TITLE: GCN CIRCULAR NUMBER: 42386 SUBJECT: GRB 251022A: GOTO optical counterpart detection DATE: 25/10/23 08:26:52 GMT FROM: d.s.oneill@bham.ac.uk
D. O’Neill, M. Wortley, B. P. Gompertz, 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 observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 251022A (Fermi GBM team, GCN 42380; Di Lalla et al. GCN 42384). Targeted observations were performed beginning at 2025-10-23 00:58:21 UT, (+2.4h post trigger) and continued through to 2025-10-23 05:04:03 UT (+6.5h post trigger). 161 images were taken, across 10 unique pointings, covering 219.8 square degrees within the 90% localisation contour. ~89.9% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.5 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, GOTO25iym, with coordinates: RA,DEC (J2000) = 65.497023, -18.9195 | 04:21:59.29, -18:55:10.19 This position lies on the 92% contour of the Fermi/GBM localisation and is offset by 0.41 degrees from the Fermi/LAT position (90% statistical uncertainty: 0.32 deg; Di Lalla et al., GCN 42384). The source was initially detected with AB magnitude L = 18.21 ± 0.03 mag (+2.4h), before fading to L = 18.82 ± 0.06 mag (+4.67h). We find no evidence of this source prior to the GRB trigger time in the previous GOTO observation taken at 2025-10-22 14:20:35 UT (8.23h pre-trigger) down to a 5-sigma depth of L > 19.54 AB mag. Despite the mild inconsistency with the Fermi/LAT position, which may be explained by the unaccounted-for systematic uncertainty, the power-law decay seen across three epochs combined with the recent deeper non-detection at the position pre-GRB strongly suggest that GOTO25iym is the afterglow of GRB 251022A. 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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