TITLE:   GCN CIRCULAR
NUMBER:  29341
SUBJECT: GRB 210120A(MASTER OT J105242.66+152355.0 / AT2021axc): Mondy optical observations
DATE:    21/01/21 09:08:59 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

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

We observed the MASTER OT J105242.66+152355.0 / AT2021axc (Lipunov et 
al., GCN 29339) in the field of Fermi GRB 210120A  (Fermi GBM Team, GCN 
29334; Biltzinger et al. GCN 29335) with AZT-33IK telescope of Sayan 
observatory (Mondy) starting on 2021-01-20 (UT) 20:57:41. The candidate 
in optical afterglow (Lipunov et al., GCN 29339) is well detected in our 
R-filter stacked image at R = 22.14 +/- 0.27 with 3 sigma upper limit of 
the field for point-like source R=22.3.  We confirm fading nature of the 
transient and support it for afterglow candidate of GRB 210112A.

Preliminary photometry of the optical transient in the stacked image is 
following

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

2021-01-20 20:57:41   0.59510    R      30*120  22.14 0.27  22.3


The photometry is based on the nearby USNO-B1.0 stars
USNO-B1.0_id R2
1053-0195054 16.13
1053-0195003 18.57