TITLE:   GCN CIRCULAR
NUMBER:  42094
SUBJECT: GRB 251003A: GOTO detection of the optical counterpart
DATE:    25/10/03 14:27:54 GMT
FROM:    I. Worssam at University of Birmingham <ixw352@student.bham.ac.uk>

I. Worssam, B. P. Gompertz, D. O’Neill, M. E. Wortley, G. Ramsay, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar,, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, 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 GRB 251003A (Fermi GBM team, GCN  42069; Beardmore et al. GCN 42070). Targeted observations were performed at 2025-10-03 02:03:05 (4.57mins post trigger) and 2025-10-03 03:11:32 (1.22h post trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). We detect the optical counterpart (Aceituno et al., GCN 42072; Méndez et al., GCN 42073; Strausbaugh et al., GCN 42074; Freeberg et al., GCN 42085; Sanchez-Ramirez et al., GCN 42086) with measured AB magnitudes of L = 19.94 ± 0.24 and L = 20.83 ± 0.28 for the two epochs respectively.

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).