TITLE:   GCN CIRCULAR
NUMBER:  29250
SUBJECT: GRB 210104B: Nanshan/NEXT optical upper limit
DATE:    21/01/05 00:33:27 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

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

We observed the field of GRB 210104B (Laha et al., GCN 29243) using the 
NEXT-0.6m optical telescope located at Nanshan, Xinjiang, China. 
Observations automatically started at  UT on 2020-12-21, i.e., 75 s 
after the BAT trigger. We obtained a series of 40 s, 60 s and 90 s 
frames in the Sloan r-filter.

No optical source is detected in our stacked image at the Swift/XRT 
position (Laha et al., GCN 29243), down to a limiting magnitude of 
r~21.0, calibrated with the nearby PanSTARRS field.