Do ultraluminous X-ray sources really contain intermediate-mass black holes?

An open question remains whether Ultraluminous X-ray Sources (ULXs) really contain intermediate-mass black holes (IMBHs). We carefully investigated the XMM-Newton EPIC spectra of four ULXs that were claimed to be strong candidates of IMBHs by several authors. We first tried fitting by the standard spectral model of disk blackbody (DBB) + power-law (PL), finding good fits to all of the data, in agreement with others. We, however, found that the PL component dominates the DBB component at similar to 0. 3 to 10 keV. Thus, the black hole parameters derived solely from the minor DBB component are questionable. Next, we tried to fit the same data by the "p-free disk model" without the PL component, assuming an effective temperature profile of T-eff proportional to r(-p), where r is the disk radius. Interestingly, in spite of one less free-model parameter, we obtained similarly good fits with much higher innermost disk temperatures, 1.8 < kT(in) < 3.2keV. More importantly, we obtained p similar to 0.5, just the value predicted by the slim (super-critical) disk theory, rather than p = 0.75 that is expected from the standard disk model. The estimated black hole masses from the p-free disk model are much smaller; M less than or similar to 40M(circle dot). Furthermore, we applied a more sophisticated slim-disk model by Kawaguchi (2003, ApJ, 593, 69), and obtained good fits with roughly consistent black hole masses. We thus conclude that the central engines of these ULXs are super-critical accretion flows to stellar-mass black holes.