GCN Circular 30187

A. S. Moskvitin (SAO RAS) on behalf of the GRB follow-up team report.

We observed the field of the GRB 210610B with the SAO RAS 1-m
telescope Zeiss-1000 + CCD-photometer in BVRc Johnson-Cousins filters.
The observations started on June 10, 20:21:53 UT, since half an hour
after the burst detection by the Swift/BAT (Page et al., GCN #30170).

We clearly detected the OT (Page et al., GCN #30170; Kumar et al.,
GCN #30174; Rumyantsev et al., GCNs #30175, #30178; Hu et al.,
GCN #30177; de Wet et al., GCN #30180; Romanov, GCN #30181;
Fynbo et al., GCN #30182) with the following brightness:

UT_mid     T_mid-T0, h  Exp., s   Rc mag (+/- 0.01 mag)
20:22:03   0.510        20        16.15
20:32:40   0.687        60        16.43
20:48:12   0.946        60        16.75
21:19:20   1.448        60        17.13
21:28:10   1.612        60        17.22
21:53:21   2.032        60        17.26
22:13:03   2.360        60        17.19
22:36:03   2.743       180        17.17
23:04:20   3.215       120        17.20
23:52:38   4.020       120        17.19

The preliminary photometry is based on the USNO-A2.0 star
from GCN #30178 (Rumyantsev et al.).
Magnitudes were not corrected for MW extinction.