GRB 260903A
GCN Circular 45548
Subject
GRB 260903A: Swift-XRT observations
Date
2026-09-10T14:57:05Z (18 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Lanava (PSU), J.P. Osborne (U. Leicester), E. Ambrosi (INAF-IASFPA)
, M. Capalbi (INAF-OAR), M. Perri (SSDC & INAF-OAR), S. Dichiara (PSU),
J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 260903A, collecting 3.9 ks of Photon
Counting (PC) mode data between T0+498.4 ks and T0+554.5 ks.
No X-ray sources have been detected at the position of the optical
afterglow (Li et al. GCN 45504, Saccardi et al. GCN 45505, Malesani et
al. GCN 45508, Russeil et al. GCN 45509, Li et al. GCN 45510, Bochenek
et al. GCN 45516). The 3-sigma upper limit in the field is 0.002 ct
s^-1, corresponding to a 0.3-10 keV observed flux of 8.1e-14 erg cm^-2
s^-1 (assuming a typical GRB spectrum).
Four uncatalogued sources were detected too far from the GRB position
to be likely afterglow candidates.
The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021910.
This circular is an official product of the Swift-XRT team.
GCN Circular 45529
Subject
GRB 260903A: Swift ToO observations
Date
2026-09-07T18:26:33Z (21 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 team:
Swift has initiated a ToO observation of the SVOM/ECLAIRs-detected event
GRB 260903A. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021910
Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the SVOM/ECLAIRs event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a
GCN Circular after manual consideration.
Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
This circular is an official product of the Swift-XRT team.
GCN Circular 45527
Subject
GRB 260903A: SAO RAS optical upper limit
Date
2026-09-07T11:59:00Z (21 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
email
A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI),
A. Volnova (IKI), N. Pankov (IKI) report on behalf of GRB follow-up team
We observed the field of the GRB 260903A discovered by the SVOM
(Brunet et al., GCNs 45499; 45517), and observed by EP (Li et al.,
GCN 45510). Observations were carried out using the SAO RAS
Zeiss-1000 telescope and a new CCD photometer. A set of 18 x 300 sec.
images in Rc band obtained on September 5, 19:34:52--21:25:55 UT
(t_mid - T0 = 2.32851 days).
We do not detect the OT (Li et al., GCN 45504; Saccardi et al.,
GCN 45505; Malesani et al., GCN 45508; Russeil et al., GCN 45509;
Moskvitin et al., GCN 45515; Bochenek and Perley, GCN 45516)
in a stacked image. Details of the observation are the following.
Date, UTstart, t-T0, Exp., Filter, Mag, Err., UL
(mid,d) (n*s) (3-sigma)
2026-09-05 19:34:52 2.32851 18*300 Rc n/d 24.1
The preliminary photometry (Vega) is based on nearby SDSS stars
(Lupton 2005 transformation) and not corrected for Galactic extinction.
GCN Circular 45519
Subject
GRB 260903A: J-band upper limit with WINTER
Date
2026-09-04T20:05:20Z (24 days ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 260903A (Brunet et al., GCN 45499 and GCN 45517) 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 on 2026-09-04 at 05:43:17.936 UTC in the J band (17.1 hr after the SVOM/ECLAIRs trigger), consisting of 15*120 sec 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 do not detect the source at the optical counterpart position (Li et al., GCN 45504; Saccardi et al., GCN 45505; Malesani et al., GCN 45508; Russeil et al., GCN 45509; Moskvitin el al., GCN 45515; Bochenek et al., GCN 45516) in our stacked image. We obtain the following 5-sigma upper limit: J = 19.07 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 45517
Subject
GRB 260903A: SVOM/ECLAIRs refined analysis
Date
2026-09-04T17:41:50Z (24 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet, H. Yang, O. Godet (IRAP) , W.-J. Xie (NAOC), R.-C. Chen (NJU), T. Xiao (GXU), report on behalf of the SVOM/ECLAIRs team:
Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260903A (SVOM burst-id sb26090302 – GCN 45499, trigger time T0 = 2026-09-03T12:37:20 UTC).
The burst that triggered ECLAIRs onboard shows a single peak lightcurve. The burst duration is T90 = 21 +6/-4 s in the 4-20 keV energy band.
The time-averaged spectrum from T0-18.98 s to T0+1.13 s in the 5-120 keV energy range is best fitted by a powerlaw model, with alpha = -3.2/-0.5. With this model, the 4-120 keV fluence is (1.6 +0.3/-0.2)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.70 +/-0.08 ph/cm^2/s.
A blackbody model also gives a good fit with kT = 1.8 +/-0.2 keV. With this model, the 4-120 keV fluence is (1.2 +/-0.2)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.54 +/-0.06 ph/cm^2/s.
The spectral parameters show that this burst could be classified as an X-Ray Flash (XRF).
All the quoted errors are at the 68% confidence level.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.
The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet et utoulouse.fr)
GCN Circular 45516
Subject
GRB260903A: Liverpool Telescope optical observations
Date
2026-09-04T16:39:14Z (24 days ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of GRB 260903A (Brunet et al., GCN 45499) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x120s exposures in SDSS gz filters and 12x120s in the SDSS r filter, starting at 2026-09-04 00:42:48 UT, approximately 12 hours after SVOM/ECLAIRS trigger.
The obtained images were stacked after discarding bad quality exposures. We detect a source in the r-band stack at a counterpart position reported by Li et al. (GCN 45504), Saccardi et al. (GCN 45505), Malesani et al. (GCN 45508), Russeil et al. (GCN 45509), Li et al. (GCN 45510), and Moskvitin et al. (GCN 45115). We also report 3-sigma upper limits in other bands.
MJD (mid) T_mid-T_0 Filter Mag. (AB)
61287.03369 12.2 h g > 22.37
61287.04889 12.6 h r 22.73 ± 0.26
61287.06252 12.9 h z > 21.38
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 45515
Subject
GRB 260903A: SAO RAS optical observations
Date
2026-09-04T15:01:32Z (24 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
email
A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI),
A. Volnova (IKI), N. Pankov (IKI) report on behalf of GRB follow-up team
We observed the field of the GRB 260903A discovered by the SVOM
(Brunet et al., GCN 45499), also followed by EP (Li et al., GCN 45510)
with the Zeiss-1000, 1-m SAO RAS telescope + new CCD photometer.
A set of 16 x 300 sec images in Rc band were obtained
on September 3, 19:58:07--21:32:51 (mid. time = 8.1358 hours).
The OT (Li et al., GCN 45504; Saccardi et al., GCN 45505; Malesani
et al., GCN 45508; Russeil et al., GCN 45509) is clearly detected
in our stacked image. Details of the observation are the following.
Date, UTstart, t-T0, Exp., Filter, Mag, Err., UL
(mid,d) (n*s) (3-sigma)
2026-09-03 19:58:07 0.33899 16*300 Rc 22.45 +/- 0.09 24.0
The preliminary photometry (Vega) is based on nearby SDSS stars
(Lupton 2005 transformation) and not corrected for Galactic extinction.
_____________________________________________
Alexander Sergeevich Moskvitin, Ph. D., researcher,
Optical Transient Physics Laboratory (#523), SAO RAS,
369167, Nizhnij Arkhyz, Karachai-Cherkessia, Russia,
+7(928)3946958, mosk@sao.ru
GCN Circular 45510
Subject
GRB 260903A: EP-FXT follow-up observation
Date
2026-09-04T06:11:05Z (24 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), Z.-M. Wang (BNU), R. Shi (PMO), H. Sun (NAO,CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of GRB 260903A, detected by SVOM/ECLAIRs (sb26090302; Brunet et al., GCN 45499), at 2026-09-03 14:46:48 (UTC), about 2.16 hours after the SVOM/ECLAIRs trigger, with an exposure time of 6005 s.
On-ground analysis of the EP-FXT data found an uncataloged fading X-ray source within the SVOM/VT error region (Li et al., GCN 45504). The preliminary results for the X-ray source are as follows:
Source name: EPF_J014238.7+112211
R.A., Decl. (J2000) = 25.6614, 11.3696 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 9.76e-13 erg/cm^ 2/s
Flux error = 1.94e-13 erg/cm^2/s (1 sigma)
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).
GCN Circular 45509
Subject
GRB 260903A: OHP/T193 optical afterglow detection
Date
2026-09-04T01:52:51Z (24 days ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
Web form
D. Russeil (LAM/Pytheas/AMU), C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), A. Saccardi (CEA/Irfu), F. Schüssler (CEA/DPhP), S. Basa (LAM/OHP/Pytheas/AMU) report on behalf of a larger collaboration:
We observed the field of the SVOM/ECLAIRs GRB 260903A (Brunet et al., GCN 45499) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 19 exposures of 300s (for a total of 95 min) in the r-band starting at 20260903_223831 UT (observations mid-time at T-To=11h). Target was close to the moon.
In the stacked images, we detect the optical counterpart reported and followed by Li et al. (GCN 45504), Saccardi et al. (GCN 45505), and Malesani et al. (GCN 45508).
The preliminary magnitude derived for that source is:
r’ = 23.0 +/- 0.2 mag (AB)
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025).
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular S. Favard.
GCN Circular 45508
Subject
GRB 260903A: NOT optical observations and afterglow fading
Date
2026-09-04T00:54:37Z (24 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI), J. An (NAOC), G. Corcoran (UCD), A. Saccardi (CEA/Irfu), F. Mustafaj (NOT), O. Stalder (NOT), report on behalf of a larger collaboration:
We observed the optical counterpart (Li et al., GCN 45504; Saccardi et al., GCN 45505) of the SVOM/ECLAIRs GRB 260903A (Brunet et al., GCN 45499) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A total of 3x300 s observations were secured in the SDSS r band.
The optical afterglow is detected in our images. Preliminary photometry against the Pan-STARRS catalog yields a magnitude r = 22.88 +- 0.16 (AB) on 2026 Sep 4.00 UT, that is 11.45 hr after the trigger.
Our measurement indicates significant fading compared to previously reported measurements (Li et al., GCN 45504; Saccardi et al., GCN 45505), thus confirming that this object is the afterglow of GRB 260903A.
GCN Circular 45506
Subject
SVOM GRB260903A: Global MASTER-Net observations report
Date
2026-09-03T21:45:45Z (24 days ago)
Edited On
2026-09-04T19:32:33Z (24 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Vladimir Lipunov at Lomonosov Moscow State University, MASTER Global Robotic NET PI <lipunov2007@gmail.com>
Via
email
O.Gress, N.Budnev(ISU)
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 (Lomonosov MSU)
D.Buckley (SAAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
A.Sosnovskij (CrAO RAS)
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory)
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
R.Rebolo (The Instituto de Astrofisica de Canarias)
M. Segura, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA)
MASTER-Tunka robotic telescope [1] located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the SVOM GRB260903.53 (trigger No 1788439007,01h 42m 14.86s , +11d 13m 58.1s, R=0.198) errorbox 7913 sec after notice time and 9200 sec after trigger time at 2026-09-03 15:10:07 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 70 deg. The sun altitude is -25.7 deg.
The galactic latitude b = -49 deg., longitude l = 143 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3498603
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
9230 | 2026-09-03 15:10:07 | MASTER-Tunka | (01h 39m 56.83s , +10d 45m 37.2s) | C | 60 | 13.9 |
9303 | 2026-09-03 15:11:19 | MASTER-Tunka | (01h 40m 02.09s , +10d 45m 48.3s) | C | 60 | 16.4 |
9379 | 2026-09-03 15:12:35 | MASTER-Tunka | (01h 39m 58.94s , +10d 47m 16.3s) | C | 60 | 16.9 |
9454 | 2026-09-03 15:13:51 | MASTER-Tunka | (01h 39m 59.00s , +10d 45m 42.4s) | C | 60 | 16.0 |
9528 | 2026-09-03 15:15:04 | MASTER-Tunka | (01h 40m 01.06s , +10d 47m 25.9s) | C | 60 | 16.3 |
9603 | 2026-09-03 15:16:19 | MASTER-Tunka | (01h 39m 55.32s , +10d 46m 25.1s) | C | 60 | 16.9 |
9709 | 2026-09-03 15:18:05 | MASTER-Tunka | (01h 39m 55.09s , +10d 47m 22.5s) | C | 60 | 16.9 |
9789 | 2026-09-03 15:19:25 | MASTER-Tunka | (01h 40m 02.37s , +10d 46m 21.1s) | C | 60 | 17.3 |
9867 | 2026-09-03 15:20:43 | MASTER-Tunka | (01h 39m 56.82s , +10d 45m 15.9s) | C | 60 | 16.5 |
9945 | 2026-09-03 15:22:01 | MASTER-Tunka | (01h 40m 01.29s , +10d 45m 27.8s) | C | 60 | 17.6 |
9991 | 2026-09-03 15:22:48 | MASTER-Tunka | (01h 39m 59.00s , +10d 46m 53.3s) | C | 60 | 18.0 |
10064 | 2026-09-03 15:24:01 | MASTER-Tunka | (01h 39m 58.96s , +10d 45m 21.6s) | C | 60 | 17.4 |
10140 | 2026-09-03 15:25:16 | MASTER-Tunka | (01h 40m 02.07s , +10d 47m 05.4s) | C | 60 | 17.2 |
10212 | 2026-09-03 15:26:28 | MASTER-Tunka | (01h 39m 55.16s , +10d 46m 05.9s) | C | 60 | 17.8 |
10285 | 2026-09-03 15:27:41 | MASTER-Tunka | (01h 39m 56.15s , +10d 47m 03.1s) | C | 60 | 17.1 |
10360 | 2026-09-03 15:28:56 | MASTER-Tunka | (01h 40m 02.39s , +10d 46m 01.4s) | C | 60 | 17.6 |
10432 | 2026-09-03 15:30:08 | MASTER-Tunka | (01h 39m 55.93s , +10d 44m 58.5s) | C | 60 | 17.2 |
10504 | 2026-09-03 15:31:21 | MASTER-Tunka | (01h 40m 00.97s , +10d 45m 24.5s) | C | 60 | 17.5 |
10580 | 2026-09-03 15:32:36 | MASTER-Tunka | (01h 39m 58.90s , +10d 46m 27.8s) | C | 60 | 18.3 |
10653 | 2026-09-03 15:33:49 | MASTER-Tunka | (01h 39m 58.86s , +10d 44m 58.2s) | C | 60 | 18.1 |
10725 | 2026-09-03 15:35:01 | MASTER-Tunka | (01h 40m 02.87s , +10d 46m 32.3s) | C | 60 | 17.0 |
10832 | 2026-09-03 15:36:48 | MASTER-Tunka | (01h 39m 55.80s , +10d 45m 47.7s) | C | 60 | 17.4 |
10911 | 2026-09-03 15:38:07 | MASTER-Tunka | (01h 39m 56.49s , +10d 46m 44.1s) | C | 60 | 18.1 |
10956 | 2026-09-03 15:38:53 | MASTER-Tunka | (01h 40m 01.47s , +10d 45m 43.1s) | C | 60 | 18.1 |
11065 | 2026-09-03 15:40:41 | MASTER-Tunka | (01h 39m 55.52s , +10d 44m 39.4s) | C | 60 | 18.2 |
11143 | 2026-09-03 15:41:59 | MASTER-Tunka | (01h 40m 02.63s , +10d 44m 49.8s) | C | 60 | 18.8 |
11221 | 2026-09-03 15:43:17 | MASTER-Tunka | (01h 39m 58.80s , +10d 46m 16.3s) | C | 60 | 18.3 |
11266 | 2026-09-03 15:44:03 | MASTER-Tunka | (01h 39m 58.83s , +10d 44m 53.9s) | C | 60 | 18.1 |
11337 | 2026-09-03 15:45:14 | MASTER-Tunka | (01h 40m 01.35s , +10d 46m 05.4s) | C | 60 | 18.3 |
11412 | 2026-09-03 15:46:28 | MASTER-Tunka | (01h 39m 55.82s , +10d 45m 27.4s) | C | 60 | 18.2 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 45505
Subject
GRB 260903A: LCO optical observations
Date
2026-09-03T20:50:07Z (25 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), M. Pillas (IAP), H. L. Li (NAOC), Y.X. Guan, S.J. Yin (GXU), C. Wu, L.P. Xin (NAOC), Y.Y. Shi (GXU), D. Turpin (CEA/Irfu), R.-C. Chen (NJU), T. Xiao (GXU) report on behalf of the SVOM mission team:
Following the report of an optical counterpart (Li et al., GCN 45504) of the SVOM GRB 260903A (Brunet et al., GCN 45499), we inspected again our images taken at the Siding Spring Observatory with the LCO telescope (Guan et al., GCN 45501; Guan et al., GCN 45503).
In total, three epochs of observations in the SDSS r filter were secured (the first and the second consisting of 3x200 s exposures while the third of 6x200 s). The optical counterpart is detected in all three epochs, and we measure the following magnitudes (calibrated against the Pan-STARRS catalog and not corrected for Galactic extinction):
r = 20.92 +/- 0.19 AB (mid-time 38.66 min after the trigger);
r = 20.46 +/- 0.19 AB (mid-time 2.30 hr after the trigger);
r = 21.8 +/- 0.4 AB (mid-time 2.74 hr after the trigger).
Overall, there is no strong evidence for variability, though the source is only barely detected in our last epoch, suggesting some fading compared to our earlier epochs and to the SVOM/VT measurement (Li et al., GCN 45504).
This project is funded by the SVOM collaboration.
GCN Circular 45504
Subject
GRB 260903A: SVOM/VT optical candidate
Date
2026-09-03T19:00:22Z (25 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, C. Wu, L. P. Xin, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu, L. S. Chen, 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), J. T. Palmerio, A. Saccardi (CEA/Irfu), R.-C. Chen (NJU), T. Xiao (GXU) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations of GRB 260903A detected by SVOM/ECLAIRs (sb26090302, Brunet et al., GCN 45499). Our observation started on 2026-09-03T14:41:46 UT (about 2.1 hr after the trigger) in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously .
An uncatalogued optical source compared to the Legacy Survey catalogue was identified in our images within the ECLAIRs localization error circle (Brunet et al., GCN 45499). Its position is R.A., Dec. = 25.662357, 11.370481 degrees:
R.A. (J2000) = +01:42:38.96
Dec. (J2000) = +11:22:13.73
with a positional uncertainty of ~0.5 arcsec.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
2.18 hr VT_B 11*70 s 21.9 +/- 0.2 mag
2.18 hr VT_R 11*70 s 20.7 +/- 0.1 mag
No significant positional or brightness variation of the candidate was detected over our nearly 2‑hour observations.
More follow-up observations of this burst are scheduled.
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.
GCN Circular 45503
Subject
GRB 260903A: Retraction of optical counterpart of GCN 45501
Date
2026-09-03T17:20:54Z (25 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.X. Guan(GXU), A. Saccardi (CEA/Irfu), S.J.Yin (GXU), C. Wu, L.P. Xin (NAOC), Yunyu Shi (GXU), D. Turpin (CEA/Irfu), R.-C. Chen (NJU), T. Xiao (GXU) report on behalf of the SVOM mission team:
We obtained further imaging of the optical counterpart to GRB 260903A (Brunet et al., GCN 45499) reported in GCN 45501. The source exhibited a positional drift of ~7 arcseconds within ~2 hr. We find that the optical detection is consistent with a known MP object 2000 OQ52, as queried using the IMCCE SkyBot online service (Berthier et al., 2016) via its cone-search API.
We apologise for any inconvenience caused.
GCN Circular 45501
Subject
GRB 260903A: LCO optical counterpart detection
Date
2026-09-03T15:11:23Z (25 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.X. Guan(GXU), A. Saccardi (CEA/Irfu), S.J.Yin (GXU), C. Wu, L.P. Xin (NAOC), Yunyu Shi (GXU), D. Turpin (CEA/Irfu), R.-C. Chen (NJU), T. Xiao (GXU) report on behalf of the SVOM mission team:
We observed the field of the SVOM/ECLAIRs GRB 260903A (Brunet et al., GCN 45499) with the LCO 1m telescope at Siding Spring Observatory equipped with the Sinistro instrument.
Our observation started on 2026-09-03T13:06:41.486 UT (about 29.36 min after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.
An uncatalogued optical source compared to DESI DR10 catalogue is clearly detected in our images within the ECLAIRs localization error circle (Brunet et al., GCN 45499). Its position is R.A., Dec. 25.77241, 11.43678 degrees:
R.A. (J2000) = 01h43m05.37
Dec. (J2000) = +11d26m12.4
with an uncertainty of ~0.5 arcsec.
We measure the following magnitudes calibrated against the DESI DR10 catalog, and not corrected for Galactic extinction:
r = 17.91 +/- 0.02 AB (mid-time 38.66 min after the trigger);
z = 17.69 +/- 0.13 AB (mid-time 31.01 min after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 45499
Subject
GRB 260903A: SVOM detection of a soft faint burst
Date
2026-09-03T13:18:42Z (25 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
GRB 260903A: SVOM detection of a soft faint burst
M. Brunet, O. Godet, H. Yang (IRAP), R.-C. Chen (NJU), T. Xiao (GXU) report on behalf of the SVOM mission team:
At 2026-09-03T12:37:20 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260903A (SVOM burst-id sb26090302).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 2 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 7.70 in the [5-8] keV energy band over a time window of 20.40 seconds starting at 2026-09-03T12:36:59 UTC.
The localization of the best alert is R.A., Dec. 25.614, 11.389 degrees:
R.A. (J2000) = 1h42m27.28s
Dec. (J2000) = 11d23m21.52s
with a 90% confidence level (C.L.) radius of 10.20 arcmin (including systematic error of 2 arcmin added in quadrature).
The SVOM/ECLAIRs light curve showed a broad structure with a duration of about 20 s.
Due to the detection significance being below the slew threshold, no immediate slew was performed on this burst.
No X-ray observation could be performed by SVOM/MXT for the time being.
No optical observation could be performed by SVOM/VT for the time being.
A SVOM ToO has been scheduled.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is R.-C. Chen: chrczxx@smail.nju.edu.cn.
Please contact the BA by email if you require additional information.