TITLE: GCN CIRCULAR NUMBER: 40028 SUBJECT: GRB 250403A: GOTO detection of the optical afterglow DATE: 25/04/03 16:26:51 GMT FROM: Rhaana Starling at U of Leicester rlcs1@leicester.ac.uk
R. Starling, Y. Julakanti, A. Kumar, D. O'Neill, B. P. Gompertz, J. Lyman, G. Ramsay, M. Kennedy, K. Ackley, M. J. Dyer, K. Ulaczyk, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, 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 GRB 250403A detected by SVOM (Julakanti et al. 2025, GCN 40026) and by Fermi (Fermi GBM team, GCN 40025). Targeted observations were performed using the GOTO-South starting at 15:25:31 UT on 2025-04-03 (10.12 minutes after the SVOM ECLAIRs trigger) to to 15:30:27 UT on 2025-04-03 (15.10 minutes after the SVOM ECLAIRs trigger). The images are 90s exposures in the GOTO L-band (400-700 nm).
A new optical source within the SVOM localisation region (Julakanti et al., GCN 40026) is identified at the position: RA, Dec: 194.61570 -24.37950 deg. We report four epochs of observations, showing a clear power-law-like decay with t^-1.1.
+------------------------------------------------------------------------------------------------+ | Observation time (UT) | t-t0 (min) | Mag (AB) | +------------------------------------------------------------------------------------------------+ | 2025-04-03 15:25:31.182 | 10.12 | 17.02 ± 0.02 | | 2025-04-03 15:27:09.645 | 11.82 | 17.16 ± 0.03 | | 2025-04-03 15:28:48.308 | 13.47 | 17.35 ± 0.03 | | 2025-04-03 15:30:26.677 | 15.10 | 17.50 ± 0.03 | +------------------------------------------------------------------------------------------------+
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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