TITLE: GCN CIRCULAR NUMBER: 42115 SUBJECT: Swift/BAT trigger 1401726: GOTO optical upper limits DATE: 25/10/05 18:48:41 GMT FROM: Ben Gompertz at U of Birmingham b.gompertz@bham.ac.uk

B. P. Gompertz, D. O’Neill, G. Ramsay, P. O’Brien, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, A. Kumar, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, 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 Swift/BAT trigger 1401726. The current difficulties with the distribution of GCN circulars means we are unsure of the official designation of this burst at the time of submission.

A targeted observation was performed at 17:48:32 (1.7 hours after trigger), consisting 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 find no new optical sources consistent with the Swift/XRT localisation to a 3-sigma limiting AB magnitude of L > 19.65 mag.

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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