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 � � � � � --- -----------------------------------------------------------------------------------