TITLE:   GCN CIRCULAR
NUMBER:  44492
SUBJECT: EP260507a: GOTO optical counterpart detection
DATE:    26/05/07 19:02:11 GMT
FROM:    Amit Kumar at The Open University, UK <amitkundu515@gmail.com>

A. Kumar, S. Belkin, D. O'Neill, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, J. Casares, B. Godson, T. Killestein and M. Pursiainen on behalf of 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 EP-WXT triggered EP260507a (Fu et al., GCN 44488). The observations were conducted with GOTO-South at 2026-05-07 13:57:30 UT and 2026-05-07 15:04:48 UT (9.5 and 76.8 mins post-trigger, respectively), consisting of 4x90s exposures in the GOTO L-band (400-700 nm). Difference imaging was performed using deeper template observations of the same pointings.


We detect the optical counterpart of EP260507a (Corcoran et al., GCN 44489; Sankar et al., GCN 44490; Jiang et al., GCN 44491) in the GOTO L-band at 2026-05-07 13:57:30 UT (9.5 mins post-trigger), with a measured magnitude of 19.56 ± 0.11 AB. The source is no longer detected at the next epoch obtained at 2026-05-07 15:04:48 UT (76.8 mins post-trigger), with a 3-sigma upper limit of 20.4 AB 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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).