Radiation Magnetohydrodynamic Simulations of Sub-Eddington accretion Flows in AGN: Origin of Soft X-ray Excess and Rapid Time Variabilities

TL;DR

This study uses radiation magnetohydrodynamic simulations to explore how dense, cool regions form within hot accretion flows in AGN, explaining the origin of soft X-ray excess and rapid variability in changing-look AGN.

Contribution

It introduces a new simulation approach revealing the coexistence of hot and cool accretion flows, explaining observed X-ray features and variability in AGN.

Findings

Formation of a cool, Thomson-thick region in accretion flows at high accretion rates

Coexistence of hot and cool flows explains soft and hard X-ray emissions

Quasi-periodic oscillations in the soft X-ray region may cause rapid variability

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 ($L_{\mathrm{Edd}}$). 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.