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Magnetic spin-down of a millisecond neutron star has been proposed as thepower source of hydrogen-poor "superluminous" supernovae (SLSNe-I). However,producing an unambiguous test that can distinguish this model fromalternatives, such as circumstellar interaction, has proven challenging. Afterthe supernova explosion, the pulsar wind inflates a hot cavity behind theexpanding stellar ejecta: the nascent millisecond pulsar wind nebula.Electron/positron pairs injected by the wind cool through inverse Comptonscattering and synchrotron emission, producing a pair cascade and hard X-rayspectrum inside the nebula. These X-rays ionize the inner exposed side of theejecta, driving an ionization front that propagates outwards with time. Undersome conditions this front can breach the ejecta surface within months afterthe optical supernova peak, allowing ~0.1-1 keV photons to escape the nebulaunattenuated with a characteristic luminosity L_X ~ 1e43-1e45 erg/s. This"ionization break-out" may explain the luminous X-ray emission observed fromthe transient SCP 06F, providing direct evidence that this SLSN was indeedengine-powered. Luminous break-out requires a low ejecta mass and that thespin-down time of the pulsar be comparable to the photon diffusion timescale atoptical maximum, the latter condition similar to that required for a supernovawith a high optical fluence. These relatively special requirements may explainwhy most SLSNe-I are not accompanied by detectable X-ray emission. Globalasymmetry of the supernova ejecta increases the likelihood of an earlybreak-out along the direction of lowest density. Atomic states with lowerthreshold energies are more readily ionized at earlier times near opticalmaximum, allowing UV break-out across a wider range of pulsar and ejectaproperties than X-ray break-out, possibly contributing to the blue/UV colors ofSLSNe-I.

Ionization Break-Out from Millisecond Pulsar Wind Nebulae: an X-ray Probe of the Origin of Superluminous Supernovae