J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817

Yousuke Utsumi, Masaomi Tanaka, Nozomu Tominaga, Michitoshi Yoshida, Sudhanshu Barway, Takahiro Nagayama, Tetsuya Zenko, Kentaro Aoki, Takuya Fujiyoshi, Hisanori Furusawa, Koji S. Kawabata, Shintaro Koshida, Chien-Hsiu Lee, Tomoki Morokuma, Kentaro Motohara, Fumiaki Nakata, Ryou Ohsawa, Kouji Ohta, Hirofumi Okita, Akito Tajitsu, Ichi Tanaka, Tsuyoshi Terai, Naoki Yasuda, Fumio Abe, Yuichiro Asakura, Ian A. Bond, Shota Miyazaki, Takahiro Sumi, Paul J. Tristram, Satoshi Honda, Ryosuke Itoh, Yoichi Itoh, Miho Kawabata, Kumiko Morihana, Hiroki Nagashima, Tatsuya Nakaoka, Tomohito Ohshima, Jun Takahashi, Masaki Takayama, Wako Aoki, Stefan Baar, Mamoru Doi, François Finet, Nobuyuki Kanda, Nobuyuki Kawai, Ji Hoon Kim, Daisuke Kuroda, Wei Liu, Kazuya Matsubayashi, Katsuhiro L. Murata, Hiroshi Nagai, Tomoki Saito, Yoshihiko Saito, Shigeyuki Sako, Yuichiro Sekiguchi, Yoichi Tamura, Masayuki Tanaka, Makoto Uemura, Masaki S. Yamaguchi, the J-GEM collaboration

Published 2017-10-16Version 1

The first detected gravitational wave from a neutron star merger was GW170817. In this study, we present J-GEM follow-up observations of SSS17a, an electromagnetic counterpart of GW170817. SSS17a shows a 2.5-mag decline in the $z$-band from 1.7 days to 7.7 days after the merger. Such a rapid decline is not comparable with supernovae light curves at any epoch. The color of SSS17a also evolves rapidly and becomes redder for later epochs; the $z-H$ color changed by approximately 2.5 mag in the period of 0.7 days to 7.7 days. The rapid evolution of both the optical brightness and the color are consistent with the expected properties of a kilonova that is powered by the radioactive decay of newly synthesized $r$-process nuclei. Kilonova models with Lanthanide elements can reproduce the aforementioned observed properties well, which suggests that $r$-process nucleosynthesis beyond the second peak takes place in SSS17a. However, the absolute magnitude of SSS17a is brighter than the expected brightness of the kilonova models with the ejecta mass of 0.01 $\Msun$, which suggests a more intense mass ejection ($\sim 0.03 \Msun$) or possibly an additional energy source.