TITLE:   GCN CIRCULAR
NUMBER:  30584
SUBJECT: GRB 210731A: continued GROND observations
DATE:    21/08/02 15:06:50 GMT
FROM:    Ana Nicuesa at TLS Tautenburg  <ana@tls-tautenburg.de>

A. Nicuesa Guelbenzu, S. Klose, S. Schmidl (all TLS Tautenburg), A. Rau
(MPE Garching), and D. A. Kann (HETH/IAA-CSIC) report:

Using GROND mounted at the 2.2m MPG telescope at ESO La Silla Observatory
(Chile), we continued to observe the bright afterglow of GRB 210731A
(Swift trigger 1062336; Troja et al., GCN 30568).

A comparison of our first-epoch optical data (Nicuesa Guelbenzu et al.,
GCN 30574) with those from de Wet et al. (GCN 30570) indicated that the
optical afterglow underwent a relatively long-lasting plateau phase (see
also Zhen & Filippenko, GCN 30582). Our 2nd epoch GROND observations now
reveal that this plateau phase has finished and the afterglow has entered
a declining phase.

At a mean time of 1.22 days post burst we measure (AB magnitudes)

r' = 20.74 +/- 0.02 mag,

calibrated against the Pan-STARRS catalog. This is in good agreement with
the VLT X-shooter result at 1.19 d (Kann et al., GCN 30583).


We thank Sam Kim for excellent support and for performing the
observations.