{
  "bibcode": "2021GCN.30570....1D",
  "body": "S. de Wet (UCT), A.J. Levan (Radboud), P.J. Groot (Radboud/UCT/SAAO)\nP.M. Vreeswijk (Radboud) report on behalf of the MeerLICHT consortium:\n\nFollowing the detection of GRB210731A by Swift and its X-ray\ncounterpart (Troja et al., GCN30568), the 0.6m MeerLICHT telescope,\nlocated at Sutherland, South Africa obtained a repeating series of\n60s images in the q,u,q,g,q,r,q,i,q,z bands. Observations began at\n2021-07-31, 22:25:54 UT, 5 minutes after the Swift detection, and\n3 minutes after the distribution of the BAT alert, and continued\nuntil 2021-08-01, 02:40:21 UT.\n\nWe find a new transient candidate in our first q-band exposure (318\nseconds post-trigger) with magnitude q = 19.50 +/- 0.09 at coordinates:\n\nRA (J2000) = 20:01:13.19 (300.30494d)\nDec (J2000) = -28:03:40.10 (-28.06114d)\n\ncalibrated against Gaia DR2. This position is 0.3\" from the updated\nXRT position, and which we regard as the optical afterglow to\nGRB210731A. We note that no credible afterglow candidate was reported\nby UVOT at ~210s post burst (Troja et al., GCN30568).\n\nWe detect the afterglow in further observations in the q,g,r and i\nbands but not in u and z. Our q-band light curve shows a complex\nshape with a steep rise and drop of ~0.7 mag within the first 2000s,\nthen a gradual rise of 0.7 mag for a further 3000s and a plateau\nuntil ~10000s post-trigger.  The final q-band magnitude of the\nsource was q=18.95 +/- 0.07 at 2021-08-01, 01:06:47 UT. Our first\ng,r and i source detections and u and z non-detections are, at:\n\ng = 18.93 +/- 0.06 at 22:31:29 UT\nr = 18.63 +/- 0.06 at 22:34:59 UT\ni = 18.34 +/- 0.09 at 22:38:33 UT\nu > 19.40 at 22:28:06 UT\nz > 18.57 at 22:41:58 UT\n\nReference images of the field show no underlying host galaxy down\nto the 5-sigma limiting magnitude, at the position of the afterglow,\nat: q > 20.95. There is also no source identified at this location\nin archival PanSTARRS images of the field.\n\nMeerLICHT is built and run by a consortium consisting of Radboud\nUniversity, University of Cape Town, the South African Astronomical\nObservatory, the University of Oxford, the University of Manchester\nand the University of Amsterdam.",
  "circularId": 30570,
  "createdOn": 1627810116000,
  "email": "dwtsim002@myuct.ac.za",
  "subject": "GRB 210731A: MeerLICHT optical afterglow detection",
  "submitter": "Simon de Wet at UCT  <dwtsim002@myuct.ac.za>",
  "eventId": "GRB 210731A"
}