TITLE:   GCN CIRCULAR
NUMBER:  30281
SUBJECT: GRB 210619B: steepening of the decay observed at the D50
DATE:    21/06/21 00:55:28 GMT
FROM:    Martin Jelinek at Astro.Inst-AVCR,Ondrejov  <martin.jelinek@asu.cas.cz>

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

We observed the optical afterglow of GRB 210619B (D'Avanzo et al. GCN
30261; Jelinek et al. GCN 30263; Lipunov et al. GCN 30259; Zhao et al. GCN
30261; Kong et al. GCN 30265; Axelsson et al. GCN 30270; de Ugarte Postigo
et al. GCN 30272; Zheng et al. GCN 30273; Blazek et al. GCN 30274; Kann et
al. GCN 30275; Svinkin et al. GCN 30276; Xin et al. GCN 30277; Perley et
al. GCN 30271, Shrestha et al. GCN 30280) with the D50 in Ondrejov through
difficult conditions with some passing clouds.

On a combined (86x60s, Sloan r') image with the mean exposure time 0.98
days post burst we measure r'(AB) = 20.2 +- 0.2. This is significantly
below any extrapolation of photometric points published before, and
suggests a break and a steepening of the decay.