TITLE: GCN CIRCULAR NUMBER: 42470 SUBJECT: GRB 251026B: Tentative GOTO optical counterpart candidate DATE: 25/10/27 10:39:39 GMT FROM: Sergey Belkin at Monash University sergey.belkin@monash.edu
S. Belkin, G. Ramsay, B. P. Gompertz, D. O’Neill, M. Wortley, R. Starling 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, B. Godson, T. Killestein, A. Kumar, M. Pursiainen report on behalf of GOTO collaboration:
We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the short GRB 251026B (Fermi GBM team, GCN 42455). Targeted observations were performed beginning at 2025-10-26 17:35:03 UT, (+10.3h post trigger) and continued through to 2025-10-27 05:29:04 UT (+22.2h post trigger). 88 images were taken, across 8 unique pointings, covering 417.8 square degrees within the 90% localisation contour. ~75.1% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.1 mag. Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings.
We detect a new optical source, GOTO25jgl/AT2025absf, with coordinates: RA,DEC (J2000) = 133.2284, -6.25815 | 08:52:54.8, -06:15:29.4
This position lies on the 38% probability contour of the Fermi/GBM localisation. The source was initially detected with AB magnitude L = 18.45 ± 0.10 mag (+10.3h), before fading to L = 18.80 ± 0.06 mag (+22.2 h). Forced photometry on the difference images indicates excess flux corresponding to L = 19.06 ± 0.28 AB mag at 2025-10-23 18:09:10 (−2.55 d), but visual inspection does not reveal an obvious point source, meaning the marginally significant residual may be due to an imperfect subtraction. Earlier coverage at 2025-10-23 04:48:34 (−3.10 d) yields a 3-sigma upper limit of L > 20.50 AB. The source is coincident with the galaxy LEDA 1034956 with a redshift of z=0.055 (247.26 ± 17.34 Mpc).
The two post-trigger detections are consistent with a power-law decay with index: alpha ~0.41. The temporal coincidence and fast decay suggest that GOTO25jgl could be the optical counterpart to GRB 251026B. However, given the possible excess flux observed pre-trigger, we can not definitively rule out that it is an unrelated, fast-evolving transient. Further follow-up to determine the nature of the source is encouraged.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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 and the Instituto de Astrofisica de Canarias (IAC).
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