TITLE:   GCN CIRCULAR
NUMBER:  29708
SUBJECT: GRB 210323A: Mondy optical observations, counterpart detection
DATE:    21/03/24 12:20:09 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

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

We observed the field of short GRB 210323A (Gropp et al., GCN 29699) 
with AZT-33IK telescope of Sayan observatory (Mondy) starting on 
2021-03-23 (UT) 22:05:05, i.e. 167 s after GRB trigger. We obtained 10 
images in R-filter. In the first image we detect an object (S/N = 4.2) 
in coordinates of (J2000) 21:11:47.35, 25:22:09.4 with uncertainty of 
0.6" in each coordinate. The coordinates are in agreement with reported 
by NOT an afterglow candidate (Malesani  et al., GCN 29703)  and 
enhanced XRT position (Evans et al., GCN 29704). In the next two stacked 
images the object is barely detected (S/N =1.8), while in rest stacked 
images we do not detect the object.

Preliminary photometry of the object and the field in  stacked images is 
following

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

2021-03-23 22:05:05   0.00229    R      1*60  18.4  0.25      18.9
2021-03-23 22:06:07   0.00333    R      2*60  19.2  S/N =1.8  19.0
2021-03-23 22:08:06   0.00403    R      5*60  n/d   n/d       18.9

The photometry is based on the nearby USNO-B1.0 star
USNO-B1.0_id R2
1153-0524625 14.44

The non-detection of the object in Nanshan/NEXT observations up to 
deeper limiting magnitude of r~20.4 (Fu et al., GCN 29700) might be due 
to searching of the afterglow in a stacked image but non in early single 
images.