Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient
  Survey
  Rapidly evolving transients form a new class of transients which show shorter
timescales of the light curves than those of typical core-collapse and
thermonuclear supernovae. We performed a systematic search for rapidly evolving
transients using the 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 5 rapidly evolving transients. Our
samples are found in a wide range of redshifts (0.3 $\le$ z $\le$ 1.5) and peak
absolute magnitudes ($-$17 $\ge$ $M_i$ $\ge$ $-$20). The properties of the
light curves 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 8,000 - 18,000 K. We show that some of the transients
require power sources other than the radioactive decays of $^{56}$Ni because of
their high peak luminosities and short timescales. The host galaxies of all the
samples are star-forming galaxies, suggesting a massive star origin for the
rapidly evolving transients. The event rate is roughly estimated to be
$\sim$4,000 events yr$^{-1}$ Gpc$^{-3}$, which is about 1 $\%$ of core-collapse
supernovae.
