Rapidly Rising Transients from Subaru Hyper Suprime-Cam Transient Survey

TL;DR

This study reports the discovery of rapidly rising transients at high redshift using Subaru Hyper Suprime-Cam, revealing new insights into early supernova shock breakout phenomena and pre-supernova mass loss in massive stars.

Contribution

First high-cadence survey with Subaru HSC identifying rapidly rising transients and analyzing their origins, including shock breakout and dense wind scenarios.

Findings

Discovered five rapidly rising transients at z=0.384-0.821.

Event rate of these transients is at least 9% of core-collapse supernova rate.

Luminous objects suggest pre-supernova mass loss rates higher than typical red supergiants.

Abstract

We present rapidly rising transients discovered by a high-cadence transient survey with Subaru telescope and Hyper Suprime-Cam. We discovered five transients at z=0.384-0.821 showing the rising rate faster than 1 mag per 1 day in the restframe near-ultraviolet wavelengths. The fast rising rate and brightness are the most similar to SN 2010aq and PS1-13arp, for which the ultraviolet emission within a few days after the shock breakout was detected. The lower limit of the event rate of rapidly rising transients is ~9 % of core-collapse supernova rates, assuming a duration of rapid rise to be 1 day. We show that the light curves of the three faint objects agree with the cooling envelope emission from the explosion of red supergiants. The other two luminous objects are, however, brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout from dense wind with the mass loss rate of ~10^{-3} Msun yr^{-1}, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these luminous objects is >~1 % of core-collapse supernova rate, and thus, our study implies that more than ~1 % of massive stars can experience an intensive mass loss at a few years before the explosion.