TITLE:   GCN CIRCULAR
NUMBER:  30670
SUBJECT: GRB 210820A: NOT optical upper limit
DATE:    21/08/21 07:30:12 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

Z.P. Zhu (NAOC, HUST), D. B. Malesani (DTU space), L. Izzo (DARK/NBI), 
J.P.U. Fynbo (DAWN/NBI), D. Xu (NAOC), R. Clavero (NOT) report on behalf 
of a larger collaboration:

We observed the field of GRB210820A (Beardmore et al., GCN30664) using 
the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We 
obtained 3x200s frames in the Sloan r- filter, starting at 00:49:56 UT 
on 2021-08-21 (i.e., ~ 7.29 hr after the BAT trigger).

No optical source is detected in our stacked image at the Swift/UVOT 
position (Beardmore et al., GCN 30664), down to a limiting magnitude of 
r~22.2, calibrated with the nearby PanSTARRS field.