TITLE:   GCN CIRCULAR
NUMBER:  29236
SUBJECT: GRB 210104A: BOOTES-4/MET early optical afterglow detection
DATE:    21/01/04 12:05:43 GMT
FROM:    Youdong HU at IAA-CSIC  <huyoudong072@hotmail.com>

Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M.D. Caballero-Garcia, M. A. Castro Tirado (IAA-CSIC), C. Perez del Pulgar, A. Castellon, I. Carrasco (Univ. de Malaga), S. Guziy (Univ. of Nikolaev) and D. R. Xiong, Y.F. Fan, J.M. Bai, C. J. Wang, Y.X. Xin, X. H. Zhao (Yunnan Observatories of CAS) on behalf of a larger collaboration, report:

"Following the detection of GRB 210104A by Swift (Troja et al. GCNC 29233) and Fermi (Fermi GBM team GCNC 29232, Biltzinger et al. GCNC 29234), the 0.6m BOOTES-4/MET robotic telescope at Lijiang Astronomical Observatory (China) started to gather images in clear filter starting at 2021-01-04 11:27:52 UT (~53 s after trigger). The optical afterglow reported by Swift/UVOT (Troja et al. GCNC 29233) is detected with a magnitude of 15.15+-0.15 in the first 5 s exposure image. Additional observations are ongoing."

We thank the staff at Lijiang observatory for their excellent support.