Radiation Magnetohydrodynamic Simulations of Sub-Eddington accretion
  Flows in AGN: 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
$10^7M_{\odot}$ black hole exceeds 10% of the Eddington accretion rate ($\dot
M_{\rm Edd}=L_{\rm Edd}/c^2$, where $c$ is the speed of light)}, the dense blob
shrinks vertically because of radiative cooling and forms a Thomson thick,
relatively cool ($\sim10^{7-8}$ K) region. The cool region coexists with the
optically thin, hot ($T\sim10^{11}~\mathrm{K}$) 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. The soft X-ray emitting region
coexists with the optically thin, hot ($T \sim 10^{11}~\mathrm{K}$),
radiatively inefficient accretion flow (RIAF) near the black hole. 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 (CLAGN). 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 CLAGN.
