Thick Accretion Disk Model for Ultraluminous Supersoft Sources

Abstract

We propose a geometrically thick, super-Eddington accretion disk model, where an optically thick wind is not necessary, to understand ultraluminous supersoft sources (ULSs). For high mass accretion rates $\dot M \ga 30\dot M_{\rm Edd}$ and not small inclination angles $\theta \ga 25^{\circ}$, where $\dot M_{\rm Edd}$ is the Eddington accretion rate, the hard photons from the hot inner region may be shaded by the geometrically thick inner disk, and therefore only the soft photons from the outer thin disk and the outer photosphere of the thick disk can reach the observer. Our model can naturally explain the approximate relation between the typical thermal radius and the thermal temperature, $R_{\rm bb} \propto T_{\rm bb}^{-2}$. Moreover, the thick disk model can unify ULSs and normal ultraluminous X-ray sources, where the different observational characteristics are probably related to the inclination angle and the mass accretion rate. By comparing our model with the optically thick outflow model, we find that less mass accretion rate is required in our model.