  We present the previously unanalysed high quality XMM-Newton spectrum of an
ultraluminous X-ray source candidate in NGC 4517. As with other high quality
ULX spectra, a downturn in the spectrum is observed at ~6 keV. Both of the
recent disc reflection and Comptonisation interpretations of this feature are
applied, in order to present a direct comparison, and are found to provide
statistically equivalent representations of the current data. We find that the
reflection model requires the accretion disc to have a highly super-solar iron
abundance, while the Comptonisation model requires low temperature Comptonising
electrons, and for the corona to be optically thick. These physical
requirements are discussed in detail, and physically motivated scenarios are
highlighted in which each model can be considered a viable explanation for the
observed emission. By extending our consideration of these two interpretations
to high energies, we demonstrate that observations of ULXs at energies above
~10 keV should be extremely useful when attempting to distinguish between them.
With current instrumentation, it is only viable to perform these observations
for M82 X-1, but future high angular resolution hard X-ray imaging
spectrometers, such as the Hard X-ray Imaging System due to fly on Astro-H,
should go a long way to resolving this issue.
