Do Ultraluminous X-Ray Sources Really Contain Intermediate-Mass Black Holes?

An open question remains whether Ultraluminous X-ray Sources (ULXs) reallycontain intermediate-mass black holes (IMBHs). We carefully investigated theXMM-Newton EPIC spectra of the four ULXs that were claimed to be strongcandidates of IMBHs by several authors. We first tried fitting by the standardspectral model of disk blackbody (DBB) + power-law (PL), finding good fits toall of the data, in agreement with others. We, however, found that the PLcomponent dominates the DBB component at $\sim$ 0.3 to 10 keV. Thus, the blackhole 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 thePL component, assuming the effective temperature profile of $T_{\rm eff}\propto r^{-p}$ where $r$ is the disk radius. Interestingly, in spite of oneless free model parameters, we obtained similarly good fits with much higherinnermost disk temperatures, $1.8 < kT_{\rm in} < 3.2$ keV. More importantly,we obtained $p \sim 0.5$, just the value predicted by the slim (super-critical)disk theory, rather than $p = 0.75$ that is expected from the standard diskmodel. The estimated black hole masses from the $p$-free disk model are muchsmaller; $M\ltsim 40 M_\odot$. Furthermore, we applied a more sophisticatedslim disk model by Kawaguchi (2003, ApJ, 593,69), and obtained good fits withroughly consistent black hole masses. We thus conclude that the central enginesof these ULXs are super-critical accretion flows to stellar-mass black holes.Comment: Appear in PASJ, 10 pages, 10 figures (in total) grouped into 3 figure captions, 3 table