TITLE: GCN CIRCULAR NUMBER: 11189 SUBJECT: GRB100901A: MITSuME Okayama Optical Observation at 1 day after the burst DATE: 10/09/03 05:48:08 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 2010-09-2 16:51:50 UT (~1.14 day 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; Kuroda et al., GCN 11172) in all the three bands. Photometric results of the OT are listed below. The magnitudes are obtained from the sum of exposures. No significant variation was detected during these exposures, though the S/N was not so good. We used SDSS catalog for flux calibration. #T0+[day] MID-UT T-EXP[sec] g' g'_err Rc Rc_err Ic Ic_err ---------------------------------------------------------------------- 1.17902 17:51:57 6060.0 19.4 0.1 19.0 0.1 18.5 0.1 ---------------------------------------------------------------------- T0+ : Elapsed time after the burst [day] T-EXP: Total Exposure time [sec]