  The discovery of a population of superluminous supernovae (SLSNe), with peak
luminosities a factor of ~100 brighter than normal SNe (typically SLSNe have
M_V <-21), has shown an unexpected diversity in core-collapse supernova
properties. Numerous models have been postulated for the nature of these
events, including a strong interaction of the shockwave with a dense
circumstellar environment, a re-energizing of the outflow via a central engine,
or an origin in the catastrophic destruction of the star following a loss of
pressure due to pair production in an extremely massive stellar core (so-called
pair instability supernovae). Here we consider constraints that can be placed
on the explosion mechanism of Hydrogen-poor SLSNe (SLSNe-I) via X-ray
observations, with XMM-Newton, Chandra and Swift, and show that at least one
SLSNe-I is likely the brightest X-ray supernovae ever observed, with L_X ~
10^45 ergs/s, ~150 days after its initial discovery. This is a luminosity 3
orders of magnitude higher than seen in other X-ray supernovae powered via
circumstellar interactions. Such high X-ray luminosities are sufficient to
ionize the ejecta and markedly reduce the optical depth, making it possible to
see deep into the ejecta and any source of emission that resides there.
Alternatively, an engine could have powered a moderately relativistic jet
external to the ejecta, similar to those seen in gamma-ray bursts. If the
detection of X-rays does require an engine it implies that these SNe do create
compact objects, and that the stars are not completely destroyed in a pair
instability event. Future observations will determine which, if any, of these
mechanisms are at play in superluminous supernovae.
