TITLE: GCN CIRCULAR NUMBER: 21685 SUBJECT: LIGO/Virgo G298048: Subaru HSC z-band imaging of the OT candidate SSS17a DATE: 17/08/25 07:38:17 GMT FROM: Michitoshi Yoshida at J-GEM Yoshida, M., Tanaka, M. (NAOJ), Utsumi, Y. (Hiroshima Univ.), Tominaga, N. (Konan Univ.), Ohsawa, R., Motohara, K. (Univ. of Tokyo), Terai, T., Nakata, F., Lee, C. H., Furusawa, H., Koshida, S. (NAOJ), Ohta, K. (Kyoto Univ.), Kawabata K. S. (Hiroshima Univ.), Morokuma, T., Yasuda, N. (Univ. of Tokyo), Kawai, N. (Tokyo Tech.), and Asakura, Y. (Nagoya Univ.) on behalf of the J-GEM collaboration We have performed z-band imaging observations of the OT candidate of G298048, SSS17a (Coulter et al., GCN 21529) with Hyper Suprime-Cam attached to the Subaru telescope on August 25.2 2017 UT. We found that the z-band magnitude of the source was significantly fainter compared to our previous observations on August 19.2 2017 UT (Yoshida et al. GCN 21586; Tominaga et al. GCN 21595). Further analysis is ongoing.