TITLE: GCN CIRCULAR NUMBER: 29286 SUBJECT: GRB 210104A: Mondy optical observations, light curve DATE: 21/01/10 22:34:47 GMT FROM: Alexei Pozanenko at IKI, Moscow S. Belkin (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), V. Kim (FAI, Pulkovo Observatory), Y. Aimuratov (FAI), M. Krugov (FAI) report on behalf of GRB IKI FuN: We observed GRB 210104A (Fermi GBM team, GCN 29232; Troja et al., GCN 29233; Frederiks et al., GCN 29258; Cherry et al., GCN 29268) with AZT-33IK telescope of Sayan observatory (Mondy). We detect the optical afterglow (Troja et al., GCN 29233; Xin et al., GCN 29235; Hu et al., GCN 29236; Hosokawa et al., GCN 29237; Kim et al., GCN 29238, 29265; Lipunov et al., GCN 29239; Horiuchi et al., GCN 29241; Breeveld et al., GCN 29247; Zhu et al., GCN 29252; Paek et al., GCN 29254; Kumar et al., GCN 29257; Smartt et al., GCN 29262; Romanov, GCN 29269; Anandagoda et al., GCN 29273; Gokuldass, GCN 29274; Mao et al., GCN 29275; Moskvitin et al., GCN 29277). Preliminary photometry of the afterglow is following Date UT start t-T0 Filter Exp. OT Err. UL(3 sigma) (mid, days) (s) 2021-01-07 15:10:23 3.19681 R 80*90 n/d n/d 22.1 2021-01-08 15:22:18 4.20438 R 59*120 22.2 0.2 22.9 2021-01-09 11:51:54 5.07980 R 90*120 22.7 0.3 22.7 The photometry is based on nearby stars of USNO-B1.0 USNO-B1.0_id R2 1546-0139059 15.53 1546-0139029 14.82 A light curve of the afterglow based on our observations including reported early (Kim et al., GCN 29238, 29265, 29283) can be found in http://grb.rssi.ru/GRB210104A/GRB210104A_LC.png