TITLE:   GCN CIRCULAR
NUMBER:  29520
SUBJECT: GRB 210204A (ZTF21aagwbjr/AT2021buv): continued Assy and Mondy optical observations
DATE:    21/02/17 22:55:05 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

S. Belkin (IKI), A. Pozanenko (IKI), V. Kim (FAI, Pulkovo Observatory), 
E. Klunko (ISTP),  N. Pankov (HSE), M. Krugov (FAI) report  on behalf of 
GRB IKI FuN:

We continued observation of the ZTF21aagwbjr/AT2021buv transient (Kool 
et al., GCN 29405) at the redshift of 0.876 (Xu et al., GCN 29432) which 
is  an afterglow (Kennea et al., GCN 29413; Teja  et al., GCN 29414;
Belkin  et al., GCNs 29417, 29438, 29499;   Rossi et al., GCN 29433; 
Gupta et al., GCN 29490) of  GRB 210204A (Fermi GBM team, GCN 29390; Li 
et al., GCN 29392; Hurley et al., GCN 29408; Waratkar et al., GCN 29410; 
  Frederiks et al., GCN 29415). Images in R-filter were obtained at 
AZT-33IK telescope of Sayan observatory (Mondy) on Feb.12, 16 and  in 
r'-filter at AZT-22 telescope of Assy-Turgen observatory on Feb.15, 16.

Preliminary photometry of the optical transient in combined images is 
following

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

2021-02-12 16:11:09  8.41787  R   20*120 21.66 0.09  22.9  AZT-33IK
2021-02-15 15:40:01 11.40667  r'  70*60  21.86 0.15  23.6  AZT-20
2021-02-16 14:39:53 12.36212  R   31*120 21.8  0.4   21.8  AZT-33IK
2021-02-16 16:57:06 12.44700  r'  32*60  22.18 0.14  23.3  AZT-20

The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id R2
1014-0149033 15.43
1014-0149115 17.00

and the same stars from SDSS-DR12
SDSS-DR12
RA DEC g dg r dr
117.03151400 +11.42180300 17.036 0.005 15.624 0.005
117.07973400 +11.41971200 17.909 0.006 17.191 0.006

The updated light curve can be found in
http://grb.rssi.ru/GRB210204A/GRB210204A_lc.png