TITLE:   GCN CIRCULAR
NUMBER:  28719
SUBJECT: GRB 201020B: Nanshan/NEXT optical afterglow confirmation
DATE:    20/10/21 13:20:48 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

D. Xu (NAOC), Z.P. Zhu (NAOC, HUST), S.Y. Fu, X. Liu (NAOC), X. Gao 
(Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:

We observed the field of GRB 201020B detected by Fermi/GBM, Fermi/LAT, 
and AGILE (Fermi GBM team, GCN 28702; Malacaria et al., GCN 28710; 
Arimoto et al., GCN 28716; Ursi et al., GCN 28714), starting at 12:35:28 
UT on 2020-10-21, i.e., 0.7927 day after the Fermi trigger. Photometry 
has been done in Sloan r-filter with 60s for each exposure, and 
observations are ongoing.

The optical transient reported by MASTER (Lipunov et al., GCN 28718) is 
clearly detected in our single image at improved coordinates

R.A. (J2000) = 5:01:52.80
Dec. (J2000) = +77:04:06.23

with a positional uncertainty of ~0.5 arcsec. It has decayed to m_r = 
19.0 +/- 0.1 at T_mid = 0.7944 day post-trigger, calibrated with nearby 
PS1 stars.

Based on the quick decay behaviour, we confirm that the source is very 
likely the afterglow of the burst.