TITLE:   GCN CIRCULAR
NUMBER:  40522
SUBJECT: GRB 250521D: GOTO detects LAST counterpart candidate prior to the GRB
DATE:    25/05/22 06:31:47 GMT
FROM:    Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>

D. O'Neill, A. Kumar, 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, 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) in response to the Fermi/GBM alert for GRB 250521D (GCN 40516, Fermi GBM team).

During targeted follow-up of GRB 250521D (Fermi GBM team, GCN 40516), we detected the counterpart candidate reported by LAST in GCN 40520 (Konno et al., GCN 40520) with L = 19.72 ± 0.10 AB mag (+3.19h post trigger), and L = 19.68 ± 0.19 AB mag (+5.08h post trigger). We note however that we have a pre-trigger 5-sigma detection of the source at 2025-05-21 01:24:41 (-20.95h pre-trigger ) with L = 19.38 ±0.19 AB mags, indicating this is likely an unrelated event. The GOTO-L (400-700nm) and LAST unfiltered (comparable to Gaia Bp) bandpasses are roughly equivalent, and the combined GOTO and LAST observations suggest a variable source.

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).