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

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 ( ${L}_{\mathrm{Edd}}$ ). 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 ${10}^{7}\,{M}_{\odot }$ black hole exceeds 10% of the Eddington accretion rate ( ${\dot{M}}_{\mathrm{Edd}}={L}_{\mathrm{Edd}}/{c}^{2}$ , where c is the speed of light), the dense blob shrinks vertically because of radiative cooling and forms a Thomson thick, relatively cool (∼10^7-8 K) region. The cool region coexists with the optically thin, hot ( $T\sim {10}^{11}\,{\rm{K}}$ ) 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.