Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey

Rapidly evolving transients form a new class of transients that show shorter timescales of light curves than those of typical core-collapse and thermonuclear supernovae. We performed a systematic search for rapidly evolving transients using deep data taken with the Hyper Suprime-Cam Subaru Strategic Program Transient Survey. By measuring the timescales of the light curves of 1824 transients, we identified five rapidly evolving transients. Our samples are found in a wide range of redshifts (0.3 ≤ z ≤ 1.5) and peak absolute magnitudes (-17 ≥ M _i ≥-20). The light-curve properties are similar to those of the previously discovered rapidly evolving transients. They show a relatively blue spectral energy distribution, with the best-fit blackbody of 8000-18,000 K. We show that some of the transients require power sources other than the radioactive decays of ⁵⁶Ni because of their high peak luminosities and short timescales. The host galaxies of all of the samples are star-forming galaxies, suggesting a massive star origin for the rapidly evolving transients. The event rate is roughly estimated to be ∼4000 events yr^-1 Gpc^-3, which is about 1% of core-collapse supernovae.