Radiation MHD Simulations of Soft X-ray Emitting Regions in Changing Look AGN

TL;DR

This study uses 3D radiation magnetohydrodynamic simulations to model the soft X-ray excess in changing look AGN, revealing a warm, optically thick region formed by inverse Compton cooling that matches observed luminosities.

Contribution

It presents the first global 3D MHD simulation demonstrating the formation of a warm, Thomson-thick region in AGN during state transitions, aligning with observed soft X-ray excess in CLAGN.

Findings

Simulated luminosity matches observed CLAGN luminosities (0.01-0.08 L_Edd).

Warm region formed by inverse Compton cooling in the simulation.

Magnetic reconnection heats the warm region, affecting its temperature.

Abstract

Strong soft X-ray emission called soft X-ray excess is often observed in luminous active galactic nuclei (AGN). It has been suggested that the soft X-rays are emitted from a warm ($T=10^6\sim10^7\ \rm{K}$) region that is optically thick for the Thomson scattering (warm Comptonization region). Motivated by the recent observations that soft X-ray excess appears in changing look AGN (CLAGN) during the state transition from a dim state without broad emission lines to a bright state with broad emission lines, we performed global three-dimensional radiation magnetohydrodynamic simulations assuming that the mass accretion rate increases and becomes around $10$\% of the Eddington accretion rate. The simulation successfully reproduces a warm, Thomson-thick region outside the hot radiatively inefficient accretion flow near the black hole. The warm region is formed by efficient radiative cooling due to inverse Compton scattering. The calculated luminosity $0.01L_{{\rm Edd}}-0.08L_{{\rm Edd}}$ is consistent with the luminosity of CLAGN. We also found that the warm Comptonization region is well described by the steady model of magnetized disks supported by azimuthal magnetic fields. When the anti-parallel azimuthal magnetic fields supporting the radiatively cooled region reconnect around the equatorial plane of the disk, the temperature of the region becomes higher by releasing the magnetic energy transported to the region.