The application of slim disk models to ULX: the case of M33 X-8
  A comparative XMM-Newton archival data spectral study of the ultraluminous X-ray source (ULX) M33 X-8 has been performed by using both the standard disk model and the newly developed slim disk models. The results of this analysis tend to confirm the hypothesis that M33 X-8 is an X-ray binary with a stellar-mass black hole accreting at super-Eddington rate. 
L. Foschini, K. Ebisawa, T. Kawaguchi, N. Cappelluti, P. Grandi, G.   Malaguti, J. Rodriguez, T.J-L. Courvoisier, G. Di Cocc...