TITLE: GCN CIRCULAR NUMBER: 10881 SUBJECT: GRB 100621A: IRSF/SIRIUS NIR Observation DATE: 10/06/22 15:37:21 GMT FROM: Hiroyuki Naito at Nagoya U H. Naito, T. Sako, D. Suzuki, S. Kobara and K. Omori (Nagoya Univ.) on behalf of the MOA Collaboration, T. Nagayama, M. Kurita (Nagoya Univ.) and N. Oi (The Graduate University for Advanced Studies/NAOJ) on behalf of the IRSF Collabration report: We searched for a NIR afterglow of GRB 100621A (Ukwatta et al., GCN 10870) starting from 04:02 to 04:51 UT on 2010 June 21 with the SIRIUS on the IRSF 1.4m telescope at SAAO in South Africa. In images of a 300 sec exposure with J, H and Ks filter, we detected an new source within the error circle of the enhanced Swift XRT position (Evans et al., GCN 10873) and at the location of the GROND source (Updike et al., GCN 10874) . The estimated magnitudes are the followings. --------------------------------------------------------------- Mid time from Total Exp. J-mag* H-mag* K-mag* trigger (min) (s) --------------------------------------------------------------- 61.41 300 16.56 15.22 14.18 67.61 300 16.24 14.63 13.84 73.84 300 15.27 13.78 12.63 80.14 300 15.07 13.57 12.40 86.36 300 14.93 13.45 12.23 92.56 300 14.90 13.45 12.32 98.81 300 14.93 13.47 12.34 105.0 300 15.04 13.55 12.36 --------------------------------------------------------------- *Typical error is ~ 0.1 mag This photometry was done by using the DoPhot and calibrated against the 2MASS catalog stars, and not corrected for the Galactic extinction. The source was brightened up by >1mag during our observation period while the other field stars were reasonably stable.