Program: GN-2019B-FT-205

In this decade, the time scale shorter than 1 day is a new frontier intensively investigated by transient surveys with wide-field cameras. The short-timescale transients offer a new insight, especially, on the final stage of the evolution of massive stars, i.e., supernova (SN) progenitors and circumstellar medium (CSM). In contrast to the past HSC-SSP transient survey, the first opportunity for a g-band high-cadence survey with long-term multicolor observations with HSC is available from the end of Sep 2019 to the end of Jan 2020. We fully utilize this unique opportunity to explore the early SN emission. We propose ToO spectroscopic follow-up observations with Gemini/GMOS. We pick up a relatively low-z and bright (g<24 mag) short-timescale (<0.1 day/mag) transients and perform follow-up observations. The rapid evolution of early spectrum and spectral energy distribution is obtained in the first and second observations, while the SN type is identified in the third observation. The spectroscopic observations clarify the CSM structure, the mass-loss rate shortly prior to the explosion, the presupernova radius, and the temperature evolution of the ejecta. The spectroscopic observations in addition to the imaging observations with HSC are essential to classify the mechanisms of early SN emission and reveal the last moments of massive stars.