TITLE:   GCN CIRCULAR
NUMBER:  29689
SUBJECT: GRB 210321A: Mondy optical observations
DATE:    21/03/22 11:15:29 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

N. Pankov (HSE), A. Pozanenko (IKI), E. Klunko (ISTP), S. Belkin (IKI) 
report  on behalf of GRB IKI FuN:

We observed the field of GRB 210321A (Dichiara  et al., GCN 29677)  with 
AZT-33IK telescope of Sayan observatory (Mondy) starting on 2021-03-21 
(UT) 14:20:37. The optical afterglow (Siegel  et al., GCN 29679; Butler 
et al., GCN 29680;   Lipunov et al., GCN 29683; Heintz et al., GCN 
29684; de Ugarte Postigo et al., GCN 29685; Xin et al. GCN 29687) is 
clearly detected in a combined image. Preliminary photometry of the 
afterglow in the combined image is following

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

2021-03-21 14:20:37   0.31199    R      45*60  20.1 0.14  20.5

The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id R2
1601-0068003 15.84
1600-0068970 16.18
1601-0068018 16.54
1601-0067995 16.23