TITLE:   GCN CIRCULAR
NUMBER:  29895
SUBJECT: GRB 210420B: follow-up photometry of optical afterglow at INASAN/Simeiz
DATE:    21/04/24 12:33:31 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

M. Ibrahimov, S. Naroenkov, I. Nikolenko (all from INASAN, Moscow, Russia) and
O. Pons (IGA, Habana, Cuba) report on behalf of a larger team:

Using Zeiss 1m telescope and 4K FLI PL16803 CCD of Simeiz Obserbatory
(Collective Usage Center of INASAN), we observed the field of GRB 210420B
(Moss et al., GCN 29844; Lien et al., GCN 29854; Osborne et al. GCN 29876;
Stamatikos et al., GCN29879). 15x180sec images were acquired through Bessel R
filter on 2021-04-21/20:47:03 UT (midpoint time, 1.09 day after trigger) with
a total exposure time of 2700 sec.


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; Vinko et al., GCN 29883; Tianrui Sun, GCN 29884; Guelbenzu et
al., GCN 29890) was marginally detected on the stacked R image.


Photometry of the detected optical afterglow, calibrated against USNO-B1
photometry of nearby stars, resulted in: R = 21.8 +/- 0.5 mag.


Research was supported by Project No. RFMEFI61319X0093 (Russian Ministry of
Science and High Education, Agreement No. 075-15-2019-1716 by 2019 Nov 20) and
Project No. 19-29-11013 (Russian Foundation of Fundamental Investigation,
Agreement No. 19-29-11013\20 by 2021 Jan 21).

On behalf of M. Ibrahimov.