TITLE:   GCN CIRCULAR
NUMBER:  29901
SUBJECT: GRB 210420B: Assy optical observations
DATE:    21/04/25 22:56:09 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

S. Belkin (IKI), V. Kim (FAI, Pulkovo Observatory), M. Krugov (FAI), A. 
Pozanenko (IKI),  N. Pankov (HSE, IKI) report on behalf of GRB IKI FuN:

We observed the field of  GRB 210420B (Moss et al., GCN 29833; Lien et 
al., GCN 29854; Osborne et al. GCN 29876; Stamatikos et al., GCN29879) 
with AZT-20 telescope of Assy-Turgen observatory starting on 2021-04-23 
(UT) 18:39:00.

We marginally detect the optical afterglow  (Lipunov et al., GCN 29845; 
Oksanen et al., GCN 29846; Hu et al., GCN 29847; Tianrui Sun et al., GCN 
29848; Leonini et al., GCN 29851; de Ugarte Postigo et al., GCN 29852, 
29853; Watson et al., GCN 29858; Zheng et al., GCN 29861; Breeveld et 
al., GCN 29862; Stecklum et al., GCN 29863; Butler et al.,GCN 29864; 
Jelinek et al., GCN 29866; Watson, GCN 29867; Leonini et al., GCN 29872; 
Guelbenzu et al., GCN 29890; Pankov et al., GCN 29894; Ibrahimov et al., 
GCN 29895).

Preliminary photometry of the source (afterglow or afterglow + host 
galaxy)  is following

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

2021-04-23 18:39:00  2.98478    113*60  r'      23.2  0.3   23.3

The photometry is based on the nearby SDSS-DR12 stars
RA DEC r
16:56:42.09384 +42:38:35.9628 15.541
16:57:24.93456 +42:34:14.6388 16.642
16:56:58.03392 +42:33:40.5468 16.987