# Global Radiation Magneto-hydrodynamic Simulations of Sub-Eddington Accretion Disks around Supermassive Black Holes

## TL;DR

This study employs 3D radiation magneto-hydrodynamic simulations to explore the structure and behavior of sub-Eddington accretion disks around supermassive black holes, revealing differences from standard models and implications for AGN coronae.

## Contribution

It provides new insights into the magnetic and radiation-driven dynamics of accretion disks at sub-Eddington rates, challenging standard thin disk assumptions.

## Key findings

- Disks are supported by magnetic pressure with larger scale height.
- No thermal instability observed over many thermal timescales.
- Coronae are generated only in the inner regions, with sizes consistent with observations.

## Abstract

We use global three dimensional radiation magneto-hydrodynamic simulations to study the properties of inner regions of accretion disks around a 5\times 10^8 solar mass black hole with mass accretion rates reaching 7% and 20% of the Eddington value. This region of the disk is supported by magnetic pressure with surface density significantly smaller than the values predicted by the standard thin disk model but with a much larger disk scale height. The disks do not show any sign of thermal instability over many thermal time scales. More than half of the accretion is driven by radiation viscosity in the optically thin corona region for the lower accretion rate case, while accretion in the optically thick part of the disk is driven by the Maxwell and Reynolds stresses from MRI turbulence. Coronae with gas temperatures > 10^8 K are generated only in the inner \approx 10 gravitational radii in both simulations, being more compact in the higher accretion rate case. In contrast to the thin disk model, surface density increases with increasing mass accretion rate, which causes less dissipation in the optically thin region and a relatively weaker corona. The simulation results may explain the formation of X-ray coronae in Active Galactic Nuclei (AGNs), the compact size of such coronae, and the observed trend of optical to X-ray luminosity with Eddington ratio for many AGNs.