GCN Circular 41567
Subject
EP250827a: GOTO observation of the optical afterglow
Event
Date
2025-08-27T16:51:59Z (a month ago)
From
d.s.oneill@bham.ac.uk
Via
Web form
D. O'Neill, B. P. Gompertz, G. Ramsay, A. Kumar, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, 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 optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the EP250827a (Hua et al. GCN 41553). Targeted observations were taken at 2025-08-27 15:50:54 UT (+8.17h post trigger). The observations consisted 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.
We detect the optical counterpart (Levan et al., GCNs 41554, 41557; Li et al., GCN 41555; Lipunov et al., GCN 41558; Gritsevich et al., GCN 41559, An et al., GCN 41660, Ghosh et al. GCN 41564) at L = 18.63 ± 0.04 AB mag.
We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2025-08-25 12:46:35 UT down to a 5-sigma limit L>19.42 AB mag. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and 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).