Spectral Analyses of the Nearest Persistent Ultraluminous X-Ray Source M 33 X-8

We provide a detailed analysis of 12 XMM observations of the nearest persistent extragalactic ultraluminous X-ray source (ULX), M 33 X-8. No significant spectral evolution has been detected for the period of the observations, and therefore we combine the individual observations to increase the signal-to-noise ratio for a spectral fitting. The combined spectra are best fitted by a self-consistent p-free disk plus power-law component model with p = 0.571^+0.032_-0.030, kT_in (inner disk temperature) = 1.38^+0.09_-0.08 keV, and the flux ratio of the p-free disk component to the power-law component being 0.63 : 0.37 in the 0.3-10 keV band. The fitting indicates that the black hole in M 33 X-8 is of ∼10_odot, and accretes at a super-Eddington rate (∼1.5L_Edd); also, the phase of the accretion disk is close to that of a slim disk (p = 0.5). We report, for the first time, that an extra power-law component is required in addition to the p-free disk model for ULXs. In super-Eddington cases, the power-law component may possibly result from an optically thin inner region of the disk or a Comptonized corona, similar to that of a standard thin disk.