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Since the discovery of superluminous supernovae (SLSNe) in the last decade,it has been known that these events exhibit bluer spectral energy distributionsthan other supernova subtypes, with significant output in the ultraviolet.However, the event Gaia16apd seems to outshine even the other SLSNe atrest-frame wavelengths below $\sim 3000$ \AA. Yan et al (2016) have recentlypresented HST UV spectra and attributed the UV flux to low metallicity andhence reduced line blanketing. Here we present UV and optical light curves overa longer baseline in time, revealing a rapid decline at UV wavelengths despitea typical optical evolution. Combining the published UV spectra with our ownoptical data, we demonstrate that Gaia16apd has a much hotter continuum thanvirtually any SLSN at maximum light, but it cools rapidly thereafter and isindistinguishable from the others by $\sim 10$-15 days after peak. Comparingthe equivalent widths of UV absorption lines with those of other events, weshow that the excess UV continuum is a result of a more powerful central powersource, rather than a lack of UV absorption relative to other SLSNe or anadditional component from interaction with the surrounding medium. Thesefindings strongly support the central-engine hypothesis for hydrogen-poorSLSNe. 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 spinperiod $P=2$ ms, and $B=2\times10^{14}$ G (lower than other SLSNe withcomparable rise-times) can consistently explain the light curve evolution andhigh temperature at peak. The host metallicity, $Z=0.18$ Z$_\odot$, iscomparable to other SLSNe.

An ultraviolet excess in the superluminous supernova Gaia16apd reveals a powerful central engine