TITLE: GCN CIRCULAR NUMBER: 39891 SUBJECT: GRB 250327B: GOTO optical counterpart detection DATE: 25/03/27 23:19:12 GMT FROM: d.s.oneill@bham.ac.uk
D. O’Neill, A. Kumar, B. Godson, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco 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 the SVOM/ECLAIRS sb2503270 (Bouchet et al. 39888). The observations were conducted between 2025-03-27 21:14:55 (208 sec post-trigger) and 2025-03-27 22:24:37.72 (76.42 min post-trigger). Each observation 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 of the same pointings.
We detect the optical counterpart reported by SAO RAS and SVOM/VT (Moskvitin et al., GCN 39889; Xin et al., GCN 39890) as an initially rising source that reached a peak magnitude of L=15.098 ± 0.012 at 8.43 min after trigger, before decaying at a rate of approximately t^-0.8 to a magnitude of 17.011 ± 0.004 at t0+76.42 min post-trigger.
We find no evidence of the source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
Observations are ongoing.
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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