Subaru Hyper Suprime-Cam Survey for an optical counterpart of GW170817

Nozomu Tominaga, Masaomi Tanaka, Tomoki Morokuma, Yousuke Utsumi, Masaki S. Yamaguchi, Naoki Yasuda, Masayuki Tanaka, Michitoshi Yoshida, Takuya Fujiyoshi, Hisanori Furusawa, Koji S. Kawabata, Chien Hsiu Lee, Kentaro Motohara, Ryou Ohsawa, Kouji Ohta, Tsuyoshi Terai, Fumio Abe, Wako Aoki, Yuichiro Asakura, Sudhanshu BarwayIan A. Bond, Kenta Fujisawa, Satoshi Honda, Kunihito Ioka, Youichi Itoh, Nobuyuki Kawai, Ji Hoon Kim, Naoki Koshimoto, Kazuya Matsubayashi, Shota Miyazaki, Tomoki Saito, Yuichiro Sekiguchi, Takahiro Sumi, Paul J. Tristram, J. Gem Collaboration

Research output: Contribution to journalReview articlepeer-review

21 Citations (Scopus)


We perform a z-band survey for an optical counterpart of the binary neutron star coalescence GW170817 with Subaru/Hyper Suprime-Cam. Our untargeted transient search covers 23.6 deg2 corresponding to the 56.6% credible region of GW170817 and reaches the 50% completeness magnitude of 20.6 mag on average. As a result, we find 60 candidate extragalactic transients, including J-GEM17btc (also known as SSS17a/DLT17ck). While J-GEM17btc is associated with NGC 4993, which is firmly located inside the 3D skymap of GW170817, the other 59 candidates do not have distance information in the GLADE v2 catalog or NASA/IPAC Extragalactic Database. Among 59 of the candidates, 58 are located at the center of extended objects in the Pan-STARRS1 catalog, while one candidate has an offset. We present location, z-band apparent magnitude, and time variability of the candidates and evaluate the probabilities that they are located within the 3D skymap of GW170817. The probability for J-GEM17btc is 64%, which is much higher than for the other 59 candidates (9.3 × 10−3–2.1 × 10−1%). Furthermore, the possibility that at least one of the other 59 candidates is located within the 3D skymap is only 3.2%. Therefore, we conclude that J-GEM17btc is the most likely and distinguished candidate to be the optical counterpart of GW170817.

Original languageEnglish
Article number28
JournalPublication of the Astronomical Society of Japan
Issue number2
Publication statusPublished - 2018 Mar


  • Gravitational waves
  • Nuclear reactions, nucleosynthesis, abundances
  • Stars: neutron
  • Surveys


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