TITLE:   GCN CIRCULAR
NUMBER:  29770
SUBJECT: GRB 210406A: Mondy optical upper limit
DATE:    21/04/07 10:25:25 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

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

We observed the field of GRB 210406A (Mereghetti  et al., GCN 29767) 
with AZT-33IK telescope of Sayan observatory (Mondy) starting on 
2021-04-06 (UT) 17:54:58. We obtained 15 images in R-filter. We do not 
find new sources in a stacked image in comparison with Pan-STARRS PS1 or 
Legacy Survey which is consistent with observations (Lipunov et al., GCN 
29766; Hu et al., GCN 29768; Vinko  et al., GCN 29769). So we have no 
immediate OT candidate. More observation is necessary to investigate 
variability of field sources.
Preliminary photometry of the field in the stacked image is following

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

2021-04-06 17:54:58   0.04069    R     15*120  n/d   n/d  22.9

The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id RA DEC R2
1665-0048950 132.414156 +76.555625 16.49
1664-0050141 132.551831 +76.468300 18.01