  We present DES14X3taz, a new hydrogen-poor super luminous supernova (SLSN-I)
discovered by the Dark Energy Survey (DES) supernova program, with additional
photometric data provided by the Survey Using DECam for Superluminous
Supernovae (SUDSS). Spectra obtained using OSIRIS on the Gran Telescopio
CANARIAS (GTC) show DES14X3taz is a SLSN-I at z=0.608. Multi-color photometry
reveals a double-peaked light curve: a blue and relatively bright initial peak
that fades rapidly prior to the slower rise of the main light curve. Our
multi-color photometry allows us, for the first time, to show that the initial
peak cools from 22,000K to 8,000K over 15 rest-frame days, and is faster and
brighter than any published core-collapse supernova, reaching 30% of the
bolometric luminosity of the main peak. No physical Nickel powered model can
fit this initial peak. We show that a shock-cooling model followed by a
magnetar driving the second phase of the light curve can adequately explain the
entire light curve of DES14X3taz. Models involving the shock-cooling of
extended circumstellar material at a distance of ~400 solar radii are preferred
over the cooling of shock-heated surface layers of a stellar envelope. We
compare DES14X3taz to the few double-peaked SLSN-I events in the literature.
Although the rise-times and characteristics of these initial peaks differ,
there exists the tantalizing possibility that they can be explained by one
physical interpretation.
