TITLE:   GCN CIRCULAR
NUMBER:  30214
SUBJECT: GRB 210610A: CrAO/ZTSH optical observations
DATE:    21/06/11 22:36:11 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

S. Belkin (IKI, HSE), A. Pozanenko (IKI), V. Rumyantsev (CrAO),  N. 
Pankov (IKI, HSE) report on behalf of IKI GRB FuN:

We observed the GRB 210610A (Page  et al., GCN  30160) with ZTSH 2.6m 
telescope of CrAO observatory on June 10 starting on (UT) 18:29:17, and 
June 11 starting on (UT) 18:53:09.  The optical afterglow  first 
reported by UVOT (Page  et al., GCN 30160) is clearly detected in each 
of a single images in R filter in both epochs.
Preliminary photometry of the afterglow in the first image on June 10, 
and combined image on the June 11 is following

Date       UT start   t-T0    Filter Exp.    OT      err   UL(3 sigma)
                      (mid, days)       (s)

2021-06-10 18:29:17   0.14310 R       1*60   19.47   0.12  20.7
2021-06-11 18:53:09   1.16766 R      12*120  22.19   0.17  23.0

The photometry is based on nearby SDSS DR12 stars
SDSS-DR12_id RA DEC B(Lupton transformation) R(Lupton transformation)
J133717.13+142854.9 204.32139900 +14.48192200 17.8777 17.0295
J133703.62+142832.7 204.26512000 +14.47576300 19.2842 17.7239
J133658.05+142404.4 204.24188500 +14.40123200 20.3290 18.3616

The light curve of can be found in 
http://grb.rssi.ru/GRB210610A/GRB210610A_LC.png

Initial power law index  of the light curve in the first epoch is about 
-0.82, and it is steepening between the two epochs down to -1.03 which 
is compatible with broken power-law fit reported by D.A. Kann  et al. 
(GCN 30211).