GCN Circular 45552
Subject
GRB 260910B: GOTO tentative optical counterpart candidates
Event
Date
2026-09-11T12:41:32Z (4 days ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
B. P. Gompertz, S. Belkin, R. Starling, B. Godson, G. Ramsay, D. O’Neill, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, T. Killestein, A. Kumar and 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 GRB 260910B (Fermi GBM Team, GCN 45550).
Observations covering the localisation area began at 2026-09-11 09:02:43 UT (+9.77 h post trigger) and continued through to 2026-09-11 10:30:32 UT (+11.23 h post trigger). Observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm). 131 images were taken, across 10 unique pointings, covering 396.4 sq. deg within the 90% localisation contour. ~87.3% of the total 2D localisation probability was covered, with an average 5-sigma depth of 21.0 mag.
Images were processed immediately after acquisition using the GOTO pipeline (Lyman et al. 2026). 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.
We identify 2 tentative candidate optical counterparts within the 90% probability contour. We find no evidence of these sources prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).
+--------------------------------------------------------------------------+
| Internal name | IAU name | RA (J2000) | Dec (J2000) | Discovery mag (AB) |
+--------------------------------------------------------------------------+
| GOTO26jjj | | 264.309139 | 10.532956 | 20.28 +/- 0.18 |
| GOTO26jjg | AT 2026abfp | 261.677596 | 2.050792 | 20.58 +/- 0.19 |
+--------------------------------------------------------------------------+
GOTO26jjg / AT 2026abfp is spatially coincident with a galaxy on the 13% contour of the Fermi/GBM localisation region. The source is detected in two epochs with magnitudes of L = 20.58 ± 0.19 at 09:02:46 UTC on 2026-09-11 (t0 + 9.77 h) and L = 20.68 ± 0.23 at 10:10:06 UTC on 2026-09-11 (t0 + 10.90 h). We therefore do not have evidence for fading in this candidate, though observations are consistent with this possibility. The most recent GOTO non-detection was L > 20.75 (3-sigma) at 09:54:30 on 2026-09-01 (t0 - 9.56 days). The discovery image and finding chart are available at https://goto-observatory.warwick.ac.uk/gotoweb/marshall/GOTO26jjg/public
GOTO26jjj is spatially coincident with a galaxy on the 89% probability contour of the Fermi/GBM localisation region. The source was only covered in the single epoch reported above, beginning at 09:30:34 UTC on 2026-09-11 (t0 + 10.22h). The most recent GOTO non-detection was L > 20.61 (3-sigma) at 21:17:42 UTC on 2026-08-31 (t0 - 10.08 days).
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction. Follow-up observations to determine the nature of these sources and any relationship with GRB 260910B are 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).