How Do We See a Relativistic Accretion Disk during a Thermal Instability?

Abstract

We calculated bolometric images of relativistic slim disks during a radiation-pressure-driven thermal instability. When the mass-accretion rate exceeds the critical one, an inner region of the standard accretion disk bursts to change to a slim disk state having a large scale height. That is, the inner region of the disk becomes high temperature, and the thickness of the disk increases due to an increase of the radiation pressure. As a result, we found that the observed image of the disk during the burst strongly depends on the inclination angle. That is, radiation from the innermost disk would be occulted by the disk outer rim for high inclination angles ($i \gtrsim 70^\circ$). We also calculated the spectral energy distribution during a thermal instability. The Wien peak of the spectrum of high inclination angles becomes softer than that of low inclination angles due to the geometrical thickness. From these facts, even if a burst occurs in a black-hole candidate, we may not observe the burst when the inclination angle is large. We may suggest that numerous luminous black-hole candidates are still hidden in our Galaxy.