TITLE: GCN CIRCULAR NUMBER: 12378 SUBJECT: GRB 110921A: MITSuME Akeno Optical Observation DATE: 11/09/21 16:38:54 GMT FROM: Yoichi Yatsu at Tokyo Tech. K. Tokoyoda, M. Hayashi, Y. Yatsu, Y. Aoki, K. Kawakami, S. Song, R. Usui, and N. Kawai (Tokyo Tech) report on behalf of the MITSuME collaboration: We observed GRB 110921A (W. H. Baumgartner et al, GCN12373) with the optical three color (g, Rc, and Ic) CCD camera attached to the MITSuME 50 cm telescope of Akeno Observatory, Yamanashi, Japan. The follow-up observation was started at 13:53:08, ~108 sec after the BAT trigger. We did not find optical counterpart in g'-band and R-band within the enhanced XRT error circle (W. H. Baumgartner et al, GCN12373). While, in I-band, we detected a very faint OT candidate in the error circle only in the earliest data-set (T0+108s ~ T0 + 348s). Further data analysis for the I-band data is now ongoing. Photometric results are listed below. We used GSC2.3 catalog for flux calibration. T0+[s] MID-UT T-EXP[sec] g' Rc Ic --------------------------------------------------------------------------------- +108 14:07:40 1530 >20.0 >19.5 --- --------------------------------------------------------------------------------- T0+ : Elapsed time after the burst [sec] T-EXP: Total Exposure time [sec]