TITLE: GCN CIRCULAR NUMBER: 40172 SUBJECT: GRB 250419A: GOTO optical counterpart detection DATE: 25/04/19 07:52:06 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK amitkundu515@gmail.com
A. Kumar, G. Ramsay, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, D. O'Neill, 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 observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the SVOM GRB 250419A (sb25041901; Wang et al., GCN 40168). The observations were conducted with GOTO-North between 2025-04-19 02:55:03 (25.5 min post-trigger) and 2025-04-19 04:03:07 (93.6 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 reported optical afterglow, discovered by SVOM/COLIBRÍ FM-GFT (López et al., GCN 40169) and confirmed by SVOM/VT (Xin et al., GCN 40170), and KAIT (Zheng et al., GCN 40171). We obtained the following 3-sigma detections of the optical counterpart:
Date | T-T0 (min) | Filter | Mag (AB) --------------------------------------------------------------------------- 2025-04-19T02:55:03 | 25.5 | L | 18.22 ± 0.05 2025-04-19T04:03:07 | 93.6 | L | 18.39 ± 0.07
Based on our observations, the afterglow exhibits a power-law decay with an index of α = -0.12 between these two epochs.
We find no evidence of the source prior to the GRB trigger time in archival GOTO observations taken 14.07 min before the trigger, down to a 3-sigma L-band upper limit of 20.26 mag.
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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