X-ray spectral evolution in M33 X-8
  The bright ultraluminous X-ray source (ULX), M33 X-8, has been observed
several times by XMM-Newton, providing us with a rare opportunity to `flux bin'
the spectral data and search for changes in the average X-ray spectrum with
flux level. The aggregated X-ray spectra appear unlike standard sub-Eddington
accretion state spectra which, alongside the lack of discernible variability at
any energy, argues strongly against conventional two-component, sub-Eddington
models. Although the lack of variability could be consistent with
disc-dominated spectra, sub-Eddington disc models are not sufficiently broad to
explain the observed spectra. Fits with a ~ Eddington accretion rate slim disc
model are acceptable, but the fits show that the temperature decreases with
flux, contrary to expectations, and this is accompanied by the appearance of a
harder tail to the spectrum. Applying a suitable two-component model reveals
that the disc becomes cooler and less advection dominated as the X-ray flux
increases, and this is allied to the emergence of an optically-thick
Comptonisation medium. We present a scenario in which this is explained by the
onset of a radiatively-driven wind from the innermost regions of the accretion
disc, as M33 X-8 exceeds the Eddington limit. Furthermore, we argue that the
direct evolution of this spectrum with increasing luminosity (and hence
radiation pressure) leads naturally to the two-component spectra seen in more
luminous ULXs.
