paper

Slim Disk Model for Ultra-Luminous X-Ray Sources

arXiv:astro-ph/0011434 · doi:10.1086/319125

Abstract

The Ultra Luminous X-ray Sources (ULXs) are unique in exhibiting moderately bright X-ray luminosities, , and relatively high blackbody temperatures, $\Tin \sim 1.0-2.0 {\rm keV}$. From the constraint that cannot exceed the Eddington luminosity, , we require relatively high black-hole masses, , however, for such large masses the standard disk theory predicts lower blackbody temperatures, $\Tin < 1.0$ keV. To understand a cause of this puzzling fact, we carefully calculate the accretion flow structure shining at , fully taking into account the advective energy transport in the optically thick regime and the transonic nature of the flow. Our calculations show that at high accretion rate ($\dot M \ga 30~L_{\rm E}/c^2$) an apparently compact region with a size of $\Rin \simeq (1-3)\rg$ (with $\rg$ being Schwarzschild radius) is shining with a blackbody temperature of $\Tin \simeq 1.8 (M/10M_\odot)^{-1/4}$ keV even for the case of a non-rotating black hole. Further, $\Rin$ decreases as increases, on the contrary to the canonical belief that the inner edge of the disk is fixed at the radius of the marginally stable last circular orbit. Accordingly, the loci of a constant black-hole mass on the "H-R diagram" (representing the relation between and $\Tin$ both on the logarithmic scales) are not straight but bent towards the lower direction in the frame of the standard-disk relation.

5 pages, 2 figures, 2 tables, accepted for publication in the Astrophysical Journal Letters

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