Radiation Magnetohydrodynamic Simulations of Sub-Eddington Accretion Flows in AGNs: Origin of Soft X-Ray Excess and Rapid Time Variabilities

Abstract

Abstract We investigate the origin of the soft X-ray excess component in Seyfert galaxies observed when their luminosity exceeds 0.1% of the Eddington luminosity ( ). The evolution of a dense blob in radiatively inefficient accretion flow (RIAF) is simulated by applying a radiation magnetohydrodynamic code, CANS+R. When the accretion rate onto a black hole exceeds 10% of the Eddington accretion rate ( , where c is the speed of light), the dense blob shrinks vertically because of radiative cooling and forms a Thomson thick, relatively cool (∼107–8 K) region. The cool region coexists with the optically thin, hot ( ) RIAF near the black hole. The cool disk is responsible for the soft X-ray emission, while hard X-rays are emitted from the hot inner accretion flow. Such a hybrid structure of hot and cool accretion flows is consistent with the observations of both hard and soft X-ray emissions from “changing-look” active galactic nuclei (CLAGNs). Furthermore, we find that quasi-periodic oscillations (QPOs) are excited in the soft X-ray-emitting region. These oscillations can be the origin of rapid X-ray time variabilities observed in CLAGNs.