Y.-D. Hu, T.-R. Sun, 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 210504A by Swift (Beardmore et al. GCNC 29929), we triggered the 0.6m BOOTES-4/MET robotic telescope at Lijiang Astronomical Observatory (China) in order to follow up this burst, starting on May 4, 16:05 UT (~2.2 hr after the BAT trigger, according to Lien et al. GCNC 29930). No optical source is detected within the XRT error box (Beardmore et al. GCNC 29929) down to 19.1 mag in the co-added image (630 s in total, clear filter). This non-detection is consistent with the UVOT (Beardmore et al. GCNC 29929) and MASTER (Lipunov et al. GCNC 29931) limits.
We thank the staff at Lijiang observatory for their excellent support.