TITLE:   GCN CIRCULAR
NUMBER:  41907
SUBJECT: GRB 250920C: GOTO optical counterpart candidate
DATE:    25/09/20 16:16:16 GMT
FROM:    d.s.oneill@bham.ac.uk


M. E. Wortley, D. O'Neill, B. P. Gompertz, G. Ramsay, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, A. Kumar, 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 GRB 250920C (GCN  #41903, Fermi GBM team, #41904, Gupta et al.). Targeted observations were performed at 2025-09-20 15:33:10 (starting 0.13 hours after 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. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. Human vetting was carried out in real time on any candidates that passed the above checks.

We identify a candidate optical counterpart GOTO25hju with L = 15.91±0.01 AB mag, within the Swift/BAT error region with coordinates:

RA,DEC (J2000) = 51.465773, -29.65547 | 03:25:51.79, -29:39:19.67

We find no evidence of these sources prior to the GRB trigger time in previous GOTO observations taken at 2025-09-17 16:39:05 UT (2.95d pre-trigger) down to a depth of L<19.7 AB mag, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). There is a faint, point-like object coincident at the source location in the Legacy catalogue.

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).