GRB 260131A, GRB 260131B
GCN Circular 43708
Subject
GRB260131A/B: 3.6m DOT Optical Observations
Date
2026-02-12T07:33:33Z (6 days ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Kuntal Misra, Debalina Kar, Pankaj Pawar, and Dhruv Jain (ARIES) report:
We observed the field of GRB 260131A/B detected by (Fermi GBM team; GCN 43579) with the 3.6m Devasthal Optical Telescope (DOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2026-02-08 at 14:20:45 UT, i.e., ~8.32 days after the Fermi trigger. We have taken multiple frames with an exposure time of 300s in the R filter. We stacked the images after the alignment. We detect an optical counterpart in our stacked image at the location reported by (Gompertz et al. 2026; GCN 43586). We obtain the following preliminary magnitude in the stacked image:
Date Start_UT T_start-T0(days) Filter Exp time(s) Magnitude
==================================================================
2026-02-08 14:20:45 ~8.32 R 300s*16 21.76 +/-0.03
Our detection is consistent with Gompertz et al. 2026 (GCN 43586); Lee et al. 2026 (GCN 43587); Saccardi et al. 2026 (GCN 43595); García García et al. 2026 (GCN 43599); Shi et al. 2026 (GCN 43601); Jayaraman et al. 2026 (GCN 43606); Zhu et al. 2026 (GCN 43607); Zheng et al. 2026 (GCN 43609); Adami et al. 2026 (GCN 43610); Volnova et al. 2026 (GCN 43632, 43670).
The magnitude is not corrected for the Galactic extinction in the direction of the burst.
Photometric calibration is performed using the standard stars from the USNO-B1.0 catalogue.
GCN Circular 43670
Subject
GRB 260131A/B: continued Mondy optical observations
Date
2026-02-09T15:13:09Z (9 days ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
email
A. Volnova (IKI), E. Klunko (ISTP), N. Pankov (IKI), A. Pozanenko (IKI)
report on behalf of IKI-GRB-FuN:
We continued observations of the field of GRB 260131A/B (Fermi GBM Team,
GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et
al., GCN 43590; Luo et al., GCN 43591; Jiang et al., GCN 43600; Li et al.,
GCN 43605) with the AZT-33IK 1.5m telescope of the Mondy observatory on
Feb. 8 starting at 13:03:46 UT and taking several frames in R band. The
optical counterpart reported by Gompertz et al., GCN 43586; Lee et al., GCN
43587; Saccardi et al., GCN 43595; García García et al., GCN 43599, Shi et
al., GCN 43601; Sanchez-Ramirez et al., GCN 43604; Jayaraman et al., GCN
43606; Zhu et al., GCN 43607; Zheng et al., GCN 43609; Adami et al., GCN
43610; Volnova et al., GCN 43632 is detected in the stacked frame.
Preliminary photometry and observational details are the following:
Date UT start t-T0 Exp. Filter Obj. Err. UL
Site/Telescope
(mid,days) (n*s) (3sigma)
2026-02-08 13:03:46 8.30854 56*120 R 21.70 0.09 22.8
Mondy/AZT-33IK
The photometry is based on several nearby stars from the USNO-B1 catalogue
(R2 magnitudes) and is not corrected for the Galactic extinction.
GCN Circular 43632
Subject
GRB 260131A/B: Mondy and Terskol optical observations
Date
2026-02-07T15:36:31Z (11 days ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), E. Klunko (ISTP), I. Sokolov (INASAN, KIAM), N. Pankov (IKI), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of GRB 260131A/B (Fermi GBM Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et al., GCN 43590; Luo et al., GCN 43591; Jiang et al., GCN 43600; Li et al., GCN 43605) with the AZT-33IK 1.5m telescope of the Mondy observatory on Feb. 4 and 5, and with the 2-meter Zeiss-2000 telescope of the Terskol observatory on Feb. 4 taking several frames in the R band. The optical counterpart reported by Gompertz et al., GCN 43586; Lee et al., GCN 43587; Saccardi et al., GCN 43595; García García et al., GCN 43599, Shi et al., GCN 43601; Sanchez-Ramirez et al., GCN 43604; Jayaraman et al., GCN 43606; Zhu et al., GCN 43607; Zheng et al., GCN 43609; Adami et al., GCN 43610, is detected in the stacked frame of every epoch. Preliminary photometry and observation data are the following:
Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope
(mid,days) (n*s) (3sigma)
2026-02-04 12:10:33 4.26118 43*120 R 19.56 0.04 22.3 Mondy/AZT-33IK
2026-02-04 15:54:04 4.41278 30*150 R 19.87 0.07 22.2 Zeiss-2000/Terskol
2026-02-05 13:40:24 5.32702 49*120 R 19.83 0.04 22.5 Mondy/AZT-33IK
The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction.
GCN Circular 43610
Subject
GRB 260131A/B: OHP/T193 optical observations
Date
2026-02-03T14:29:32Z (15 days ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
Web form
C. Adami (LAM/Pytheas/AMU), S. Basa (UAR Pytheas), N. Globus (UNAM), A. Watson (UNAM), B. Schneider (LAM), E. Le Floc'h (CEA-Saclay) report on behalf of the MISTRAL GRB collaboration:
We carried out observations of GRB 260131A/B (Fermi GBM Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et al., GCN 43590; Luo et al., GCN 43591; Jiang et al., GCN 43600; Li et al., GCN 43605) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 7 exposures for a total of 14 min in the r-band at a midtime of 2026-02-01 19:48UT, corresponding to T-T0 = 37.15 hours.
The afterglow reported by Gompertz et al., GCN 43586; Lee et al., GCN 43587; Saccardi et al., GCN 43595; García García et al., GCN 43599, Shi et al., GCN 43601; Sanchez-Ramirez et al., GCN 43604; Jayaraman et al., GCN 43606; Zhu et al., GCN 43607 ; Zheng et al., GCN 43609 is well detected in r’ with a preliminary magnitude of :
r’ = 19.09 +/- 0.08
The photometric calibration was performed using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen and Stephane Favard, and the SOPHIE observer Xavier Delfosse.
GCN Circular 43609
Subject
GRB 260131A: KAIT optical observations
Date
2026-02-03T07:55:33Z (15 days ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on
behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of GRB 260131A (Fermi GBM
Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585;
Frederiks et al., GCN 43590; Luo et al., GCN 43591; Li et al.,
GCN 43605) at ~0.914d and again at ~1.875d after the burst. A set
of clear (roughly R) filter images were obtained. We detected the
optical afterglow (Gompertz et al., GCN 43586; Lee et al., GCN 43587;
Saccardi et al., GCN 43595; García García et al., GCN 43599,
Shi et al. GCN 43601, Jayaraman et al., GCN 43606; Zhu et al.,
GCN 43607) in our coadd images and estimate the afterglow to be
17.8 +/- 0.1 and 19.2 +/- 0.2 mag (Vega) respectively.
GCN Circular 43607
Subject
GRB 260131A: FTW optical and NIR observations
Date
2026-02-02T22:00:00Z (16 days ago)
Edited On
2026-02-02T22:07:59Z (16 days ago)
From
Ziyuan.Zhu@campus.lmu.de
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Ziyuan.Zhu@campus.lmu.de
Via
Web form
Ziyuan Zhu (LMU), Malte Busmann (LMU), Julius Gassert (LMU), Xander J. Hall (CMU), Brendan O’Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the counterpart of GRB 260131A (Fermi/GBM Team, GCN 43579; Frederiks et al., GCN 43590; Luo et al., GCN 43591; Li et al., GCN 43605; Jayaraman et al., GCN 43606) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands for 15x 120s, starting at 2026-02-01T22:14:35 UT, (1.65 days after the trigger). We detect the counterpart in all bands and measure an r-band magnitude of
r = 19.18 +/- 0.03 AB mag
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Christoph Ries from the Wendelstein Observatory for obtaining these observations.
GCN Circular 43606
Subject
GRB 260131A/AT2026bwg: Observations by the Zwicky Transient Facility
Date
2026-02-02T19:28:33Z (16 days ago)
From
rj438@cornell.edu
Via
Web form
R. Jayaraman (Cornell), X. J. Hall (CMU), A. Y. Q. Ho (Cornell), I. Andreoni (UNC), M. Coughlin (UMN), A. Jacquesson (IJCLab), G. Schroeder (Cornell) report:
We report a serendipitous detection of the optical afterglow to GRB 260131A (Fermi/GBM Team, GCN 43579; Frederiks et al., GCN 43590; Luo et al., GCN 43591; Li et al., GCN 43605), which has been given the designation AT2026bwg (Gompertz et al., GCN 43586; Lee et al., GCN 43587; Malesani et al;, GCN 43595, García et al., GCN 43599; Shi et al., GCN 43601), as part of the Zwicky Transient Facility’s public survey.
The transient ZTF26aaemwcb was first detected on MJD 61072.16995 (21.4h after the GRB trigger) with a magnitude of r = 18.08 ± 0.05 in ZTF alert photometry. Roughly a day later (MJD 61073.15126), the transient had faded to r = 19.13 ± 0.10. The last upper limit was approximately 2.5h before the initial trigger, with a limiting magnitude of r > 19.411.
Separately, the transient was also observed at MJD 61072.13239 (20.5h after the GRB trigger) using multi-band imaging with the SED Machine on the Palomar P60 telescope. The table of photometry follows:
Time (MJD) | Magnitude
61072.13239 | 18.21 ± 0.14 (sdss-g)
61072.13405 | 18.03 ± 0.20 (sdss-r)
61072.13571 | 17.80 ± 0.48 (sdss-i)
These measurements from both ZTF and SEDM are consistent with a decaying afterglow, when compared to those from Gompertz et al., Lee et al., Malesani et al., and García et al. The fast rise and decay was initially flagged by a pipeline for discovering afterglows in the ZTF data (e.g., Ho et al. 2020, ApJ, 905, 98), as well as the ZTFrest filter (Andreoni et al. 2021, ApJ, 918, 63). Upon a retrospective search, we identified ZTF26aaemwcb as the afterglow to GRB 260131A.
This Circular is based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.
GCN Circular 43605
Subject
GRID detection of GRB 260131A/B
Date
2026-02-02T17:33:11Z (16 days ago)
From
GRID Student Team at Tsinghua University <grid@tsinghua.edu.cn>
Via
Web form
Longhao Li, Zirui Yang, Chenyu Wang report on behalf of the GRID Collaboration:
GRID reports the detection of GRB 260131A/B, which was also reported by Fermi and MAXI (Roberts, GCN 43585; a.k.a. GRB 260131B, Sugai et al., GCN 43580).
The event first triggered GRID at T0 = 2026-01-31 06:39:30 UTC, and was detected by both GRID-10B and GRID-11B, with a weaker signal observed by GRID-11B. A second trigger was recorded at 2026-01-31 06:43:02 UTC; however, the GRID signal associated with this later trigger is comparatively weak. Therefore, temporal analysis is based on the first trigger only.
The measured burst duration for the first trigger is T90 = 10.5 ± 1.4 s in the 30–2000 keV energy range.
The GRID light curves of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB260131A-B/.
GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w and https://doi.org/10.1007/s10686-021-09819-4.
GCN Circular 43604
Subject
GRB 260131A/B: GTC/OSIRIS+ redshift confimation
Date
2026-02-02T11:48:03Z (16 days ago)
From
Ruben Sanchez-Ramirez at IAA-CSIC <ruben@iaa.es>
Via
Web form
R. Sanchez-Ramirez, A. J. Castro-Tirado, M.D. Caballero-Garcia, S. Guziy, S.-Y. Wu, I. Perez-Garcia, and E. Fernandez-Garcia (IAA-CSIC), D. Garcia-Alvarez (GTC), Y.-D. Hu (GXI), M. Gritsevich (Univ. of Helsinki), and L. Piro (INAF/IAPS), on behalf of a larger collaboration, report:
Following the detection of GRB 260131A/B (Fermi/GBM Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et al., GCN 43590; Luo et al., GCN 43591), we observed the optical afterglow detected by several facilities (Gompertz et al., GCN 43586; Lee et al., GCN 43587; Saccardi et al., GCN 43595; García García et al., GCN 43599, Jiang et al. GCN 43600, Shi et al. GCN 43601), with the 10.4m GTC telescope, at the Spanish Roque de los Muchachos Observatory, on the island of La Palma, equipped with the OSIRIS+ instrument.
The spectroscopic data consisted of 3x900s exposures using grism R1000R, covering the spectral range between 5,000 and 10,000 A. The observations started on Feb 1 at 20:08 UT (i.e. ~37h after the burst trigger). From a preliminary reduction, we find the absorption system reported by Saccardi et al. (GCN 43595) at a consistent z=1.435. We note also the presence of FeII* lines.
GCN Circular 43601
Subject
GRB 260131A/B (AT2026bwg): J-band observations by SYSU 80cm infrared telescope
Date
2026-02-02T08:39:44Z (16 days ago)
From
shiy228@mail2.sysu.edu.cn
Via
Web form
Yun Shi, Jin-Ji Li, Pu Lin, Hao-Nan Yang, Zhong-Nan Dong, Yan Yu, Jia-Qi Lin, Wei-Sen Huang, Chun Chen, Duo-Le Cao, 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 260131A/B (Fermi/GBM Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et al., GCN 43590; Luo et al., GCN 43591), using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Feb 01 14:39 UT, 31.99 hours after the GRB trigger, using stacked images in the J band.
We clearly detect an infrared counterpart at the position of the optical afterglow candidate AT2026bwg / GOTO26akj (Gompertz et al., GCN 43586; Lee et al., GCN 43587; Saccardi et al., GCN 43595; García García et al., GCN 43599). The preliminary photometric results are summarized in the table below. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction.
Start Time (UTC) |Band | Exp | Mag (Vega)
-------------------------|-------|---------|------------------
2026/2/1 14:39:25 | J | 600s | 16.9+/-0.2
2026/2/1 14:49:48 | J | 600s | 17.0+/-0.2
2026/2/1 15:00:10 | J | 600s | 17.1+/-0.2
2026/2/1 15:10:33 | J | 840s | 17.0 +/-0.2
The SYSU 80cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 43600
Subject
GRB 260131A/B: EP/FXT X-ray afterglow detection
Date
2026-02-02T08:14:51Z (16 days ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang, D. Xu (NAOC), B.P. Gompertz (U. Birmingham), H.W. Pan (NAOC), K.R. Ni (CCNU), C.Y. Dai (NJU), B.T. Wang (YNAO) report:
EP/FXT performed a follow-up observation on GRB 260131A/B detected by Fermi (Fermi/GBM team, GCN 43579; Roberts et al., GCN 43585), MAXI (Sugai et al., GCN 43580), Konus-Wind (Frederiks et al., GCN 43590), and Insight-HXMT (Luo et al., GCN 43591) at 2026-02-01T16:45:27 (UTC), about 34.1 hrs after the Fermi/GBM trigger.
An uncatalogued source is detected at R.A. = 24.7137, Dec. = 34.3284 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the optical counterpart GOTO26akj/AT2026bwg (Gompertz et al., GCN 43586).
The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.81 × 10^20 cm^-2, an intrinsic hydrogen column density upper limit of 4.66 × 10^21 cm^-2 at the redshift of 1.434 (Saccardi et al., GCN 43595), and a photon index of 2.01 (-0.09/+0.12). The derived average unabsorbed 0.5-10 keV flux is 3.35 (-0.19/+0.23) × 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
GCN Circular 43599
Subject
GRB 260131A/B (AT2026bwg): COLIBRÍ further optical observations
Date
2026-02-02T04:28:39Z (17 days ago)
From
Leonardo Enrique García García <lgarcia@astro.unam.mx>
Via
Web form
Leonardo García García (UNAM), Benjamin Schneider (LAM), Noémie Globus (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), 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), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report:
We observed the optical afterglow candidate AT2026bwg (Gompertz et al., GCN 43586) of GRB 260131A/B (Fermi/GBM Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et al., GCN 43590; Luo et al., GCN 43591) with the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-01 02:10 to 03:07 UTC (from 43.52 to 44.48 hours after the trigger) in the g/r/i/y/z bands and obtained 13 minutes of exposure in the r band.
The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
The optical counterpart (Gompertz et al., GCN 43586; Lee et al., GCN 43587; Saccardi et al., GCN 43595) is clearly detected in all filters, and has a preliminary magnitude of:
r = 19.24 +/- 0.02.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
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 43595
Subject
GRB 260131A: NOT spectroscopic redshift z = 1.434
Date
2026-02-01T22:04:19Z (17 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
A. Saccardi (CEA/Irfu), J. An (NAOC), G. Corcoran (UCD), B. Gompertz (Birmingham), L. Izzo (INAF/OAC and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), B. Schneider (LAM), A de Ugarte Postigo (LAM), K. Valeckas (NOT and NBI), report on behalf of a larger collaboration:
We observed the optical counterpart GOTO26akj / AT2026bwg (Gompertz et al., GCN 43586; Lee et al., GCN 43587) of the Fermi and MAXI GRB 260131A (Roberts, GCN 43585; a.k.a. GRB 260131B: Sugai et al., GCN 43580) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC spectrograph.
We measure the afterglow brightness to be r = 18.98 +/- 0.02 (AB) at a mean epoch 2026 Feb 01 at 19:45:24 UT (37.1 hr after the burst), calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction.
A spectrum consisting of 4x900 s exposures was secured starting on 2026 Feb 01 at 19:57 UT (37.3 hr after the Fermi/GBM trigger), using grism #4 which covers the wavelength range 3200-9600 AA.
Based on a preliminary reduction of part of the spectra, a continuum is detected down to 3600 AA, setting an upper limit to the redshift z < 1.95 based on the lack of HI absorption. From the detection of several absorption features, among which Mg II 2804/2796, Fe II 2600, Fe II 2382, and Al III 1863/1855, we infer a redshift z = 1.434. Further analysis is ongoing.
GCN Circular 43591
Subject
GRB 260131A: Insight-HXMT/HE detection
Date
2026-02-01T15:06:33Z (17 days ago)
From
Xinghao Luo at IHEP <2952704891@qq.com>
Via
Web form
Xing-Hao Luo, Chen-Wei Wang, Chao Zheng, Hao-Xuan Guo, Xiao-Bo Li and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team:
At 2026-01-31T06:39:31.400 (T0), Insight-HXMT/HE detected the long burst GRB 260131A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #43579) .
The Insight-HXMT/HE light curve mainly consists of multiple pulses with a T90 of 7.0 +2.5/-1.0 s. The 1s peak rate, measured from T0+1.75 s, is 982 cnts/sec. Insight-HXMT/HE detected a total of 5366 counts form this burst.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb260131A.png
All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/
GCN Circular 43590
Subject
Konus-Wind detection of GRB 260131A/B
Date
2026-02-01T14:06:26Z (17 days ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long GRB 260131A/B (Fermi-GBM detection:
The Fermi GBM team, GCN 43579, Roberts, GCN 43585;
MAXI/GSC detection: Sugai et al., GCN 43580)
triggered Konus-Wind (KW) at T0=23972.646 s UT (06:39:32.646).
The burst consists of two separated emission pulses,
peaked around ~T0 and to T0+215 s, respectively.
The total duration of the burst is ~250 s.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260131_T23972/
As observed by Konus-Wind, the burst had
a fluence of (4.14 ± 0.91)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.384 s,
of (4.33 ± 0.52)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum (measured from T0 to T0+238 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.15 (-0.70,+1.61),
the high energy photon index beta = -1.97 (-0.20,+0.14),
the peak energy Ep = 124 (-40,+56) keV,
chi2 = 83/97 dof.
The spectrum near the peak count rate (measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -1.17 (-0.15,+0.17),
the high energy photon index beta = -1.98 (-0.18,+0.10),
the peak energy Ep = 300 (-68,+131) keV,
chi2 = 74/79 dof.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 43587
Subject
GRB 260131A/B (AT2026bwg): COLIBRÍ optical observations and confirmation of fading
Date
2026-02-01T03:45:33Z (18 days ago)
Edited On
2026-02-02T14:28:29Z (16 days ago)
From
William H. Lee at UNAM <wlee@astro.unam.mx>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of William H. Lee at UNAM <wlee@astro.unam.mx>
Via
Web form
William H. Lee (UNAM), Benjamin Schneider (LAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (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), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report:
We observed the optical afterglow candidate AT2026bwg discovered by Gompertz et al. (GCN 43586), of GRB 260131A (Fermi/GBM Team, GCN 43579), also associated with GRB 260131B (Sugai et al., GCN 43580; Roberts et al., GCN 43585), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-01 02:06 to 03:05 UTC (from 19.45 to 20.46 hours after the trigger) and obtained 23 minutes of simultaneous exposure in the r and z filters.
The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
The optical counterpart is clearly detected with preliminary magnitudes of:
r = 17.96 +/- 0.01,
z = 17.61 +/- 0.01.
Since the start of our observations, the source has faded with an apparent average temporal power-law index of -2.01 +/ 0.26 (in r) and -1.50 +/- 0.33 (in z).
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
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 43586
Subject
GRB 260131A/B: GOTO candidate optical counterpart consistent with the MAXI localisation
Date
2026-01-31T23:32:35Z (18 days ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
B. P. Gompertz, D. O’Neill, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, R. Starling, B. Godson, T. Killestein, A. Kumar, M. Pursiainen, on behalf of GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 260131A (Fermi/GBM Team, GCN 43579), now associated with GRB 260131B (Sugai et al., GCN 43580; Roberts et al., GCN 43585).
Observations covering the initial GBM localisation area of GRB 260131A began at 2026-01-31 20:00:48 UT (+13.35h post trigger) and continued through to 2026-01-31 21:32:06 UT (+14.88h post trigger). 103 images were taken, across 10 unique pointings, covering 215.8 within the 90% localisation contour. ~89.8% of the total 2D localisation probability was covered, with an average 5-sigma depth of 19.4 mag in the GOTO-L filter (400 - 700nm).
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.
A new optical source GOTO26akj/AT2026bwg is identified at coordinates:
RA,DEC (J2000) = 24.713819, 34.328119,
01:38:51.32, +34:19:41.23
This position is consistent with the MAXI localisation of GRB 260131B (Sugai et al., GCN 43580).
The source was initially detected with L = 17.41 ± 0.03 AB mag (t0+13.62h) before fading to L = 17.59 ± 0.04 mag (t0+14.75h). We find no evidence of the source prior to the GRB trigger time in the most recent pre-GRB GOTO observations taken at 19:59:53 on 2026-01-28 (t0-58.66h) down to a 5-sigma depth of L > 19.55 AB mag. We also find no evidence of the source prior to the GRB in the ZTF observations provided by the Lasair broker (Smith et al. 2019) or the ATLAS forced photometry server (Shingles et al. 2021).
Due to its temporal coincidence with GRB 260131A/B, apparent fading, and spatial coincidence with the relatively small MAXI localisation region, we consider GOTO26akj/AT2026bwg to be a promising candidate optical counterpart. Further observations to determine the nature of the source are strongly encouraged.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham
and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular 43585
Subject
GRB 260131A/B: Fermi GBM Observation
Date
2026-01-31T23:12:56Z (18 days ago)
From
oliver.roberts@universityofgalway.ie
Via
Web form
O.J. Roberts (Uni. of Galway, Ireland) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 06:39:30.44 UT on 31 January 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260131A/B (trigger 791534375/260131277).
which was also detected by MAXI (H. Sugai et al. 2026, GCN 43580).
The Fermi GBM final real-time Localization (GCN 43579) is consistent with the MAXI position.
The MAXI burst triggered on the second pulse of 260131A.
We note both bursts have similar locations and are likely the same event.
The angle from the Fermi LAT boresight using the MAXI location is 112 degrees.
The GBM light curve consists of two separated FRED emission episodes
with a duration (T90) of about 257 s (50-300 keV). The time-averaged spectrum
from T0-2.0 to T0+280.0 s is best fit by a Band function with an
Epeak of 148 +/- 39 keV, alpha is -0.90 +/- 0.16 and
beta is -1.76 +/- 0.06.
The event fluence (10-1000 keV) in this time interval is
(2.82 +/- 0.09)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+2.6 s in the 10-1000 keV band is 8.3 +/- 0.4 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/"
GCN Circular 43580
Subject
GRB 260131B: MAXI/GSC detection
Date
2026-01-31T08:53:31Z (18 days ago)
From
Motoko Serino at Aoyama Gakuin U. <serino@phys.aoyama.ac.jp>
Via
Web form
H. Sugai (Chuo U.), D. Iijima, M. Serino (AGU),
H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, H. Nishio (Nihon U.),
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU),
Y. Tsuboi, N. Nagashima, Y. Ishihara (Chuo U.),
M. Shidatsu, C. Kang, T. Nakamoto, M. Uenishi, T. Usuki, S. Yatsuzuka (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo),
S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA),
Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.),
M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.),
K. Yamaoka (Nagoya U.),
M. Sugizaki (Kanazawa U.),
W. Iwakiri (Chiba U.),
T. Kawamuro (Osaka U.),
S. Yamada (Tohoku U)
The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient
source at 06:43:02 UT on January 31, 2026.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (24.348 deg, 34.351 deg) = (01 37 23, +34 21 03) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.11 deg and 0.09 deg, respectively.
The roll angle of long axis from the north direction is 26.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 801 +- 56 mCrab
(4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (23.945, 34.685) deg = (01 35 46, +34 41 06) (J2000)
(R.A., Dec) = (23.826, 34.499) deg = (01 35 18, +34 29 56) (J2000)
(R.A., Dec) = (25.021, 33.970) deg = (01 40 05, +33 58 11) (J2000)
(R.A., Dec) = (25.142, 34.154) deg = (01 40 34, +34 09 14) (J2000)
There was no significant excess flux in the previous transit at 05:10 UT
and in the next transit at 08:16 UT with an upper limit of 20 mCrab for each.
GCN Circular 43579
Subject
GRB 260131A: Fermi GBM Final Real-time Localization
Date
2026-01-31T06:50:08Z (18 days ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 06:39:30 UT on 31 Jan 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260131A (trigger 791534375.436908 / 260131277).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 21.9, Dec = 26.9 (J2000 degrees, equivalent to J2000 01h 27m, 26d 53'), with a statistical uncertainty of 1.8 degrees.
The angle from the Fermi LAT boresight is 104.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260131277/quicklook/glg_skymap_all_bn260131277.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/bn260131277/quicklook/glg_healpix_all_bn260131277.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260131277/quicklook/glg_lc_medres34_bn260131277.gif