Radiation and Magnetic Pressure Support in Accretion Disks around Supermassive Black Holes and The Physical Origin of the Extreme Ultraviolet to Soft X-ray Spectrum

TL;DR

This study uses 3D radiation magnetohydrodynamic simulations to explore how radiation and magnetic pressures influence accretion disks around supermassive black holes, explaining the origin of the extreme ultraviolet to soft X-ray spectrum in quasars.

Contribution

It demonstrates the physical mechanisms producing the EUV to soft X-ray continuum in accretion disks, highlighting the role of magnetic and radiation pressures at different accretion rates.

Findings

Disks can be radiation or magnetic pressure dominated depending on cooling and inflow timescales.

High accretion rates produce a power-law spectrum between 10 eV and 1 keV, similar to observations.

Low accretion rate disks lack the high-energy power-law component.

Abstract

We present the results of four three-dimensional radiation magnetohydrodynamic simulations of accretion disks around a $10^8$ solar mass black hole, which produce the far ultraviolet spectrum peak and demonstrate a robust physical mechanism to produce the extreme ultraviolet to soft X-ray power-law continuum component. The disks are fed from rotating tori and reach accretion rates ranging from $0.03$ to $4$ times the Eddington value. The disks become radiation pressure or magnetic pressure dominated depending on the relative timescales of radiative cooling and gas inflow. Magnetic pressure supported disks can form with or without net poloidal magnetic fields as long as the inflowing gas can cool quickly enough, which can typically happen when the accretion rate is low. We calculate the emerging spectra from these disks using multi-group radiation transport with realistic opacities and find that they typically peak around $10$ eV. At accretion rates close to or above the Eddington limit, a power-law component can appear for photon energies between $10$ eV and 1 keV with a spectral slope varying between $L_\nu\propto\nu^{-1}$ and $\nu^{-2}$, comparable to what is observed in radio quiet quasars. The disk with $3\%$ Eddington accretion rate does not exhibit this component. These high energy photons are produced in an optically thick region $\approx 30^{\circ}-45^{\circ}$ from the disk midplane by compressible bulk Comptonization within the converging accretion flow. Strongly magnetized disks that have a very small surface density will produce a spectrum that is very different from what is observed.