2020ApJ...902..103I

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 10⁷M_☉ black hole exceeds 10% of the Eddington accretion rate ({dot}M_Edd=L_Edd/c², 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∼10¹¹K) 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.