TITLE:   GCN CIRCULAR
NUMBER:  29252
SUBJECT: GRB 210104A: Nanshan/NEXT optical powerlaw decay
DATE:    21/01/05 03:04:27 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

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

We observed the field of GRB 210104A (Troja et al., GCN 29233) using the 
NEXT-0.6m optical telescope located at Nanshan, Xinjiang, China. 
Observations started at 11:59:19 UT (i.e., 0.538 hr post-burst) on 
2021-01-04 and ended at 22:34:07 UT (i.e., 11.11 hr post-burst) on 
2021-01-04, and a series of 120 s, 200 s, 300 s Sloan r-band frames were 
obtained.

Preliminary analysis of the data shows that afterglow is decaying as 
F_rband ~ t^(-\alpha), where \alpha = 0.73 since the very beginning of 
our observations.

If the afterglow continues to decay in this way, it would be of r ~ 20.3 
mag at 1 day post-burst, and still bright enough for optical spectroscopy.