A systematic search for rapid transients in the Subaru HSC-SSP transient survey

TL;DR

This study systematically searches for rapid transients at medium-high redshifts using Subaru HSC-SSP data, employing machine learning to classify candidates and estimate their rates, revealing diverse properties and potential progenitors.

Contribution

Introduces a machine learning-based classification method for rapid transients in a large survey, and provides the first rate estimates for these events at medium-high redshifts.

Findings

Identified 14 rapid transient candidates at redshifts 0.34 to 1.85.

Estimated the event rate of rapid transients as ~6,000 events per year per Gpc^3.

Detected candidates of Type Ibn supernovae and superluminous supernovae at high redshifts.

Abstract

Recent high-cadence transient surveys have discovered rapid transients whose light curve timescales are shorter than those of typical supernovae. In this paper, we present a systematic search for rapid transients at medium-high redshifts among 3381 supernova candidates obtained from the Subaru HSC-SSP transient survey. We developed a machine learning classifier to classify the supernova candidates into four types (Type Ia, Ibc, II supernovae, and rapid transients) based on the features derived from the light curves. By applying this classifier to the 3381 supernova candidates and by further applying the quality cut, we selected 14 rapid transient samples. They are located at a wide range of redshifts ($0.34 \leq z \leq 1.85$) and show a wide range of the peak absolute magnitude ($-17 \geq M \geq -22$). The event rate of the rapid transients is estimated to be $\sim 6\times10^3~\rm{events~yr^{-1}~Gpc^{-3}}$ at $z \sim 0.74$, which corresponds to about $2$ $\%$ of the event rate of normal core-collapse supernovae at the similar redshift. Based on the luminosity and color evolution, we selected two candidates of Type Ibn supernovae at $z\sim0.75$. The event rate of Type Ibn SN candidates is more than 1 $\%$ of Type Ib SN rate at the same redshift, suggesting that this fraction of massive stars at this redshift range eruptively ejects their He-rich envelope just before the explosions. Also, two objects at $z=1.37$ and 1.85 show high luminosities comparable to superluminous supernovae. Their event rate is about 10-25 $\%$ of superluminous supernovae at $z\sim 2$.