GCN Circular 29277

A. S. Moskvitin, V. V. Vlasyuk (SAO RAS)
on behalf of GRB follow-up team report.

We observed the field of the GRB 210104A (Fermi GBM Team, GCN #29232;
Troja et al., GCN #29233; Biltzinger et al., GCN #29234)
with the 0.5m Astrosib RC500-2 telescope of SAO RAS
equipped with the Rc filter and FLI Proline PL16801 camera.
The observations carried out on 2021 Jan. 4, 0.32--0.38 days
after the trigger (T_mid - T0 = 0.35d).


In the 37 x 20 sec. stacked image we clearly detect the GRB OT
(Troja et al., GCN #29233; Xin et al., GCN #29235; Hu et al.,
GCN #29236; Hosokawa et al., GCN #29237; Kim et al., GCN #29238;
Lipunov et al., GCN #29239; Horiuchi et al., GCN #29241;
Breeveld et al., GCN #29247; Zhu et al., GCN #29252; Paek et al.,
GCN #29254; Kumar et al., GCN #29257; Smartt et al., GCN #29262;
V. Kim, GCN #29265; S. Anandagoda, GCN #29273; P. Gokuldass,
GCN #29274; J. Mao, GCN #29275)
with the brightness of R = 19.4 +/- 0.1. The preliminary photometry
is based on R2 magnitudes of nearby USNO-B1 stars.