GRB 210104A: MASTER OT rebrightening detection

MASTER-Tavrida robotic telescope (Global MASTER-Net http://observ.pereplet.ru, Lipunov et al. 2010, Advances in Astronomy,v.2010,30L) started (Lipunov et al. GCN 29239) inspect of Fermi and Swift GRB 210104A (Fermi GCN 29232, 29246; Ttrig=11:26:59; Swift GCN Troja et al. GCN 29233, Beardmore et al. GCN 29245, Breeveld et al. GCN 29247 ; Barthelmy https://gcn.gsfc.nasa.gov/other/210104A.gcn3 ) at sunset at Lomonosov MSU Crimean observatory at 2021-01-04 15:19:29 UT. We detect rebrightening of OT (discovered by Swift GCN 29233 and observed by Xin et al., GCN Circ. 29235; Hosokawa etal. GCN 29237; Kim et al. GCN 29238; Lipunov et al. GCN 29239, Horiuchi et al. GCN 29241, Breeveld et al. GCN 29247) MASTER OT J065505.45+644035.0 at 2021-01-04 15:48:32UT with unfiltered m_OT=17.3. The message may be cited.