GRB 260809A
GCN Circular 45443
Subject
GRB 260809A: SVOM/ECLAIRs refined analysis
Date
2026-08-26T08:39:21Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet, H. Yang, O. Godet (IRAP), F. Piron (LUMP)
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 the faint burst GRB 260809A (SVOM burst-id sb26080907, GCN 45331, trigger time T0 = 2026-08-09T14:57:33 UTC), also detected as a sub-threshold event by Fermi/GBM (GCN 45366).
The burst that triggered ECLAIRs shows a multiple peak light curve, and is detected from T0-24 s to T0+19 s in the 4-120 keV energy band.
The time-averaged spectrum from T0-24 s to T0+19 in the 5-120 keV is well fitted by a power-law model with a photon index of -1.3+/-0.2, a 4-120 keV photon flux of 0.76 +0.06/-0.20 photons/cm^2/s, and a corresponding 4-120 keV fluence of (1.4 +/-0.3)e-6 erg/cm^2.
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@utoulouse.fr).
GCN Circular 45366
Subject
Fermi GBM Sub-Threshold Detection of GRB 260809A
Date
2026-08-12T19:55:21Z (16 days ago)
From
Bagrat Mailyan at Florida Tech <mbagrat@gmail.com>
Via
Web form
B. Mailyan (Florida Tech) reports on behalf of the Fermi-GBM Team:
The SVOM/ECLAIRs detected GRB 260809A on 2026-08-09 at 14:57:33 UTC (Brunet et al. 2026, GCN 45331). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified three candidates.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient starting approximately 12 s after the ECLAIRs best image SNR time of 2026-08-09T14:57:13 UTC. The Targeted Search candidate was found most significantly on the 32 s timescale, with a false alarm rate of 1.6e-04 Hz. The Fermi-MET of the start time of this transient is 807980250.351 s. The Targeted Search localization is found to be spatially consistent with the ECLAIRs localization, with a spatial association probability of 95.8%.
Additionally, the GBM Targeted Search event was found with the highest significance using a "normal" spectrum (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3) for a GRB.
The Targeted Search data release product for this event can be found here:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/targeted_search/trigger_report.php?trigger_id=807980238&sample=SVOM_GCNs&hash=312d5789ab3c4b28
[1] Goldstein et al. 2019, arXiv:1903.12597
GCN Circular 45361
Subject
GRB 260809A: SAO RAS optical observations
Date
2026-08-12T11:58:37Z (16 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
email
A. Moskvitin (SAO RAS), N. Pankov (IKI), A. Pozanenko (IKI),
A. Volnova (IKI), O. Spiridonova (SAO RAS)
report on behalf of GRB follow-up team
We observed the field of the GRB 260809A discovered by the SVOM
(Brunet et al., GCN 45331), also detected by EP-FXT (Sun et al.,
GCN 45344) with the Zeiss-1000, SAO RAS 1-m telescope + CCD photometer
on 2026-08-09T23:20:36--2026-08-10T00:49:19 UT (t_mid = 9.1234 hours).
The OT (Li et al., GCN 45336; Adami et al., GCN 45338; Rajabov et al.,
GCN 45339; Izzo, Saccardi and Corcoran, GCN 45340; Basa et al.,
GCN 45342) 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)
2026-08-09 23:20:36 0.38014 14*300 Rc 22.60 +/- 0.17 23.4
This preliminary photometry (Vega) is based on nearby SDSS stars
(Lupton 2005 transformation) and not corrected for Galactic extinction.
GCN Circular 45350
Subject
GRB 260809A: J-band upper limit with WINTER
Date
2026-08-10T17:58:29Z (18 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 260809A (Brunet et al., GCN 45331; also see Sun et al., GCN 45344) 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 2026-08-10T05:11:10.816 UTC in the J band (14.23 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 optical counterpart (Li et al., GCN 45336; Adami et al., GCN 45338; Rajabov et al., GCN 45339; Izzo et al., GCN 45340; Basa et al., GCN 45342) in our stacked image. We obtain the following 5-sigma upper limit: J = 18.6 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 45344
Subject
GRB260809A/EPF_J234751.2+210212: EP-FXT follow-up observation
Date
2026-08-10T12:12:39Z (18 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
X. X. Sun(NAO, CAS), Q. J. Huang (PMO, CAS), K. J. Zhang(YNU), J. P. Chen (SYSU), Z. X. Ling(NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of GRB 260809A detected by SVOM/ECLAIRs (Brunet et al., GCN 45331). The observation started at 2026-08-09 17:30:29 (UTC), about 2.55 hr after the SVOM trigger, with a total exposure time of around 3 ks.
On-ground analysis of the FXT data found a source EPF_J234751.2+210212 at R.A. = 356.9632, deg and Dec. = 21.0368 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The source position is consistent with the optical counterpart (Li et al., GCN 45336; Adami et al., GCN 45338; Rajabov et al., GCN 45339; Izzo et al., GCN 45340; Basa et al. 45342). The observed flux is approximately 4.5 × 10^-12 erg s^-1 cm^-2 in the 0.5-10 keV energy band.
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 45342
Subject
GRB 260809A: SVOM/COLIBRÍ (FM-GFT) optical observation
Date
2026-08-10T11:01:44Z (18 days ago)
From
Stephane Basa at UAR Pytheas/OHP, LAM <stephane.basa@lam.fr>
Via
Web form
Stéphane Basa (UAR Pytheas), Noémie Globus (UNAM), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) and Frédéric Piron (LUPM) report:
We imaged the field of the SVOM GRB 260809A (Brunet et al., GCN 45331) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-08-1 06:18:16
to 09:10:59 UTC (from 15.3 to 18.2 hours after the trigger) and obtained 80x60 seconds of exposure in the z band.
The data were reduced, coadded and analyzed with the ASU and CPPM COLIBRÍ pipelines. The photometry was calibrated using nearby stars from the PanSTARRS DR1r catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we detect the optical counterpart (Li et al., GCN 45336; Adami et al., GCN 45338; Rajabov et al., GCN 45339, Izzo et al., GCN 45340) at a preliminary magnitude of:
z = 23.11 +/- 0.16
Further observations are ongoing.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
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 45340
Subject
GRB 260809A: NOT optical observations
Date
2026-08-10T08:11:19Z (18 days ago)
From
Andrea Saccardi at CEA/Irfu <andrea.saccardi@cea.fr>
Via
Web form
L. Izzo (INAF/OACN and DARK/NBI), A. Saccardi (CEA/Irfu), G. Corcoran (UCD), report on behalf of a larger collaboration:
We observed the field of GRB 260809A detected by SVOM/ECLAIRs (Brunet et al., GCN 45331), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 3x300 s exposures in the SDSS r-band and 5x200s in the SDSS z-band.
In the stacked images, we detect the optical afterglow (Li et al., GCN 45336; Adami et al., GCN 45338; Rajabov et al., GCN 45339). We measure the following magnitudes calibrated against the Pan-STARRS catalog, and not corrected for Galactic extinction:
r = 23.78+/-0.18, midtime: 2026-08-10 at 04:29:51 UT (13.54 hr post trigger)
z = 22.25+/-0.13, midtime: 2026-08-10 at 04:47:47 UT (13.84 hr post trigger).
GCN Circular 45339
Subject
GRB260809A: MAO/AZT-22 optical observations
Date
2026-08-10T04:16:25Z (18 days ago)
Edited On
2026-08-10T13:20:36Z (18 days ago)
From
Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Via
Web form
Y. Rajabov, O. Burkhonov, B.Abidkhanov, S. Ehgamberdiev, Y. Tillayev (UBAI), A. Shaymanov, T. Boyqobilov (Maidanak Observatory/UBAI), M. Coughlin (UMN), P. Hello (IJCLAB), S. Antier (IJCLAB), M. Pillas (IAP), and S. Karpov (FZU) report on behalf of UBAI team and GRANDMA collaboration.
We observed the field of GRB 260809A detected by SVOM (Brunet et al., GCN 45331) with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2026-08-09 at 22:25:45 UT, i.e, ~7.47 hours after the SVOM/ECLAIRs trigger. In total we obtained 4x300 s exposures in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010).
We detect the afterglow in our stacked frame at the position reported by Li et al. (GCN 45336).
Our preliminary measurements consist with detections reported by Li et al. GCN 45336 and Adami et. al. GCN 45338.
Preliminary photometry is as follows:
Date UTstart Exptime t-T0 Filter OT Err. UL SNR Site/Telescope
(nxs) (mid, hours)
2026-08-09 22:25:45 4x300 7.47 R 22.97 0.1 23.24 6 MAO/AZT-22
All the data have been reduced by a single data processing pipeline, STDWeb (Karpov 2025). Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot catalog. Our measurements in Vega magnitude system and the data has not corrected for the Galactic extinction.
Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/).
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/). Astrophysics Center for Multimessenger studies in Europe is supported Skyportal/ICARE (contact camille.douzet@ijclab.in2p3.fr, skyportal.io).
GCN Circular 45338
Subject
GRB260809A: OHP/T193 optical observations
Date
2026-08-10T03:11:58Z (18 days ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
Web form
C. Adami (LAM/Pytheas/AMU),S. Basa (LAM/OHP/Pytheas/AMU), A. Saccardi (CEA/Irfu), B. Schneider (LAM), F. Schussler (CEA Paris-Saclay) report on behalf of a larger collaboration:
We observed the field of GRB 260809A (Brunet et al., GCN45331) using the T193cm telescope at Observatoire de Haute-Provence (France) with the MISTRAL spectro-imager. We obtained 5x600 sec of exposure in the i-band under a ~2arcsec seeing, starting at 20260809_233302UT (~8.6 hr after the Brunet et al. trigger) and ending at 20260810_010555UT (~10.1 hr after the Brunet et al. trigger).
We obtained 3x600 + 4x1000 sec of exposure in the r-band under a ~2arcsec seeing, starting at 20260810_001106UT (~9.2 hr after the Brunet et al. trigger) and ending at 20260810_021627UT (~11.3 hr after the Brunet et al. trigger).
We obtained 900 sec of exposure in the Y-band under a ~2arcsec seeing at 20260809_234858UT (~8.9 hr after the Brunet et al. trigger), 1200 sec at 20260810_023727UT (~11.7 hr after the Brunet et al. trigger), and 1200 sec at 20260810_025838UT (~12 hr after the Brunet et al. trigger).
In the preliminary stacked images, we detect in r and i bands the afterglow at the position of Li et al. (GCN 45336) with preliminary magnitudes of:
r = 23.36 +/- 0.10 mag
i = 22.03 +/- 0.06 mag
We do not detect the source in our Y-band image. This image has a preliminary estimated limiting magnitude of Y=21.2 (3sigma).
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction.
We acknowledge the excellent support from Observatoire de Haute-Provence and in particular Stephane Favard.
GCN Circular 45336
Subject
GRB 260809A: SVOM/VT optical counterpart
Date
2026-08-09T22:39:24Z (19 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, 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 (CEA/Irfu), F. Piron (LUPM) report on behalf of the SVOM/VT team.
SVOM/VT performed observations with automatic slew for GRB 260809A, which was triggered by SVOM/ECLAIRs (sb26080907, Brunet et al., GCN 45331) and also detected by SVOM/MXT (Brunet et al., GCN 45331). The observation started at 2026-08-09T15:01:31 UTC, i.e., 238 seconds post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
An uncatalogued optical counterpart was detected within the error circles of MXT (Brunet et al., GCN 45331)and ECLAIRs (Brunet et al., GCN 45331), compared with the Legacy Survey. The position is at R.A., Dec. = 356.962498, 21.037855 degrees, equivalent to:
R.A. (J2000) = 23:47:50.99
Dec. (J2000) = +21:02:16.28
with an uncertainty of 0.5 arcsec.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Exposure Time | Band | Brightness
263 s | 50 s | VT_B | 21.0+/-0.2
263 s | 50 s | VT_R | 18.8+/-0.1
3.02 h | 25*100 s | VT_B | 23.2+/-0.2
3.02 h | 27*100 s | VT_R | 21.5+/-0.1
The source showed a slow brightening trend in both channels during our early observations taken within 600 seconds post trigger. It then faded with a temporal decay slope of -1.2 since 1.02 hr.
Given the fading of the source in our observations, we propose that it is the optical counterpart of the burst.
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 45331
Subject
GRB 260809A: SVOM detection of a faint burst
Date
2026-08-09T15:24:53Z (19 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet (IRAP), F. Piron (LUPM), D.Gotz (CEA), A. Saccardi (CEA) report on behalf of the SVOM mission team:
At 2026-08-09T14:57:33 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260809A (SVOM burst-id sb26080907).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was only detected by the Image Trigger (IMT), which produced a sequence of 2 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 7.20 in the [8-120] keV energy band over a time window of 20.48 seconds starting at 2026-08-09T14:57:13.
The localization of the best alert is R.A., Dec. 356.820, 20.969 degrees (J2000) with a 90% confidence level (C.L.) radius of 10.88 arcmin (including systematic error of 2 arcmin added in quadrature).
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2026-08-09T15:00:23 UTC, 170 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 356.9664, 21.0347 degrees:
R.A. (J2000) = 23h47m51s
Dec. (J2000) =21d02m04s
with a 90% C.L. radius of 94 arcseconds.
This location is 9.09 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.
VT began observing the field after the slew. The analysis of the data will be published in a future circular.
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 Frédéric Piron: piron@in2p3.fr.
Please contact the BA by email if you require additional information.