X-ray spectral evolution in the ultraluminous X-ray source 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 Comptonization 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.