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