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), R. Fernandez-Munoz (IHSM/UMA-CSIC), M. Jelinek (ASU-CAS), 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 210402A by Swift (Page et al. GCNC 29743), the 0.6m BOOTES-4/MET robotic telescope at Lijiang Astronomical Observatory (China) automatically followed up this event at 22:10:15 UT (~ 19 s after notice) gathering images (60 s exposure time, clear filter). Furthermore, the 0.6 m BOOTES-2/TELMA robotic telescope at IHSM La Mayora (UMA-CSIC) in Algarrobo Costa (Malaga, Spain) also responded to this event, with the first image (60 s exposure, clear filter) obtained at 22:12:04 UT on April 2 (~ 2 min after notice).
In any case, no optical transient is detected down to 19.2 and 18.9 mag respectively within the Swift/XRT error region (Evans et al, GCNC 29750). These two non-detections are consistent with the limits reported by MASTER (Lipunov et al, GCNC 29741), Swift/UVOT (Page et al. GCNC 29743), Montarrenti (Leonini et al. GCNC 29747), NOT (Zhu et al, GCNC 29745) and DDOTI (Butler et al, GCNC 29752).
We thank the staff at both La Mayora and Lijiang Astronomical Observatory for their excellent support.