[vsnet-grb-info 28467] GRB 210731A: MeerLICHT optical afterglow detection

[GCN Circulars]

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  <dwtsim002 at myuct.ac.za>

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.