GCN 44169: EP260329b: GOTO detection of optical counterpart

D. O'Neill, R. Starling, A. Kumar, 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. Gompertz, B. Godson, T. Killestein, M. Pursiainen, 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 the  EP/WXT alert EP260329b (Song et al; GCN 44142).

We detect the counterpart candidate previously reported (Li et al., GCN 44139; Eyles-Ferris et al., GCN 44144; Magnani et al., GCN 44146; Aryan et al., GCN 44147; Lipunov et al., GCN 44150) with an L-band magnitude of 18.73 ± 0.09 AB mag (+0.74h) at 2026-03-29 11:20:38 UT. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2026-03-28 11:41:26 (-22.91h) down to a 3-sigma depth of L>20.10 AB mag.

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).