TITLE: GCN CIRCULAR NUMBER: 6430 SUBJECT: GRB 070520A: MITSuME Akeno optical upper limits DATE: 07/05/21 04:19:51 GMT FROM: Nobuyuki Kawai at Tokyo Tech T. Ishimura, T. Shimokawabe, N. Vasquez, Y. Yatsu, Y. Kudou and N. Kawai (Tokyo Tech) report on behalf of the MITSuME collaboration: We observed the field of GRB070520A (Romano et al., GCN 6423) with the 3-color 50cm MITSuME Telescope at Akeno, Japan starting at 13:15:15 UT, 10 min after the trigger, under a clear, but windy condition. In the co-added images of Ic, Rc, and g' bands, we did not detect any new source in the XRT error circle. The 3-sigma limiting magnitudes based on USNO-B1.0 (I-band) and NOMAD (R-band,g'-band) stars are following. Filter start time end time Exp(s) Mag (3-sigma UL) ----------------------------------------------------------- Ic 13:15:15 15:06:06 60s * 91 20.3 Rc 13:15:15 15:06:06 60s * 91 21.2 g' 13:15:15 15:06:06 60s * 91 21.1 -----------------------------------------------------------