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-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-circle dot. black hole exceeds 10% of the Eddington accretion rate ((M) over dot(Edd) = L-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 (similar to 10(7-8) K) region. The cool region coexists with the optically thin, hot (T similar to 10(11) 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 quasiperiodic 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.