Geometrical effect of supercritical accretion flows: Observational implications of galactic black-hole candidates and ultraluminous X-ray sources

Abstract

We consider the dependence of the viewing angle in supercritical accretion flows and discuss the observational implications of galactic black-hole candidates and ultraluminous X-ray sources. Model spectra of supercritical accretion flows strongly depend on the inclination angle. For example, the maximum temperature of the supercritical disk (the accretion rate and the black-hole mass are M = 1000LE/c2 and M = 10M ⊙, respectively) is kTin ∼ 2.0keV for a low-inclination angle, i ≤40°, while kTin ∼0.6keV for a high-inclination angle, i ≥ 60°. This spectral softening originates from self-occultation of the disk, i.e., the outer disk blocks emission from the disk inner region, even if we take into account the effect of general relativity (light bending, Doppler boosting). This is because, when the mass accretion rate exceeds the critical rate, then the shape of the disk is geometrically thick due to enhanced radiation pressure. We also find that the spectral properties of low-i and low accretion-rate disks are very similar to those of high-i and high accretion-rate disks. That is, if an object has a high i and a high accretion rate, such a system suffers from self-occultation and the spectrum will be extremely soft. Therefore, we cannot distinguish these disks by only the difference in their spectrum shapes. Conversely, if we use the self-occultation properties, we could constrain the inclination angle of the system. We suggest that some observed high-temperature uhraluminous X-ray sources have low-inclination angles, i.e., near face-on geometry, i ≤ 40°, and the Galactic black-hole candidate XTE J1550-564 possesses relatively high-inclination angles, i ≥ 60°. © 2005. Astronomical Society of Japan.