(Proposal ID) S16B-059

(PI) Tanaka, Masaomi

(Proposal Title)
Unveiling the Nature of Luminous Short-Timescale Transients

(Abstract)
The transient sky has been intensively explored by various surveys in the last 
decade. One of the important areas of discovery for transient surveys is a 
phenomenon with a short timescale (i.e., rise/decay time of < several days). 
The past attempts with PTF, Pan-STARRS1, and HSC have shown the existence of 
luminous short-timescale transients, but their nature is still unclear due to 
difficulty of simultaneous spectroscopy. We propose to perform Gemini/GMOS ToO 
observations coordinated with the HSC-SSP transient survey. HSC-SSP Ultra Deep 
transient survey will be performed in the S16B and S17A semesters for the 
COSMOS field. Typical g- and r-band depths are 26.5 mag, with the observing 
cadence of 3 days, which corresponds to about 2-day sampling in the restframe 
of the transients (typical redshift of z ~ 0.5). With this unprecedentedly 
deep transient survey, we can detect about 2 short-timescale transients even 
only in the 2-month survey period. Thanks to the photometric redshift catalog 
of the COSMOS field, candidates of short-timescale transients can be selected 
immediately after the observations, which enables the rapid spectroscopic 
follow-up. We aim at the first identification of absorption/emission line 
features in the luminous short-timescale transients themselves.