TITLE: GCN CIRCULAR NUMBER: 41406 SUBJECT: GRB 250818B: GOTO optical counterpart candidate DATE: 25/08/18 05:44:49 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK amitkundu515@gmail.com

A. Kumar, S. Belkin, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, D. O'Neill, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the 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 SVOM/ECLAIRs GRB 250818B (sb25081801, Wang et al. GCN 41405). Targeted observations were performed at 2025-08-18 from 04:01:34 UT to 05:09:40 UT (from +0.54 to 1.67 h post-trigger, respectively). 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. 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 GOTO25fzq/AT 2025ukm as a new candidate optical counterpart within the SVOM/ECLAIRs 90% localisation region. We find no evidence of this source 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). The GOTO observations, obtained 9.23 hours before the trigger, show no source at this position, with a 3-sigma limiting magnitude of L > 20.3 (AB). The source shows a clear decay and is a strong candidate optical counterpart to GRB 250818B.

Name | RA(J2000) | Dec(J2000) | T-T0 (h) | Filter | Mag (AB) GOTO25fzq | 03:04:13.52 | -03:07:30.82 | 0.54 | L | 18.71 ± 0.14 | 1.67 | L | 19.49 ± 0.18

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 and the Instituto de Astrofisica de Canarias (IAC).

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