The Hyper Suprime-Cam SSP Transient Survey in COSMOS: Overview
  We present an overview of a deep transient survey of the COSMOS field with
the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77
deg$^2$ ultra-deep layer and 5.78 deg$^2$ deep layer in the Subaru Strategic
Program over 6- and 4-month periods from 2016 to 2017, respectively. The
ultra-deep layer shows a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and
24.6 mag in $g$, $r$, $i$, $z$, and $y$ bands, respectively; the deep layer is
$\sim0.6$ mag shallower. In total, 1,824 supernova candidates were identified.
Based on light curve fitting and derived light curve shape parameter, we
classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129
objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects
are located at $z > 1$. Our unique dataset doubles the number of Type Ia SNe at
$z > 1$ and enables various time-domain analyses of Type II SNe, high redshift
superluminous SNe, variable stars, and active galactic nuclei.
