TITLE: GCN CIRCULAR NUMBER: 17347 SUBJECT: GRB 150120B: Mondy optical observations DATE: 15/01/23 19:00:55 GMT FROM: Alexei Pozanenko at IKI, Moscow E. Mazaeva (IKI), A. Volnova (IKI), E. Klunko (ISTP), I. Korobtsev (ISTP), M. Eselevich (ISTP), A. Pozanenko (IKI) report on behalf of larger GRB follow-up collaboration: We observed the field of GRB 150120B (D'Elia et al., GCN 17314) with AZT-33IK telescope of Sayan observatory (Mondy) Jan., 20 starting on (UT) 11:42:02. We obtained 65 images in R-filter of 120 s exposure. In two combined images within enhanced Swift-XRT error circle (Goad et al., GCN 17320) we clearly detect optical afterglow (Perley et al. GCN 17315, Guidorzi et al. GCN 17316). Photometry of the afterglow is following Date UT start t-T0 Filter Exp. OT Err. (mid, days) (s) 2015-01-20 11:42:02 0.20149 R 30*120 21.25 0.22 2015-01-20 12:42:04 0.24348 R 30*120 21.43 0.25 Photometry is based on nearby USNO B1.0 (R2) stars: USNO-B1.0_id R 0980-0031064 15.57 0980-0031070 14.16 0980-0031080 16.35 0980-0031095 15.41