[vsnet-grb-info 27763] GRB 210420B: Ondrejov D50 optical afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29866
SUBJECT: GRB 210420B: Ondrejov D50 optical afterglow detection
DATE:    21/04/21 15:32:16 GMT
FROM:    Martin Jelinek at Astro.Inst-AVCR,Ondrejov  <martin.jelinek at asu.cas.cz>

M. Jelinek, J. Strobl, R. Hudec, C. Polasek (ASU CAS Ondrejov)

report:

We observed the position of the bright GRB 210420B (Moss et al., GCN 29833
& Lien et al., GCN 29854) with the D50 telescope of the Astronomical
Institute Ondrejov, near Prague, Czech  Republic. We performed a series of
120 s unfiltered exposures starting 21:10 UT, between 2.6 and 4.9 h after
the trigger.

The optical afterglow (Lipunov et al., GCNC 29845; Oksanen et al., GCNC
29846; Hu et al., GCNC 29847; Tianrui Sun et al., GCNC 29848; Leonini et
al., GCNC 29851; de Ugarte Postigo et al., GCNC 29852; de Ugarte Postigo et
al., GCNC 29853; Watson et al., GCNC 29858; Zheng et al., GCNC 29861;
Breeveld et al., GCNC 29862; Stecklum et al., GCNC 29863 and Butler et al.,
GCNC 29864) is clearly detected in single 120 s images, at least at the
beginning of the series.

We can confirm a decaying nature of the object - the afterglow faded > 0.4
mag during the course of our observations, providing an alpha decay of 0.69
+- 0.17.