GRB 260125A
GCN Circular 43527
Subject
GRB 260125A: Swift-XRT observations
Date
2026-01-27T15:36:17Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Dichiara (PSU), J.A.
Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), S. Campana (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
Fermi/GBM-detected burst GRB 260125A (Fermi GBM Team, GCN 43510),
collecting 3.1 ks of Photon Counting (PC) mode data between T0+64.9 ks
and T0+86.3 ks.
No X-ray sources have been detected consistent with the GOTO
counterpart position (Gompertz et al., GCN 43514). The 3-sigma upper
limit in the field is 0.003 ct s^-1, corresponding to a 0.3-10 keV
observed flux of 1.0e-13 erg cm^-2 s^-1 (assuming a typical GRB
spectrum).
Five uncatalogued sources were detected too far from the GOTO position,
and not showing significant signs of fading to be likely afterglow
candidates.
The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021899.
This circular is an official product of the Swift-XRT team.
GCN Circular 43518
Subject
GRB 260125A: NOT optical observations on GOTO26acv / AT 2026bjf
Date
2026-01-26T14:19:11Z (2 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), B. P. Gompertz (Birmingham), G. Corcoran (UCD), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), S. Bijavara Seshashayana (NOT), report on behalf of a larger collaboration:
We observed the optical candidate GOTO26acv / AT 2026bjf (Gompertz et al., GCN 43514) of the Fermi GRB 260125A (Fermi GBM team, GCN 43510) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out using the SDSS griz filters, and started on 2026 Jan 25.995 UT (16.27 hr after the Fermi/GBM trigger).
Visual inspection reveals no obvious point source at the coordinates of GOTO26acv / AT 2026bjf, on top of the galaxy PGC 139241 (LEDA 139241), in any of the observed bands, though we note that the bright and complex morphology of the galaxy makes a clear identification tough.
Image subtraction against the Legacy survey archival images yields an upper limit r > 22.75 (AB) at a mean time of 16.47 hr after trigger (5x150 s exposure time). This value is calibrated against the Pan-STARRS catalog and is not corrected for Galactic extinction.
Our limit is notionally deeper than the extrapolation of the power-law decay reported by Gompertz et al. (GCN 43514), even after accounting for a GOTO/ALFOSC color term and the quoted decay index uncertainty.
GCN Circular 43515
Subject
GRB 260125A: Swift ToO observations
Date
2026-01-26T02:44:30Z (2 months 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 Fermi/GBM-detected event
GRB 260125A. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021899
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 Fermi/GBM 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 43514
Subject
GRB 260125A: GOTO candidate optical counterpart
Date
2026-01-25T20:20:30Z (2 months ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
B. P. Gompertz, S. Moran, D. O’Neill, R. Starling, 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, B. Godson, T. Killestein, A. Kumar, M. Pursiainen report 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 the GRB 260125A (Fermi GBM Team, GCN 43510).
Observations from GOTO-South (Siding Spring) covering the localisation area began at 2026-01-25 12:06:29 UT (+4.49h post trigger) and continued through to 2026-01-25 15:46:04 UT (+8.54h post trigger). 292 images were taken, across 16 unique pointings, covering 510.2 within the 90% localisation contour. ~78.0% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.8 mag.
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.
Within the 90% probability region, we identify a new transient, GOTO26acv (AT 2026bjf), lying on the 20th percentile contour of the GBM localisation map at coordinates:
RA = 10:06:16.71, Dec = -06:34:33.55 (J2000), equivalent to RA = 151.56962, Dec = -6.57599 degrees.
GOTO26acv / AT 2026bjf was discovered with an AB magnitude of L = 20.17 ± 0.16 in an epoch taken at 12:29:25 UT, i.e. 4.87h post-trigger. Across three epochs of detection, it was observed to fade by 0.42 ± 0.25 magnitudes in 2.35 hours, equivalent to a power-law decay rate of t^-0.92 ± 0.38. The final detection magnitude was L = 20.59 ± 0.19 at 14:50:26 UT. The source is spatially coincident with the galaxy PGC 139241 at a luminosity distance of 53 Mpc. It is offset from the nucleus by ~9”.
The source was not present in the most recent survey image of the field, taken at 13:37:58 UT on 2026-01-24 (0.75 days prior to the GRB trigger), to a 3-sigma limiting magnitude of L > 20.95. We also find no evidence of GOTO26acv / AT 2026bjf prior to the GRB trigger time in the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).
Further follow-up observations to establish the nature of GOTO26acv / AT2026bjf and its potential association with GRB 260125A are encouraged.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
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 43510
Subject
GRB 260125A: Fermi GBM Final Real-time Localization
Date
2026-01-25T07:47:31Z (2 months 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 07:36:58 UT on 25 Jan 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260125A (trigger 791019423.079276 / 260125317).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 148.7, Dec = -2.9 (J2000 degrees, equivalent to J2000 09h 54m, -2d 53'), with a statistical uncertainty of 8.9 degrees.
The angle from the Fermi LAT boresight is 67.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260125317/quicklook/glg_skymap_all_bn260125317.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/bn260125317/quicklook/glg_healpix_all_bn260125317.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260125317/quicklook/glg_lc_medres34_bn260125317.gif