TITLE: GCN CIRCULAR NUMBER: 32460 SUBJECT: GRB 220810A: MeerLICHT detection of two afterglow candidates DATE: 22/08/12 15:23:28 GMT FROM: Simon de Wet at UCT S. de Wet (UCT), P.J. Groot (Radboud/UCT/SAAO), D. Pieterse (Radboud), P.M. Vreeswijk (Radboud), A.J. Levan (Radboud) and D.B. Malesani (Radboud and DAWN/NBI) report on behalf of the MeerLICHT consortium: Following the detection of GRB 220810A by Fermi/GBM (Fermi GBM team, GCN 32455), the 0.6 m wide-field MeerLICHT optical telescope began a series of automatically-triggered 60 s q-band exposures of a number of fields encompassing the GBM error box. Observations began approximately 14 minutes post-trigger at 23:30:01 UT on 2022 August 10, and continued for an hour. A total of 20 fields encompassing a cumulative probability of 58% were observed once each. Within our difference-imaging products for this dataset we identify two new transient candidates with the following coordinates, brightness, and observing times: +---------------------------+-------------------------+----------------+------------------+----------+ | RA (J2000) | Dec (J2000) | Mag (AB) | 5-sigma limiting | UT time | | | | | mag (AB) | | +---------------------------+-------------------------+----------------+------------------+----------+ | 20:45:15.77 (311.31569d) | +05:06:39.95 (5.11110d) | 11.51 +/- 0.06 | q > 15.79 | 23:59:19 | +---------------------------+-------------------------+----------------+------------------+----------+ | 20:49:24.58 (312.35241d) | +01:56:09.64 (1.93601d) | 17.52 +/- 0.07 | q > 18.77 | 23:44:42 | +---------------------------+-------------------------+----------------+------------------+----------+ These positions are calibrated against Gaia DR2 with a positional uncertainty of 0.1" in each coordinate. We note that the first, bright candidate was identified in an image in which the sources are trailed. However, human inspection confirms that the source is not an artefact. The position of both transients is within the 50% probability region of the GBM error box, and no objects consistent with these positions and observing times were found in the Minor Planet Centre database. We also note that no underlying source is visible in archival PanSTARRS images at these positions. We obtained further observations of the second, fainter source on the following night and obtained a 5-sigma limiting magnitude of q > 19.22 at the transient position, consistent with fading behaviour. Further follow-up observations are encouraged in order to determine if either of these sources is the afterglow of GRB 220810A. MeerLICHT is built and run by a consortium consisting of Radboud University, the University of Cape Town, the South African Astronomical Observatory, the University of Oxford, the University of Manchester and the University of Amsterdam.