GRB 261108A

A. C. Trigg (NPP ORAU, NASA MSFC) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 01:13:19.35 UT on 08 November 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 261108A (trigger 815793204/261108051), which was also detected by SVOM/ECLAIRs and SVOM/GRM (Xie et al. 2026, GCN 46186
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 ).
The Fermi GBM real-time localization (GCN 46185
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 ) is consistent with the SVOM/ECLAIRS position.

The angle from the Fermi LAT boresight is 40 degrees.

The GBM light curve consists of a single emission with a duration (T90) of about 24 s (50-300 keV). The time-averaged spectrum from T0-2.2 to T0+17.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.26 +/- 0.04 and the cutoff energy, parameterized as Epeak, is 87 +/- 4 keV.

The event fluence (10-1000 keV) in this time interval is (8.1 +/- 0.6)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.3 s in the 10-1000 keV band is 3.1 +/- 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/"

S.J. Yin (GXU), Z. Kang (CHO), C. Wu , L.P. Xin (NAOC), Y.X. Guan, Y.Y. Shi(GXU), 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), Y.D. Hu(GXU), Clara Plasse(HKU)  report on behalf of the SVOM/C-GFT team:
 
We observed the field of GRB 261108A detected by SVOM/ ECLAIRs (Xie et al., GCN 46186
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 ) with LATIOS on SVOM/C-GFT. Observations started at 2026-11-08T09:20:33.7 UTC, ~ 8.12 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 provided by SVOM/Eclairs (Xie et al., GCN 46186
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 ) in our images. We measure the following upper limit calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:

g > 19.54 AB (5-sigma, mid-time 8.65 hr after the trigger).
r > 19.53 AB (5-sigma, mid-time 8.84 hr after the trigger).
i > 19.46 AB (5-sigma, mid-time 8.46 hr after the trigger).


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.

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), Youdong Hu (GXU), Clara Plasse (HKU) report on behalf of the SVOM/VT team.

SVOM/VT performed observation with automatic slew on the field of GRB 261108A triggered by SVOM/ECLAIRs (sb26110802, Xie et al., GCN 46186
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 ). The observation started at 2026-11-08T01:25:28 UTC, about 12.5 minutes post trigger in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
 
No credible uncatalogued candidates were identified in the stacked images within the ECLAIRs error box (Xie et al., GCN 46186
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 ), compared with the Legacy Survey, down to 22.0 mag (7 sigma) in AB magnitudes, at the mid-time of 1.4 hr post trigger.

Our non-detection is consistent with the LCO report (Guan et al., GCN 46190
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 ).

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.

Y.X. Guan, Y.Y. Shi(GXU), A. Saccardi, D. Turpin (CEA/Irfu),  C. Wu , L.P. Xin,(NAOC), S.J. Yin(GXU), X.H. Han(NAOC), Y.D. Hu(GXU), Clara Plasse(HKU),
 report on behalf of the SVOM mission team:

We observed the field of GRB 261108A (detected by SVOM burst-id sb26110802
 (Xie et al., GCN 46186
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 ) with the LCO 1m telescope at Cerro Tololo Inter-American Observatory equipped with the Sinistro instrument.

Our observation started on 2026-11-08T01:17:20 UTC (about 4.2 min after the trigger) and we obtained 3x200 s exposures in the SDSS r filter 3x200 s exposures in the Pan-STARRS z 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 Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:

r > 22.6  AB (5-sigma, mid-time 9.6 min after the trigger);
z > 20.5  AB (5-sigma, mid-time 34.8 min after the trigger).

This project is funded by the SVOM collaboration.

Martin Jelinek, Jan Strobl and Filip Novotny
(ASU CAS Ondrejov, CZ),

Martin Masek, Sergey Karpov, Petr Janecek, Jakub Jurysek,
Jan Ebr, Ronan Cunniffe, Petr Travnicek and Michael Prouza
(Institute of Physics, Prague, CZ)

report:

The 30cm robotic telescope FRAM-Auger at the Pierre Auger
Observatory (Cerro Los Leones, Malargue, Argentina) reacted
robotically to the SVOM/ECLAIRs alert of GRB 261108A (SVOM
GRB 261108.05, cf. Lipunov et al., GCN 46188
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 ; Fermi GBM also
triggered at the same time, trigger 815793204), obtaining a
series of 32 R-band images of 60 s each, starting at
01:29:44 UT, i.e. ~16 min after the trigger and ~2 min after
the alert was received, and continuing until ~55 min after
the trigger. The field was at an elevation of ~60 deg.

We do not detect any new source within the ECLAIRs error
circle, neither in single images nor in the combined frames,
when comparing to the catalogue. Neither does a subtraction
of the coadd of the second half of the series from the coadd
of the first half reveal any variable source. We obtain the
following limits:

T-T0[mid]  Images  Limit (Vega)
~16 min    1       R_C > 17.0
~26 min    16      R_C > 18.7
~45 min    16      R_C > 18.7

Single images reach R_C ~ 17.0 throughout the series. The
photometry is calibrated against the Atlas-REFCAT2 catalogue
(Tonry et al. 2018, ApJ, 867, 105) and is not corrected for
Galactic extinction.

Wenjin Xie (NAOC), Xin Feng  (GXU), Youdong Hu (GXU), Clara Plasse (HKU), Hatsune Goto (RIKEN/CEA) report on behalf of the SVOM mission team:

At 2026-11-08T01:13:20 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 261108A (SVOM burst-id sb26110802). 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 18 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 17.68 in the [8-50] keV energy band over a time window of 10.20 seconds starting at 2026-11-08T01:13:17.

The localization of the best alert is R.A., Dec. 336.5157, -20.2077 degrees:
R.A. (J2000) = 22h26m03.78s
Dec. (J2000) = -20d12m27.78s
 with a 90% confidence level (C.L.) radius of 4.79 arcmin (including systematic error of 2 arcmin added in quadrature).

This burst was also detected by SVOM/GRM with a significance of 17.30. The SVOM/GRM light curve showed a single peak structure with a T90 duration of about 9.33 (-1.42 +1.95) s.

This burst is consistent in time and position with Fermi/GBM detection at 01:13:19 UTC (GCN #46185
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 ).

SVOM slewed to the burst. SVOM/MXT and VT started observation. 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 Clara Plasse: cplasse@hku.hk.
Please contact the BA by email if you require additional information.

The Fermi GBM team reports the detection of a likely LONG GRB

At 01:13:19 UT on 8 Nov 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 261108A (trigger 815793204.346972 / 261108051).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 338.7, Dec = -22.4 (J2000 degrees, equivalent to J2000 22h 34m, -22d 23'), with a statistical uncertainty of 15.6 degrees.

The angle from the Fermi LAT boresight is 40.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261108051/quicklook/glg_skymap_all_bn261108051.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261108051/quicklook/glg_healpix_all_bn261108051.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261108051/quicklook/glg_lc_medres34_bn261108051.gif