TITLE: GCN CIRCULAR NUMBER: 11175 SUBJECT: GRB 100901A : HCT optical observations DATE: 10/09/02 13:34:34 GMT FROM: D.K. Sahu at Indian Inst of Astrophysics,Bangalore D.K. Sahu, B.C. Bhatt, Sonam Arora and Pepsi Anto (IIA, Bangalore) report. Optical afterglow of GRB 100901A (Immler et al. GCN 11159) was obsrved with the 2m. Himalayan Chandra Telescope of the Indian Astronomical Observatory, Hanle, India. A series of exposures were taken in Bessell V, R and I bands, between 17:35 UT and 22:15 UT on 01/09/2010. The optical afterglow was detected in all our frames. The preliminary R and I magnitudes of the optical afterglow calibrated using USNO B1.0 1127-0027231 reference star (R=15.37, I=15.13) is as under: Mid UT Exposure Filter Mag 17:42 300sec R 17.34 +/- 0.05 19:44 300sec R 17.03 +/- 0.05 22:05 300sec R 17.08 +/- 0.04 17:51 300sec I 17.05 +/- 0.05 19:51 300sec I 16.76 +/- 0.04 This message may be cited.