TITLE: GCN CIRCULAR NUMBER: 40347 SUBJECT: GRB 2050504A: MeerLICHT optical afterglow detection DATE: 25/05/05 08:04:54 GMT FROM: Simon de Wet at DTU Space simdewet@gmail.com
S. de Wet (DTU Space), P.M. Vreeswijk (Radboud) and P.J. Groot (Radboud/UCT/SAAO) report on behalf of the MeerLICHT consortium:
The 0.6 m wide-field MeerLICHT optical telescope located in Sutherland, South Africa, obtained a repeating series of 60 s exposures in the q,u,g,r,i,z bands of GRB 250504A following the Swift detection (Moss et al., GCN 40343). Observations started at 23:29:27 UT on 2025 May 5 (210 seconds post-trigger) and continued for a further 3 hours, following the filter sequence quqgqrqiqz.
We find a new transient candidate in our first q-band exposure at the following coordinates:
RA (J2000) = 17:58:28.38 (269.61827d) Dec (J2000) = -40:21:59.58 (-40.36655d)
calibrated against Gaia DR2. The astrometric uncertainty is ~0.3" in each coordinate. This position is outside the initial XRT error circle reported by Moss et al. (GCN 40343) but within the updated XRT error circle reported by Osborne et al. (GCN 40345). We regard this object as the optical afterglow to GRB250504A. We report the following AB magnitudes and non detections from our first exposures:
q = 19.32 +/- 0.05 at 23:29:59 UT u > 19.34 (3sigma) at 23:31:20 UT g = 19.69 +/- 0.10 at 23:34:04 UT r = 19.52 +/- 0.11 at 23:36:48 UT i = 19.32 +/- 0.13 at 23:39:31 UT z = 18.85 +/- 0.20 at 23:42:14 UT
MeerLICHT is built and run by a consortium consisting of Radboud University, University of Cape Town, the South African Astronomical Observatory, the University of Oxford, the University of Manchester and the University of Amsterdam.
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