An ultraviolet excess in the superluminous supernova Gaia16apd reveals a
  powerful central engine
  Since the discovery of superluminous supernovae (SLSNe) in the last decade,
it has been known that these events exhibit bluer spectral energy distributions
than other supernova subtypes, with significant output in the ultraviolet.
However, the event Gaia16apd seems to outshine even the other SLSNe at
rest-frame wavelengths below $\sim 3000$ \AA. Yan et al (2016) have recently
presented HST UV spectra and attributed the UV flux to low metallicity and
hence reduced line blanketing. Here we present UV and optical light curves over
a longer baseline in time, revealing a rapid decline at UV wavelengths despite
a typical optical evolution. Combining the published UV spectra with our own
optical data, we demonstrate that Gaia16apd has a much hotter continuum than
virtually any SLSN at maximum light, but it cools rapidly thereafter and is
indistinguishable from the others by $\sim 10$-15 days after peak. Comparing
the equivalent widths of UV absorption lines with those of other events, we
show that the excess UV continuum is a result of a more powerful central power
source, rather than a lack of UV absorption relative to other SLSNe or an
additional component from interaction with the surrounding medium. These
findings strongly support the central-engine hypothesis for hydrogen-poor
SLSNe. An explosion ejecting $M_{\rm ej} = 4 (0.2/\kappa)$ M$_\odot$, where
$\kappa$ is the opacity in cm$^2$g$^{-1}$, and forming a magnetar with spin
period $P=2$ ms, and $B=2\times10^{14}$ G (lower than other SLSNe with
comparable rise-times) can consistently explain the light curve evolution and
high temperature at peak. The host metallicity, $Z=0.18$ Z$_\odot$, is
comparable to other SLSNe.
