TITLE: GCN CIRCULAR NUMBER: 11172 SUBJECT: GRB100901A: MITSuME Okayama Optical Observation DATE: 10/09/02 06:13:46 GMT FROM: Daisuke Kuroda at OAO/NAOJ D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ), S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto) and N. Kawai(Tokyo Tech) report on behalf of the MITSuME collaboration: We observed the field of GRB 100901A (Immler et al., GCN 11159) with the optical three color (g', Rc and Ic) CCD camera attached to the MITSuME 50cm telescope of Okayama Astrophysical Observatory. The observation started on 2010-09-1 13:36:08 UT (~2.0 min after the burst). We detected the previously reported afterglow (Immler et al., GCN 11159; Guidorzi et al., GCN 11160; Klunko et al., GCN 11162; Ivanov et al., GCN 11165; Andreev et al., GCN 11166, GCN 11168; De Cia et al., GCN 11170) in all the three bands. Our results show that the brightness of the afterglow began to increase at ~0.14 days after the burst. Photometric results and three sigma upper limits of the OT are listed below. We used GSC2.3 catalog for flux calibration. #T0+[day] MID-UT T-EXP[sec] g' g'_err Rc Rc_err Ic Ic_err ---------------------------------------------------------------------- 0.01195 13:51:23 1560.0 18.6 0.5 17.4 0.1 16.8 0.2 0.03343 14:22:18 1560.0 18.1 0.1 17.9 0.1 17.4 0.1 0.05475 14:53:00 1560.0 18.7 0.1 18.5 0.1 17.6 0.1 0.07594 15:23:32 1440.0 18.7 0.1 18.4 0.1 18.0 0.2 0.09685 15:53:38 1500.0 18.9 0.1 18.5 0.1 18.0 0.2 0.11790 16:23:57 1560.0 19.1 0.2 18.7 0.2 17.9 0.2 0.13912 16:54:30 1560.0 18.8 0.1 18.4 0.1 17.8 0.1 0.16013 17:24:46 1500.0 18.7 0.1 18.0 0.1 17.2 0.1 0.18107 17:54:55 1560.0 18.2 0.1 17.6 0.1 17.1 0.1 0.20223 18:25:23 1500.0 18.0 0.1 17.7 0.1 17.0 0.1 0.22363 18:56:12 1500.0 18.2 0.1 17.6 0.1 17.0 0.1 ---------------------------------------------------------------------- T0+ : Elapsed time after the burst [day] T-EXP: Total Exposure time [sec]