GRB 261104A
GCN Circular 46166
Subject
GRB 261104A：EP-FXT follow-up observation
Date
2026-11-06T03:57:29Z (a day ago)
Edited On
2026-11-06T19:53:19Z (20 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B. B. Zhang, C. L. Guo, Q.Y. Wu, W.D. Zhang (NAO, CAS), Y. Q. Zhao(PRIC, USTC)
on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of GRB 261104A, detected by SVOM/ECLAIRs (Bernardini et al., GCN 46150), which was also observed in optical by LCO 1m telescope (Yin et al., GCN 46154), SVOM/VT (Li et al., GCN 46161), and DECam (Davis et al., GCN 46164). The observation started at 2026-11-05 19:31:07 (UTC), about 23 hrs after the SVOM/ECLAIRs trigger, with an exposure time of 1871 s.
On-ground analysis of the EP-FXT data found six uncataloged X-ray source within the SVOM/ECLAIRs error region (Bernardini et al., GCN 46150). None of these sources has a position consistent with that of the optical counterpart candidate. The preliminary results for the X-ray source are as follow:
Source 1: EPF_J002511.2-542737
R.A., Decl. (J2000) = 6.2972, -54.4603 deg
Localization error radius = 10 arcsec 0.5-10 keV
Flux = 3.13e-13 erg/cm^ 2/s
Flux error = 7.15e-14 erg/cm^2/s (1 sigma)
Separation = 0.13 arcmin
Source 2: EPF_J002543.1-543011
R.A., Decl. (J2000) = 6.4296, -54.5032 deg
Localization error radius = 10 arcsec 0.5-10 keV
Flux = 1.98e-13 erg/cm^ 2/s
Flux error = 5.99e-14 erg/cm^2/s (1 sigma)
Separation = 5.3 arcmin
Source 3: EPF_J002439.7-542315
R.A., Decl. (J2000) = 6.1655, -54.3874 deg
Localization error radius = 10 arcsec 0.5-10 keV
Flux = 1.10e-13 erg/cm^ 2/s
Flux error = 8.14e-14 erg/cm^2/s (1 sigma)
Separation = 6.4 arcmin
Source 4: EPF_J002458.6-543125
R.A., Decl. (J2000) = 6.2441, -54.5236 deg
Localization error radius = 10 arcsec 0.5-10 keV
Flux = 1.12e-13 erg/cm^ 2/s
Flux error = 4.23e-14 erg/cm^2/s (1 sigma)
Separation = 4.1 arcmin
Source 5: EPF J002428.2-542321
R.A., Decl. (J2000) = 6.1177, -54.3891 deg
Localization error radius = 10 arcsec 0.5-10 keV
Flux = 1.83e-13 erg/cm^ 2/s
Flux error = 5.22e-14 erg/cm^2/s (1 sigma)
Separation = 7.6 arcmin
Source 6: EPF J002557.1-542320
R.A., Decl. (J2000) = 6.4879, -54.3890 deg
Localization error radius = 10 arcsec 0.5-10 keV
Flux = 1.48e-13 erg/cm^ 2/s
Flux error = 5.01e-14 erg/cm^2/s (1 sigma)
Separation = 8.0 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 46164
Subject
GRB 261104A: DECam Detection of an Optical Counterpart Candidate
Date
2026-11-05T20:24:37Z (2 days ago)
From
Michael C. Davis at University of Minnesota <davi4614@umn.edu>
Via
Web form
Michael C. Davis (UMN), Tomas Ahumada (NOIRLab), Michael Coughlin (UMN), Xander J. Hall (CMU), and Anirudh Salgundi (UNC) report:
We observed the field of GRB 261104A, detected by SVOM/ECLAIRs (Bernardini et al., GCN 46150; SVOM burst-id sb26110402), using the Dark Energy Camera (DECam) on the 4-m Blanco telescope at CTIO. Observations began on 2026-11-05 at 01:57:06.43 UTC (approximately 5.34 hours after the trigger) under program 2026B-398365 (PI Davis).
Observations consisted of 1x300 s exposures in each of the SDSS r and i bands. The images were reduced using the NOIRLab DECam Community Pipeline and processed with the DESIRT difference-imaging pipeline (Hu et al. 2026). We additionally performed independent forced photometry and image subtraction using Legacy Surveys DR10 reference images.
We identify an optical transient candidate, AT 2026aigl, at the following coordinates:
RA (J2000): 6.336822 deg
Dec (J2000): -54.412962 deg
The candidate lies approximately 3.17 arcmin from the SVOM/ECLAIRs localization center, within the reported 9.81 arcmin 90% confidence region. The DESIRT difference-imaging pipeline reports the following AB magnitudes:
| Start UT | T_mid - T0 (days) | Filter | Exposure (s) | Magnitude (AB) |
|---|---|---|---|---|
| 2026-11-05T01:57:06.43 | 0.2242 | r | 1x300 | 23.35 +/- 0.12 |
| 2026-11-05 02:02:35.34 | 0.2280 | i | 1x300 | 23.08 +/- 0.13 |
We note that a faint, cataloged point source (magnitudes: r=24.04, i=23.64) is present in the archival Legacy Surveys DR10 images within approximately 0.16 arcsec of the candidate position. The archival source is classified as a point source by Tractor modelling at a photometric redshift of z_ph = 1.04 +/- 0.19. The archival source has no archival WISE photometry, nor is it in the Milliquas catalog.
Our detections are consistent with earlier optical observations reported non-detections by SVOM/VT (Li et al., GCN 46161) and LCO (Yin et al., GCN 46154). At present, no fading has been established, and the association of this candidate with GRB 261104A remains tentative.
Further time-resolved optical observations are encouraged to establish the nature of the candidate.
GCN Circular 46163
Subject
GRB 261104A: SVOM/ECLAIRs refined analysis
Date
2026-11-05T17:29:51Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
R. Hamburg (USRA), M. Brunet (IRAP), F. Piron (LUPM), M.-G. Bernardini (INAF/LUPM), and M. Gnaoui (IAP) 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 261104A (SVOM burst-id sb26110402 – Bernardini et al. 2026, GCN 46150, trigger time T0 = 2026-11-04T20:36:44 UTC).
The burst that triggered ECLAIRs onboard shows a single peak lightcurve with most of the emission seen below 20 keV. The burst duration is T90 = 20.1 +3.3/-3.8 s in the 4-120 keV energy band.
The time-averaged spectrum from T0+7.6 s to T0+27.5 s in the 5-120 keV energy range is best fitted by a power law model with a photon index of -1.76 +/- 0.19. With this model, the 4-120 keV fluence is (1.38 ± 0.43)e-7 erg/cm^2 and the 4-120 keV photon flux is (2.55 ± 0.45)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: R. Hamburg (USRA) (rhamburg@usra.edu)
GCN Circular 46161
Subject
GRB 261104A: SVOM/VT optical upper limits
Date
2026-11-05T16:20:26Z (2 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), M. G. Bernardini (INAF/LUPM), M. Gnaoui (IAP) and R. Hamburg (USRA) report on behalf of the SVOM/VT team.
SVOM/VT performed an automatic slew on the burst GRB 261104A triggered by SVOM/ECLAIRs (sb26110402, Bernardini et al., GCN 46150). The observation started at 2026-11-04T21:31:59 UTC, about 0.92 hr in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No credible uncatalogued candidates were identified the stacked images within the ECLAIRs error box (Bernardini et al., GCN 46150), compared with the Legacy Survey.
Preliminary upper limits are estimated in AB magnitudes and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | 5-sigma upper limit
1.10 h VT_B 50*20 s > 22.5 mag
1.10 h VT_R 50*18 s > 22.5 mag
Our non-detection is consistent with the reports from LCO (Yin et al., GCN 46154).
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 46154
Subject
GRB 261104A: LCO optical upper limits
Date
2026-11-05T01:30:27Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
S.J. Yin(GXU),D. Turpin, A. Saccardi , J. T. Palmerio (CEA/Irfu),Y.X. Guan(GXU), C. Wu , L.P. Xin,(NAOC), Y.Y. Shi(GXU), X.H. Han(NAOC), M. Gnaoui (IAP), L.Zhang (IHEP), A. Li (BNU),
report on behalf of the SVOM mission team:
We observed the field of GRB 261104A (detected by SVOM burst-id sb26110402
(Bernardini et al., GCN 46150) with the LCO 1m telescope at South Afican Astronomical Observatory equipped with the Sinistro instrument.
Our observation started on 2026-11-04T21:51:12.583 UTC (about 1.24 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r filter.
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 DESI DR10 catalog, and not corrected for Galactic extinction:
r > 22.31 AB (5-sigma, mid-time 1.33 hr after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 46150
Subject
GRB 261104A: SVOM detection of burst
Date
2026-11-04T21:03:59Z (3 days ago)
Edited On
2026-11-04T22:46:41Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. G. Bernardini (INAF/LUPM), M. Gnaoui (IAP), and R. Hamburg (USRA) report on behalf of the SVOM/ECLAIRs team
At 2026-11-04T20:36:44 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 261104A (SVOM burst-id sb26110402).
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 5 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 8.02 in the [5-20] keV energy band over a time window of 20.48 seconds starting at 2026-11-04T20:36:31.
The localization of the best alert is R.A., Dec. 6.2999, -54.4612 degrees:
R.A. (J2000) = 0h25m11.98s
Dec. (J2000) = -54d27m40.50s
with a 90% confidence level (C.L.) radius of 9.81 arcmin (including systematic error of 2 arcmin added in quadrature).
The SVOM/ECLAIRs light curve showed a single broad peak structure with a T90 duration of about 40 s.
SVOM slewed to the burst.
MXT and 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 Mouad GNAOUI: mouad.gnaoui@iap.fr.
Please contact the BA by email if you require additional information.