TEC160FL operated by M. Freeberg. Our observations started at TGRB+1.2hr.
In our stacked frames, subtracted from the PanSTARRS DR2 template image, we detect the optical counterpart reported by OSN (Aceituno et al., GCN 42072), COLIBRI (Moreno Méndez et al., GCN 42073), LCO-1m (Strausbaugh et al., GCN 42074).
We report our follow-up results in the table below
+---------------+-----------+--------+----------------+------------+
| Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument |
+===============+===========+========+================+============+
| 1.71 | 20 x 180s | r (AB) | 21.07 +/- 0.37 | TEC160FL |
| 3.58 | 25 x 180s | i (AB) | 19.14 +/- 0.18 | TEC160FL |
+---------------+-----------+--------+----------------+------------+
As also pointed out by (Moreno Méndez et al., GCN 42073, Strausbaugh et al., GCN 42074), the r-i color index we measure indicates a very red afterglow and further observations at infrared wavelengths and spectroscopic observations are encouraged to disentangle whether this burst is highly extinguished or localized at a moderately high redshift.
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 42074
Subject
GRB 251003A: LCOGT Optical Detection
Date
2025-10-03T04:04:06Z (10 days ago)
From
Robert Strausbaugh at Eastern Illinois University <rstrausbaugh@eiu.edu>
Via
email
R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:
We observed the Swift GRB 251003A field (Beardmore et al., GCN 42070) with the LCOGT 1-meter Sinistro instrument at the McDonald Observatory, USA site, on October 3, from 03:07 to 03:39 UT (corresponding to 1.15 to 1.68 hours after the GRB trigger time) with the SDSS i and r filters.
We performed a series of 3x300s exposures in i-band and r-band. We detect an uncatalogued source in the XRT enhanced error region (Evans et al. GCNC 42071) consistent with other optical counterpart observations (Aceituno et al., GCN 42072; Méndez et al., GCN 42073) in both bands. We note that the r-i color of this event may indicate a moderate redshift source (z>4).
The following magnitudes are calculated using the PanSTARRS catalog as reference:
r = 20.97 +/- 0.12
i = 19.25 +/- 0.04
These magnitudes are not corrected for galactic extinction.
GCN Circular 42073
Subject
GRB 251003A: COLIBRÍ optical detection of the fading afterglow
Date
2025-10-03T03:55:12Z (10 days ago)
From
enriquemm@astro.unam.mx
Via
Web form
Enrique Moreno Méndez (UNAM), Benjamin Schneider (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), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):
We imaged the field of the Swift GRB 251003A (Beardmore et al., GCN Circ. 42070) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-03 02:19 to 02:22 UTC (from 0.3 to 0.7 hours after the trigger) and obtained 3 minutes of simultaneous 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.
We detected the optical counterpart reported by Aceituno et al., GCN Circ. 42072, at a position of RA, Dec = 18.52939, 56.49110 (J2000), with preliminary magnitudes of:
r = 20.34 +/- 0.11
z = 18.56 +/- 0.05
The object fades in subsequent observations, confirming that it is the optical afterglow.
The very red color suggests that this might be either a moderate redshift GRB with a drop-out in r or a very heavily reddened GRB.
Further observations in g/r/i/z/y 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.
GCN Circular 42072
Subject
GRB 251003A: 1.5m OSN afterglow candidate
Date
2025-10-03T03:06:40Z (10 days ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
F. J. Aceituno, R. Sanchez-Ramirez A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), and Y.-D. Hu (GXI), on behalf of a larger collaboration, report:
Following the detection of GRB 251003A by both Fermi (Fermi team, GCNC 42069) and Swift (Beardmore et al. GCNC 42070), we obtained a 300s exposure image (I-band) with the 1.5m OSN telescope in Sierra Nevada (Granada, Spain) starting on Oct 3 (02:34 UT, i.e. 36 min post burst). At the enhanced X-ray position (Evans et al. GCNC 42071) we identify an optical source with a preliminary magnitude of 21, which we propose to be the afterglow to GRB 251003A. Further observations are being gathered.
GCN Circular 42071
Subject
GRB 251003A: Prompt enhanced Swift-XRT position
Date
2025-10-03T02:22:53Z (10 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Using 263 s of promptly downlinked XRT event data for GRB 251003A, we
find an enhanced XRT position of the afterglow: RA, Dec: 18.52838,
56.49082 which is equivalent to:
RA (J2000) = 01 14 06.81
Dec (J2000) = +56 29 26.9
with an uncertainty of 2.3 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1400964.
Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).
This circular is an official product of the Swift-XRT team.
GCN Circular 42070
Subject
GRB 251003A: Swift detection of a burst
Date
2025-10-03T02:10:50Z (10 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
A. P. Beardmore (U Leicester), R. Gupta (NASA GSFC),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:
At 01:58:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251003A (trigger=1400964). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 18.503, +56.507 which is
RA(J2000) = 01h 14m 01s
Dec(J2000) = +56d 30' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a complex
structure with a duration of about 20 sec. The peak count rate
was ~1835 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 01:59:42.1 UT, 72.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 18.52734, 56.49009
which is equivalent to:
RA(J2000) = 01h 14m 06.56s
Dec(J2000) = +56d 29' 24.3"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 77 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 4.39
x 10^21 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 1.66e-09 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 78 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.408.
Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 42069
Subject
GRB 251003A: Fermi GBM Final Real-time Localization
Date
2025-10-03T02:09:08Z (10 days ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 01:58:31 UT on 3 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251003A (trigger 781149516.956282 / 251003082).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 31.9, Dec = 49.6 (J2000 degrees, equivalent to J2000 02h 07m, 49d 36'), with a statistical uncertainty of 18.2 degrees.
The angle from the Fermi LAT boresight is 64.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251003082/quicklook/glg_skymap_all_bn251003082.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251003082/quicklook/glg_healpix_all_bn251003082.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251003082/quicklook/glg_lc_medres34_bn251003082.gif
GCN Circular 42068
Subject
Swift GRB251003A: Global MASTER-Net observations report
Date
2025-10-03T02:04:38Z (10 days ago)
Edited On
2025-10-05T16:40:05Z (7 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Vladimir Lipunov at Lomonosov Moscow State University <lipunov@sai.msu.ru>
Via
email
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), A.Sosnovskij (CrAO RAS),
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 Swift GRB251003.08 (trigger No 1400964,01h 14m 00.72s , +56d 30m 25.2s, R=0.05) errorbox 10 sec after notice time and 34 sec after trigger time at 2025-10-03 01:59:04 UT, with upper limit up to 16.6 mag. Observations started at twilight. The observations began at zenith distance = 43 deg. The sun altitude is -13.7 deg.
The galactic latitude b = -6 deg., longitude l = 126 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3005736
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
40 | MASTER-Kislovodsk | P- | 10 | 15.8 |
50 | MASTER-Kislovodsk | P- | 30 | 16.4 | Coadd
40 | MASTER-Kislovodsk | P| | 10 | 14.6 |
58 | MASTER-Kislovodsk | P- | 10 | 16.0 |
58 | MASTER-Kislovodsk | P| | 10 | 15.0 |
77 | MASTER-Kislovodsk | P- | 10 | 16.2 |
77 | MASTER-Kislovodsk | P| | 10 | 15.1 |
100 | MASTER-Kislovodsk | P- | 20 | 16.5 |
100 | MASTER-Kislovodsk | P| | 20 | 15.8 |
129 | MASTER-Kislovodsk | P- | 20 | 16.5 |
129 | MASTER-Kislovodsk | P| | 20 | 15.6 |
162 | MASTER-Kislovodsk | P- | 30 | 16.6 |
162 | MASTER-Kislovodsk | P| | 30 | 15.9 |
The observation and reduction will continue.
The message may be cited.