TITLE: GCN CIRCULAR NUMBER: 12410 SUBJECT: GRB 110921A: MITSuME-Akeno Optical photometry DATE: 11/09/30 16:46:34 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: As reported in GCN12378 (Tokoyoda et al.), we detected a very faint object within the XRT error circle of GRB 110921A (W. H. Baumgartner et al, GCN12373) with the MITSuME-Akeno 50cm telescope. Here we report the results of aperture photometry. We started the follow-up observation at 13:53:08 (UT), ~108 sec after the BAT trigger(W. H. Baumgartner et al, GCN12373). The optical counterpart can be seen both in R and I band only in the earliest data-set from T0+108s to T0 + 348s. For zero-point calibration we used GSC2.3 catalog. The g' magnitudes of the reference stars were estimated based on Sesar et al. (2006). For I-band data we could not find any usable catalog data, therefore we performed a field photometry. The Photometric results are summarized below. Filter  T0+[s]  MID-UT  T-EXP[sec]   Mag  Mag_err   LimitMag ----------------------------------------------------------------------------------- g'        +108    13:55:31        240        ---      ---           >19.4 Rc      +108    13:55:31        240       18.8    0.2           --- Ic        +108    13:55:13        210       18.8    0.3           --- -----------------------------------------------------------------------------------