# The Hyper Suprime-Cam SSP Transient Survey in COSMOS: Overview

## TL;DR

This paper provides an overview of a deep transient survey in the COSMOS field using Subaru HSC, identifying thousands of supernova candidates and enabling studies of high-redshift supernovae and other variable objects.

## Contribution

It presents a comprehensive survey dataset that doubles the known number of high-redshift Type Ia supernovae and offers valuable data for time-domain astrophysics.

## Key findings

- Identified 1,824 supernova candidates.
- Classified 433 as Type Ia supernovae, with 129 having redshift data.
- Doubled the number of known Type Ia SNe at z > 1.

## Abstract

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.