TITLE: GCN CIRCULAR NUMBER: 41332 SUBJECT: EP 250812a: GOTO optical upper limit DATE: 25/08/12 17:59:49 GMT FROM: kendall.ackley@warwick.ac.uk

K. Ackley, A. Kumar, D. O'Neill, G. Ramsay, B.Godson, B. P. Gompertz, R. Starling, M. J. Dyer, J. Lyman, K. Ulaczyk, 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 EP250812a (Zhang et al. GCN 41327).

Three targeted observations were performed by GOTO-South beginning at 14:58:30UT and continuing up to 17:23:59UT on 2025-08-12, corresponding to 1.84 hours, 3.03 hours, and 4.18 hours after trigger. Each observation consisted of 4x90 s exposures in the GOTO L-band (400-700 nm).

We serendipitously observed the field by GOTO-South at 9.5 hours prior to the trigger time at 03:39:10UT on 2025-08-12, where each observation consisted of 4x45s 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. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues.

We do not identify any candidate optical counterparts at the location of the Swift-XRT afterglow candidate (Evans et al. GCN 41330) or within the EP/WXT localisation uncertainty region after the trigger time, down to a 3-sigma limiting magnitude for the three sets of observations of L >18.9 (AB), L >19.4 (AB), L >19.8 (AB). The observation at 9.5 hours prior to the trigger shows no source to a 3-sigma limit of L >18.1 mag (AB).

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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