We revisit the question of the nature of ultraluminous X-ray sources ( ULXs ) through a detailed investigation of their spectral shape , using the highest quality X-ray data available in the XMM-Newton public archives ( \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } { \raise 2.0 pt% \hbox { $ > $ } } } } 10 , 000 counts in their EPIC spectrum ) .
We confirm that simple spectral models commonly used for the analysis and interpretation of ULXs ( power-law continuum and multi-colour disc blackbody models ) are inadequate in the face of such high quality data .
Instead we find two near ubiquitous features in the spectrum : a soft excess and a roll-over in the spectrum at energies above 3 keV .
We investigate a range of more physical models to describe these data .
Slim discs which include radiation trapping ( approximated by a p -free disc model ) do not adequately fit the data , and several objects give unphysically high disc temperatures ( kT _ { in } > 3 keV ) .
Instead , disc plus Comptonised corona models fit the data well , but the derived corona is cool , and optically thick ( \tau \sim 5 - 30 ) .
This is unlike the \tau \sim 1 coronae seen in Galactic binaries , ruling out models where ULXs are powered by sub-Eddington accretion onto an intermediate mass black hole despite many objects having apparently cool disc temperatures .
We argue that these observed disc temperatures are not a good indicator of the black hole mass as the powerful , optically thick corona drains energy from the inner disc , and obscures it .
We estimate the intrinsic ( corona-less ) disc temperature , and demonstrate that in most cases it lies in the regime of stellar mass black holes .
These objects have spectra which range from those similar to the highest mass accretion rate states in Galactic binaries ( a single peak at 2–3 keV ) , to those which clearly have two peaks , one at energies below 1 keV ( from the outer , unComptonised disc ) and one above 3 keV ( from the Comptonised , inner disc ) .
However , a few ULXs have a significantly cooler corrected disc temperature ; we suggest that these are the most extreme stellar mass black hole accretors , in which a massive wind completely envelopes the inner disc regions , creating a cool photosphere .
We conclude that ULXs provide us with an observational template for the transition between Eddington and super-Eddington accretion flows , with the latter occupying a new ultraluminous accretion state .