TITLE: GCN CIRCULAR NUMBER: 36380 SUBJECT: GRB 240502C: GOTO possible optical counterpart candidates DATE: 24/05/03 10:21:17 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, K. Ackley; A. Kumar; G. Ramsay, B. Godson, T. Killestein, D. O'Neill, R. Starling, M. J. Dyer; J. Lyman, 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. Pall'e 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) in response to the Fermi GBM detected GRB 240502C (Fermi GBM Team, GCN 36376). Targeted observations were performed by GOTO-N, from 2024-05-02 UT 21:56:49.49 to 2024-05-03 UT 00:34:13.78 (respectively from 11.75 mins to 2.82 hours after trigger) distributed over three epochs. 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 recent survey 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 3 candidate optical counterparts within the GBM 90% localisation region. We find no evidence of these sources 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). We caution that none of these sources exhibit clear afterglow-like behaviour in the available epochs and the fields had not been covered by GOTO since 2024-04-21 prior to the GBM trigger. Hence, while a connection to GRB 240502C is not excluded by available imaging, further observations are required to ascertain their nature. Name | RA(J2000) | Dec(J2000) | Epoch(MJD) | dt_trig(hrs) | Filter | Mag(AB) | Notes GOTO24azq | 10:45:20.2 | +63:23:03.3 | 60432.915748 | 0.23 | L | 20.1 +/- 0.1 | Detected in three epochs with some evidence for variability, but low S/N. GOTO24bao | 09:57:09.5 | +62:01:04.6 | 60432.979525 | 1.76 | L | 19.4 +/- 0.1 | Detected in the only observation covering the position. Consistent with a g=22.00 mag galaxy with a photo-z of 0.3554+/-0.2158 in the PS1-STRM catalogue (Beck et al. 2021). GOTO24bar | 11:20:26.8 | +62:07:17.2 | 60432.963164 | 1.36 | L | 20.6 +/- 0.2 | Found in both available epochs. No clear evidence of fading. Consistent with a g = 20.87 mag galaxy with a photo-z of 0.1591+/-0.0317 in the PS1-STRM catalogue (Beck et al. 2021). 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/36380. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...