Accretion Disk Spectra of Ultraluminous X‐Ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies andGalactic superluminal jet sources share the common spectral characteristic thatthey have unusually high disk temperatures which cannot be explained in theframework of the standard optically thick accretion disk in the Schwarzschildmetric. On the other hand, the standard accretion disk around the Kerr blackhole might explain the observed high disk temperature, as the inner radius ofthe Kerr disk gets smaller and the disk temperature can be consequently higher.However, we point out that the observable Kerr disk spectra becomessignificantly harder than Schwarzschild disk spectra only when the disk ishighly inclined. This is because the emission from the innermost part of theaccretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seenfor a face-on disk. The Galactic superluminal jet sources are known to behighly inclined systems, thus their energy spectra may be explained with thestandard Kerr disk with known black hole masses. For ULXs, on the other hand,the standard Kerr disk model seems implausible, since it is highly unlikelythat their accretion disks are preferentially inclined, and, if edge-on Kerrdisk model is applied, the black hole mass becomes unreasonably large (> 300M_solar). Instead, the slim disk (advection dominated optically thick disk)model is likely to explain the observed super-Eddington luminosities, hardenergy spectra, and spectral variations of ULXs. We suggest that ULXs areaccreting black holes with a few tens of solar mass, which is not unexpectedfrom the standard stellar evolution scenario, and that their X-ray emission isfrom the slim disk shining at super-Eddington luminosities.Comment: ApJ, accepte