TITLE: GCN CIRCULAR NUMBER: 18435 SUBJECT: GRB 151021A: MITSuME Okayama Ks-band upper-limit DATE: 15/10/22 06:19:51 GMT FROM: Kenshi Yanagisawa at OAO/NAOJ K. Yanagisawa, D. Kuroda, Y. Shimizu, H. Izumiura(OAO/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 151021A (Melandri et al., GCN 18425) in Ks-band with a wide-field near infrared imager at Okayama Astrophysical Observatory (Japan). The imager has effective aperture of 0.91 m. Observations started from 10:42 UT on 21th October, 9.2h after the BAT trigger, to 11:11 UT. The total exposure of 11.7 min was successfully obtained. In our co-add image, we did not find any new point source within the enhanced XRT error circle (Osborne et al., GCN 18428) down to limiting magnitude of Ks=14.5 (Vega, S/N=10). The photometric calibration was made against 2MASS field stars. T0+[sec] MID-UT T-EXP[sec] Ks --------------------------------------------------- +34,049 10:56 700.0 >14.5 --------------------------------------------------- T0+ : Elapsed time after the burt [sec] T-EXP: Total Exposure time [sec]