TITLE: GCN CIRCULAR NUMBER: 30570 SUBJECT: GRB 210731A: MeerLICHT optical afterglow detection DATE: 21/08/01 09:28:36 GMT FROM: Simon de Wet at UCT S. de Wet (UCT), A.J. Levan (Radboud), P.J. Groot (Radboud/UCT/SAAO) P.M. Vreeswijk (Radboud) report on behalf of the MeerLICHT consortium: Following the detection of GRB210731A by Swift and its X-ray counterpart (Troja et al., GCN30568), the 0.6m MeerLICHT telescope, located at Sutherland, South Africa obtained a repeating series of 60s images in the q,u,q,g,q,r,q,i,q,z bands. Observations began at 2021-07-31, 22:25:54 UT, 5 minutes after the Swift detection, and 3 minutes after the distribution of the BAT alert, and continued until 2021-08-01, 02:40:21 UT. We find a new transient candidate in our first q-band exposure (318 seconds post-trigger) with magnitude q = 19.50 +/- 0.09 at coordinates: RA (J2000) = 20:01:13.19 (300.30494d) Dec (J2000) = -28:03:40.10 (-28.06114d) calibrated against Gaia DR2. This position is 0.3" from the updated XRT position, and which we regard as the optical afterglow to GRB210731A. We note that no credible afterglow candidate was reported by UVOT at ~210s post burst (Troja et al., GCN30568). We detect the afterglow in further observations in the q,g,r and i bands but not in u and z. Our q-band light curve shows a complex shape with a steep rise and drop of ~0.7 mag within the first 2000s, then a gradual rise of 0.7 mag for a further 3000s and a plateau until ~10000s post-trigger. The final q-band magnitude of the source was q=18.95 +/- 0.07 at 2021-08-01, 01:06:47 UT. Our first g,r and i source detections and u and z non-detections are, at: g = 18.93 +/- 0.06 at 22:31:29 UT r = 18.63 +/- 0.06 at 22:34:59 UT i = 18.34 +/- 0.09 at 22:38:33 UT u > 19.40 at 22:28:06 UT z > 18.57 at 22:41:58 UT Reference images of the field show no underlying host galaxy down to the 5-sigma limiting magnitude, at the position of the afterglow, at: q > 20.95. There is also no source identified at this location in archival PanSTARRS images of the field. 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.