GRB 260915A
GCN Circular 45647
Subject
GRB 260915A: VLT optical and near-infrared upper limits
Date
2026-09-19T16:40:08Z (2 hours ago)
From
M. Garnichey at LUX - Paris Observatory <marine.garnichey@obspm.fr>
Via
Web form
J. An (NAOC), R. Brivio (INAF-OAB), G. Corcoran (UCD), V. D’Elia (ASI/SSDC), M. Garnichey (LUX-Paris Obs.), N. Habeeb (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), B. Schneider (LAM), N. R. Tanvir (U. Leicester), S. D. Vergani (LUX-Paris Obs.), X. Liu, D. Xu (NAOC) report on behalf of the Stargate collaboration:
We observed the field of GRB 260915A detected by SVOM/ECLAIRs (Liang et al., GCN 45570; Yang et al., GCN 45612) using the ESO VLT UT1 (Antu) equipped with the FORS2 imager and the ESO VLT UT4 (Yepun) equipped with the HAWK-I near-infrared imager.
We obtained observations in the z-band (on two epochs) and in the YJHK filters (one epoch). A single, faint source, at approximate J2000 coordinates RA = 00:32:37.52, Dec = +08:15:00.1, is detected within the overlap region of the EP/FXT (Shi et al., GCN 45587) and Swift/XRT (Page et al., GCN 45623) error circles. This source is also faintly visible in archival Legacy Survey imaging. A few more, nearby sources are seen consistent with only one of the FXT and XRT error circles. Comparison of the two z-band epochs reveals no variability for any of these sources, so we cannot identify any compelling candidate counterpart of GRB 260915A.
A summary of our observations is provided in the table below.
| t-t0 (hr) | Exposure time (s) | Magnitude (AB) | Instrument | Band |
| ------ | ----------------- | ---------------| ---------- | ---- |
| 5.77 | 10x120 | > 24.5 | FORS2 | z |
| 6.32 | 10x60 | > 24.5 | HAWK-I | Y |
| 6.53 | 8x60 | > 24.0 | HAWK-I | J |
| 6.73 | 8x60 | > 23.8 | HAWK-I | H |
| 6.96 | 10x60 | > 23.4 | HAWK-I | K |
| 29.48 | 10x120 | > 25.0 | FORS2 | z |
Magnitude upper limits are at the 3 sigma level. Optical and NIR photometry was calibrated against the Pan-STARRS DR2 and 2MASS catalogs, respectively, and is not corrected for Galactic extinction.
Our non-detection is consistent with previous reports (Yin et al., GCN 45573; Wu et al., GCN 45575; Zhu et al., GCN 45577; Perez-Garcia et al., GCN 45578; Ducoin et al., GCN 45580; Li et al., GCN 45581; Brivio et al., GCN 45584; Li et al., GCN 45586; Shi et al., GCN 45588; Li et al., GCN 45589; Mohan et al., GCN 45600).
We acknowledge the excellent support from the observing staff in Paranal, in particular Alonso Luna, Camila de Sa Freitas, Gonzalo Burgos, Juan Carlos Olivares, Sol Troncoso, and Thomas Rivinius.
GCN Circular 45623
Subject
GRB 260915A: Swift-XRT afterglow detection
Date
2026-09-18T06:26:48Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC &
INAF-OAR), J.A. Kennea (PSU), S. Lanava (PSU), A.P. Beardmore (U.
Leicester), J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB) and
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 260915A. We searched for X-ray sources
in 2.7 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 2.7 ks, obtained between
T0+174.4 ks and T0+246.9 ks.
An uncatalogued X-ray source is detected within the estimated 3-sigma
SVOM/MXT error region (49 arcsec). While the source does not fade
during the Swift-XRT observation, the measured flux of 2.0 (+7.4,
-1.4)e-13 erg cm^-2 s^-1 (0.3-10 keV) is more than an order of
magnitude below the EP value of 7.8 (+/- 0.4)e-12 erg cm^-2 s^-1
(0.5-10 keV) at T0+6 ks, reported in GCN Circ. 45587. This source is
therefore believed to be the afterglow. The position of this source is
RA, Dec=8.1546, +8.2487 which is equivalent to:
RA (J2000): 00h 32m 37.11s
Dec(J2000): +08d 14' 55.5"
with an uncertainty of 8.5 arcsec (radius, 90% confidence). This
position is 23 arcsec from the SVOM/MXT position, and 11 arcsec from
the EP-FXT position.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021912.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021912.
This circular is an official product of the Swift-XRT team.
GCN Circular 45612
Subject
GRB 260915A: SVOM/ECLAIRs refined analysis
Date
2026-09-17T12:22:19Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
H. Yang, M. Brunet, O. Godet (IRAP), W. J. Xie (NAOC), Y. F. Liang (PMO), C. Plasse (HKU), H. Goto (RIKEN/CEA) 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 260915A (SVOM burst-id sb26091501 – GCN 45570, trigger time T0 = 2026-09-15T01:12:03 UTC), which was also detected by Fermi/GBM as a sub-threshold (Hamburg, GCN 45574).
The burst that triggered ECLAIRs onboard shows a broad bump from T0 - 14 s to T0 + 200 s in the 4-120 keV energy band, with a slew performed from T0 + 42 s to T0 + 135 s. The burst duration estimated through the imaging technique is T90 = (167 +10/-6) s in the 4-120 keV energy band.
The time-averaged spectrum from T0 - 14 s to T0 + 42 s (before the slew) in the 5-120 keV band is best fitted by a power-law model, with alpha= -1.20 +/- 0.05. With this model, the 4-120 keV fluence is (2.6 +/- 0.1)e-6 erg/cm^2 and the 4-120 keV photon flux is (9.9 +/- 0.4)e-1 ph/cm^2/s.
The time-averaged spectrum from T0 + 135 s to T0 + 200 s (after the slew) in the 5-120 keV band is best fitted by a cutoffpl model, with alpha = -1.4 +0.4/-0.8 and Epeak = 54 +80/-24 keV. With this model, the 4-120 keV fluence is (2.7 +/- 0.6)e-7 erg/cm^2 and the 4-120 keV photon flux is (1.4 +0.3/-0.2)e-1 ph/cm^2/s.
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: Hui Yang (hui.yang at cnrs.fr)
GCN Circular 45600
Subject
GRB 260915A: J-band upper limit with WINTER
Date
2026-09-17T00:45:32Z (3 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 260915A (Liang et al., GCN 45570, also see Hamburg et al., GCN 45574) 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 2026-09-15 at 04:56:16.725 UTC in the J band (3.74 hr after the SVOM/ECLAIRs trigger), consisting of 43*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 any new source in the SVOM/MXT localization (Liang et al., GCN 45570) and at EPF_J003237.9+081455 and EPF_J003224.9+081259 localization (Shi et al., GCN 45587) in our stacked image (Yin et al., GCN 45573; Wu et al., GCN 45575; Zhu et al., GCN 45577; Perez-Garcia et al., GCN 45578; Ducoin et al., GCN 45580; Li et al., GCN 45581; Brivio et al., GCN 45584; Li et al., GCN 45586; Shi et al., GCN 45588; Li et al., GCN 45589). We obtain the following 5-sigma upper limit: J = 19.9 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 45590
Subject
GRB 260915A: Swift ToO observations
Date
2026-09-15T22:26:28Z (4 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 260915A. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021912
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 45589
Subject
GRB 260915A: J-band upper limit by SYSU 80cm infrared telescope
Date
2026-09-15T18:42:53Z (4 days ago)
From
lijj328@mail2.sysu.edu.cn
Via
Web form
Jin-Ji Li, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Rui-Chen Gao, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of GRB 260915A, detected by SVOM/Eclairs (Liang et al., GCN 45570), with the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Sep 15 from 14:37:04 to 17:54:33 UT, i.e. ~13.4-16.7 hours after the trigger (mid-time 15.1 hr), using stacked images in the J band (522 x 20 s, 2.90 hr total exposure).
We do not detect any source within the error box of source 1 reported by EP-FXT (EPF_J003237.9+081455; R.A., Decl. = 00:32:37.85, +08:14:54.96, J2000; Shi et al., GCN 45587) in our stacked J-band image, down to a 5-sigma depth of J ~ 19.7 Vega mag.
Our non-detection is consistent with the non-detections reported by Yin et al. (GCN 45573), Wu et al. (GCN 45575), Zhu et al. (GCN 45577), Perez-Garcia et al. (GCN 45578), Ducoin et al. (GCN 45580), Li et al. (GCN 45581), Brivio et al. (GCN 45584) and Shi et al. (GCN 45588).
The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45588
Subject
GRB 260915A: SVOM/C-GFT optical upper limits
Date
2026-09-15T15:55:57Z (4 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.Y. Shi (GXU), Z. Kang (CHO), C. Wu , L.P. Xin, X.H. Han, P.P. Zhang, X.M. Lu (NAOC), Z.W. Li, Y. Lv (CHO), R.S. Zhang, Y.J. Xiao, Y.L. Qiu, J. Wang, J.S. Deng, L. Huang, J.Y. Wei (NAOC), W.J. Tan (IHEP), Y.N. Ma (NAOC) report on behalf of the SVOM/C-GFT team:
We observed the field of GRB 260915A detected by SVOM/Eclairs (Liang et al., GCN 45570) with LATIOS on SVOM/C-GFT.Our observations started at 2026-09-15T11:54:07 UTC, ~ 10.7 hr after the trigger.
We obtained follow-up imaging in g, r, and i bands. After preliminary analysis, no credible candidate was detected within the error box of source 1 provided by EP-FXT(Shi et al., GCN 45587) in our images. We measure the following upper limits calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
g > 20.8 AB (3-sigma, mid-time 11.89 hr after the trigger).
r > 19.9 AB (3-sigma, mid-time 11.36 hr after the trigger).
i > 21.0 AB (3-sigma, mid-time 10.83 hr after the trigger).
Our result is consistent with non-detection by Yin et al. (GCN 45573), Wu et al. (GCN 45575), Zhu et al. (GCN 45577), Perez-Garcia et al. (GCN 45578), Ducoin et al. (GCN 45580), Li et al. (GCN 45581), and Brivio et al. (GCN 45584).
We note that in source 2 position of EP/FXT (Shi et al. GCN 45587), there is a known optical source (SDSS J003224.91+081258.0 : Quasar) ~0.1 mag brighter than the PS1 catalog value in the g,r, and i band.
We thank the observation assistants B.-W. Li and H.-X. Xue at Jilin observatory for their excellent support.
The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcmin x 21 arcmin FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.
GCN Circular 45587
Subject
GRB 260915A: EP-FXT follow-up observation
Date
2026-09-15T14:28:31Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.Shi, Y. F. Liang, J. Y. Liu (PMO, CAS), Y. Q. Zhao (PRIC, USTC), F. F. Song (YNAO, CAS), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of GRB 260915A, detected by SVOM/ECLAIRs and SVOM/GRM (sb26091501; Liang et al., GCN 45570), which was also observed in optical (Yin et al., GCN 45573; Wu et al., GCN 45575; Li et al., GCN 45581; Ducoin et al., GCN 45580; Zhu et al., GCN 45577; Perez-Garcia et al., GCN 45578). The observation started at 2026-09-15 02:51:49 (UTC), about 1.66 hours after the SVOM/ECLAIRs trigger, with an exposure time of 5195 s.
On-ground analysis of the EP-FXT data found two uncataloged X-ray sources within the SVOM/ECLAIRs error region (Liang et al., GCN 45570). Both show no clear decay. The preliminary results for the X-ray sources are as follows:
Source 1: EPF_J003237.9+081455
R.A., Decl. (J2000) = 8.1577, 8.2486 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 7.80e-12 erg/cm^ 2/s
Flux error = 4.12e-13 erg/cm^2/s (1 sigma)
Separation = 1.72 arcmin
Source 2: EPF_J003224.9+081259
R.A., Decl. (J2000) = 8.1037, 8.2161 deg
Localization error radius = 10 arcsec
0.5-10 keV flux = 5.43e-13 erg/cm^ 2/s
Flux error = 1.37e-13 erg/cm^2/s (1 sigma)
Separation = 2.20 arcmin
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 45586
Subject
GRB 260915A: SVOM/VT refined early optical upper limits
Date
2026-09-15T13:53:44Z (4 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, L. J. 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 (CEA/Irfu) report on behalf of the SVOM/VT team.
With more early SVOM/VT X-band data available, the refined analysis yields an early optical upper limits for GRB 260915A (Liang et al., GCN 45570; Hamburg et al., GCN 45574) within the error box of the EP-FXT's source 1 (Shi et al., GCN 45585) and SVOM/MXT (Liang et al., GCN 45570).
Mid-time（t-T0）| Band | Exposure Time | 3-sigma upper limit
16.4 min VT_B 32*50 sec > 23.3 mag
18.3 min VT_R 30*50 sec > 23.2 mag
The above measurements are in the AB magnitude and are not corrected for Galactic extinction.
The result is consistent with the non-detections from SVOM/VT (Wu et al., GCN 45575；Li et al., GCN 45581), LCO (Yin et al., GCN 45573), NOT (Zhu et al., GCN 45577), GTC (Perez-Garcia et al., GCN 45578), and SVOM/COLIBRÍ (Ducoin et al., GCN 45580) and REM (Brivio et al., GCN 45584).
We notice that EP-FXT's source 2 is overlapped with a Quasar (SDSS J003224.91+081258.0).
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 45585
Subject
GRB 260915A: EP-FXT follow-up observation
Date
2026-09-15T13:23:06Z (4 days ago)
Edited On
2026-09-15T16:23:10Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
Web form
At the authors' request, GCN Circular 45585 is superseded by GCN Circular 45587 which contains corrections and additional information on GRB 260915A.
GCN Circular 45584
Subject
GRB 260915A: REM optical/NIR observations
Date
2026-09-15T12:24:58Z (4 days ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, F. Guidolin, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of GRB 260915A, detected by SVOM/ECLAIRs (Liang et al., GCN 45570) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile).
The observations were carried out in the g, r, i, z, J, and H bands, started on 2026-09-15 at 01:14:32 UT (i.e. 2.5 min after the burst) and lasted for about 2.5 hr.
From preliminary photometry, we do not detect any optical/NIR uncatalogued source down to the following 3sigma limits:
r > 18.3 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 7.5 min after the trigger;
H > 14.5 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 3.0 min after the trigger.
GCN Circular 45581
Subject
GRB 260915A: SVOM/VT optical upper limits
Date
2026-09-15T09:42:49Z (4 days ago)
Edited On
2026-09-15T12:52:13Z (4 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, L. J. 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 (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed an automatic slew toward the burst GRB 260915A triggered by SVOM/ECLAIRs (sb26091501, Liang et al., GCN 45570). This burst was also detected by SVOM/GRM and SVOM/MXT (Liang et al., GCN 45570). The observation started at 2026-09-15T01:16:38 UTC, i.e. 275 seconds in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band telemetry data, no uncatalogued sources were identified in our stacked images within the error box of MXT (Liang et al., GCN 45570) compared to the Legacy Survey.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | 3-sigma upper limit
1.1 hr VT_B 50*50 sec > 23.6 mag
1.0 hr VT_R 50*50 sec > 23.5 mag
Our results are consistent with the non-detections from VT-VHF data (Wu et al., GCN 45575), LCO (Yin et al., GCN 45573), NOT (Zhu et al., GCN 45577), GTC (Perez-Garcia et al., GCN 45578), and SVOM/COLIBRÍ (FM-GFT) (Ducoin et al., GCN 45580).
Given the bright X-ray emission and deep early optical upper limits, this source may be a high-z GRB candidate. Near-infrared follow-up observations are encouraged.
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 45580
Subject
GRB 260915A: SVOM/COLIBRÍ (FM-GFT) optical upper limit
Date
2026-09-15T08:54:06Z (4 days ago)
From
J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr>
Via
Web form
Jean-Grégoire Ducoin (CPPM), William H. Lee (UNAM), Stéphane Basa (UAR Pytheas), 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), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (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), Y. F. Liang(PMO), C. Plasse (HKU), report:
We imaged the field of the SVOM GRB 260915A (Liang et al., GCN Circ. 45570) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-09-15T05:23:02 to 05:42:23 UTC (from 4.14 to 4.46 hours after the trigger) and obtained 16 minutes of simultaneous exposure in the r and z filters.
The data were reduced, coadded and analysed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the MXT source position (Liang et al., GCN Circ. 45570) down to the following 5-sigma limit:
r > 23.12
z > 21.76
This upper limit is consistent with the one reported by Zhu et al. GCN Circ. 45577 and by Perez-Garcia et al. GCN Circ. 45578.
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 45578
Subject
GRB 260915A: 10.4m GTC deep optical limits
Date
2026-09-15T08:26:48Z (4 days ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
I. Perez-Garcia, A. Sota, M.D. Caballero-Garcia, M. Gritsevich, S.-Y. Wu, S. Guziy (IAA-CSIC), Y.-D. Hu (GXU), D. Slumstrup, D. Perez-Valladares (IAC, GRANTECAN) and A.J. Castro-Tirado (IAA-CSIC), on behalf of a larger collaboration
Following the detection of GRB 260915A by SVOM/ECLAIRs (Liang et al. GCN 45570), we triggered the 10.4m GTC telescope equipped with OSIRIS in La Palma (Spain). Images in the Sloan i, z and r bands were gathered on Sep. 15 starting at 05:20 UT (i.e. ~4.1 hours after the trigger), at high airmass (~1.55) and under variable transparency, covering the SVOM/MXT error circle (Liang et al. GCN 45570). No optical transient is detected within the error circle down to 3-sigma limits of r = 24.58 mag, i = 22.82 and z = 23.71 (100 s, 100 s and 240 s of exposure, respectively).
Magnitudes are in the AB system, calibrated against ATLAS-REFCAT2, and not corrected for Galactic extinction.
These non-detections are consistent with the reports from LCO (Yin et al. GCN 45573), SVOM/VT (Wu et al. GCN 45575) and NOT (Zhu et al. GCN 45577).
We thank the staff at GTC for their excellent support.
GCN Circular 45577
Subject
GRB 260915A: NOT optical upper limit
Date
2026-09-15T07:53:10Z (4 days ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form
Z. Zhu, J. An, D. Xu (NAOC), A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI), V. Neustroev (NOT), V. Vuolteenaho (NOT), report on behalf of a larger collaboration:
We observed the field of GRB 260915A detected by SVOM/ECLAIRS (Liang et al., GCN 45570), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 5x200 s exposures in the SDSS z-band starting at 05:21 UT on 2026-09-15 (about 4.2 hr after the trigger).
No uncatalogued optical transient is detected in the stacked image within the SVOM/ECLAIRs error region in agreement with previous observations (Yin et al., GCN 45573; Wu et al., GCN 45575), down to a 3-sigma limiting magnitude of z ~ 21.5, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 45575
Subject
GRB 260915A: SVOM/VT optical upper limits from VHF data
Date
2026-09-15T05:00:39Z (5 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
C. Wu, Y. N. Ma, L. P. Xin, Y.L. Qiu, H.L. Li, J.R. Xu(NAOC), J. T. Palmerio (CEA/Irfu), W. Tan (IHEP) report on behalf of the SVOM/VT team.
After the trigger by SVOM/ECLAIRs at 2026-09-15T01:12:03 UTC (T0), SVOM performed an automatic slew on the burst location ( Liang et al., GCN 45570). SVOM/VT took images of the field in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.From a preliminary analysis of the 1-bit subimage and source list downloaded via the VHF network, no uncatalogued optical source was detected within the SVOM/MXT (Liang et al., GCN 45570) localization region in our three sequence images, compared to the Pan-STARRS1 DR1. The 5-sigma upper limits are:
| start time (min) | Band | Exposure (s) | 5-sigma upper limit (mag) |
|---|---|---|---|
| 5.42 | VT_B | 6 * 50 | > 21.5 |
| 5.42 | VT_R | 6 * 50 | > 21.5 |
| 11.25 | VT_B | 6 * 50 | > 21.5 |
| 11.25 | VT_R | 6 * 50 | > 21.5 |
| 81.48 | VT_B | 6 * 50 | > 21.5 |
| 81.48 | VT_R | 6 * 50 | > 21.5 |
The non-detection is consistent with Yin et al. (GCN 45573). Radio and infrared follow-up observations are encouraged.
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/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular 45574
Subject
Fermi GBM Sub-Threshold Detection of GRB 260915A
Date
2026-09-15T04:56:50Z (5 days ago)
From
rhamburg@usra.edu
Via
Web form
R. Hamburg (USRA) reports on behalf of the Fermi-GBM Team:
SVOM ECLAIRs detected GRB 260915A on 2026-09-15 at 01:12:03 UTC (GCN 45570). There was no Fermi-GBM onboard trigger around this time.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a spatially-consistent transient at 2026-09-15T01:12:01.517 UTC. The Targeted Search event is found most significantly on the 32.768 s timescale with a false alarm rate of 8.3e-5 Hz. The Fermi-MET of this transient is 811127526.517 s. Additionally, the Targeted Search event was found with the highest significance using a normal GRB spectrum (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3).
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 45573
Subject
GRB 260915A: LCO optical upper limits
Date
2026-09-15T04:14:02Z (5 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
S.J. Yin (GXU),C. Wu, L.P. Xin (NAOC),D. Turpin (CEA/Irfu), A. Saccardi (CEA/Irfu), W. Tan (IHEP), Y. N. Ma (NAOC) report on behalf of the SVOM mission team:
We observed the field of GRB 260915A (SVOM burst-id sb26091501） detected by SVOM (Liang et al., GCN 45570) with the LCO 1m telescope at Cerro Tololo Observatory and Teide Observatory equipped with the Sinistro instrument.
Our observation started on 2026-09-15 at 01:42:33 UT (about 30.5 min after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.
In the stacked image, we do not detect any new source within the SVOM/ECLAIRs error box. We measure the following upper limit calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
r > 21.1 AB (5-sigma, mid-time 35.9 min after the trigger);
z > 20.5 AB (5-sigma, mid-time 1.37 hr after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 45570
Subject
GRB 260915A: SVOM detection of a burst
Date
2026-09-15T01:39:08Z (5 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y. F. Liang(PMO), C. Plasse (HKU), H. Goto (RIKEN/CEA), W. Xie (NAOC), W. Tan (IHEP), Y. N. Ma (NAOC) report on behalf of the SVOM mission team:
At 2026-09-15T01:12:03 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260915A (SVOM burst-id sb26091501). 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 14 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 21.38 in the [8-120] keV energy band over a time window of 40.96 seconds starting at 2026-09-15T01:12:00.
The localization of the best alert is R.A., Dec. 8.1021, 8.2587 degrees (J2000) with a 90% confidence level (C.L.) radius of 4.12 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst was also detected by SVOM/GRM with a significance of 8.
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2026-09-15T01:14:42 UTC, 159 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 8.1504, 8.2440 degrees:
R.A. (J2000) = 0h32m36.09s
Dec. (J2000) = 8d14m38.47s
with a 90% C.L. radius of 49.74 arcseconds.
This location is 3.00 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 Yinuo Ma: mayn@bao.ac.cn
Please contact the BA by email if you require additional information.