GRB 210104B: BOOTES network optical upper limit

Following the Swift trigger of GRB 210104B (Laha et al., GCNC 29243), the 0.3m BOOTES-1B robotic telescope in Mazagon (Huelva), southern Spain, automatically responded to this burst. Series images were taken started 2021-01-04 21:11:00 UT (~50 s after trigger). In the co-added of 10x10 s images, no source is detected within the XRT position (Laha et al., GCNC 29243) which down to 18.1 mag (clear filter). The 0.6m BOOTES-2/TELMA robotic telescope at IHSM La Mayora (UMA-CSIC) in Algarrobo Costa (Malaga, Spain) also automatically responded to the Swift trigger of GRB 210104B (Laha et al., GCNC 29243). A series of images were taken starting on 2021-01-04 21:11:24 UT (~74 s after trigger). No optical source is detected within the XRT position on the co-added image (10x10 s, clear filter) which down to 19.9 mag. Those non-detections are consistent with the reports from Swift (Laha et al., GCNC 29243) and MASTER-Net (Lipunov et al., GCNC 29242). Additional observations are ongoing. We thank the staff at INTA-CEDEA and La Mayora for their excellent support.