Radiation Properties of an Accretion Disk with a Non-Zero Torque on Its Inner Edge

Abstract

Abstract The structure of the inner edge of the accretion disk around a black hole can be altered, if the matter inside the marginally stable orbit is magnetically connected to the disk. In this case, a non-zero torque is exerted on its inner edge, and the accretion efficiency, $\epsilon$, can be much higher than that in the standard accretion-disk model. We explored the radiation properties of an accretion disk at its sonic point around a black hole with a time-steady torque exerted on the inner edge of the disk. The local structure of the accretion flow at the sonic point was investigated in the frame of general relativity. It was found that the accretion flow would be optically thin at its sonic point for most cases, if the additional accretion efficiency $\delta\epsilon$ caused by the torque is as high as $\sim 10\%$. The results imply that a variable torque may trigger transitions of the flow between different accretion types.