Rapidly evolving transients in the Dark Energy Survey

We present the results of a search for rapidly evolving transients inthe Dark Energy Survey Supernova Programme. These events arecharacterized by fast light-curve evolution (rise to peak in ≲10 dand exponential decline in ≲30 d after peak). We discovered 72events, including 37 transients with a spectroscopic redshift from hostgalaxy spectral features. The 37 events increase the total number ofrapid optical transients by more than a factor of two. They are found ata wide range of redshifts (0.05 < z < 1.56) and peak brightnesses(-15.75 > Mg > -22.25). The multiband photometry iswell fit by a blackbody up to few weeks after peak. The events appear tobe hot (T ≈ 10 000-30 000 K) and large (R ≈ 1014 - 2× 1015 cm) at peak, and generally expand and cool intime, though some events show evidence for a receding photosphere withroughly constant temperature. Spectra taken around peak are dominated bya blue featureless continuum consistent with hot, optically thickejecta. We compare our events with a previously suggested physicalscenario involving shock breakout in an optically thick wind surroundinga core-collapse supernova, we conclude that current models for such ascenario might need an additional power source to describe theexponential decline. We find that these transients tend to favourstar-forming host galaxies, which could be consistent with acore-collapse origin. However, more detailed modelling of the lightcurves is necessary to determine their physical origin