Ionization structure and Fe K$\alpha$ energy for irradiated accretion disks

TL;DR

This paper models the ionization structure and iron Kα line energies in accretion disks around black holes, comparing predictions with observations and proposing methods to estimate disk viscosity using variable line features.

Contribution

It provides a detailed theoretical framework for the ionization and iron line energies in accretion disks, and compares these with observational data from multiple X-ray observatories.

Findings

Identifies a forbidden region where highly-ionized iron lines cannot originate at low Eddington ratios.

Shows consistency between predicted and observed iron line energies in AGN.

Suggests using variable iron Kα lines to estimate accretion disk viscosity.

Abstract

We study the radial ionization structure at the surface of an X-ray illuminated accretion disk. We plot the expected iron K$\alpha$ line energy as a function of the Eddington ratio and of the distance of the emitting matter from the central source, for a non-rotating and a maximally-rotating black hole. We compare the predicted disk line energies with those measured in an archival sample of active galactic nuclei observed with {\it Chandra}, {\it XMM-Newton} and {\it Suzaku}, and discuss whether the line energies are consistent with the radial distances inferred from reverberation studies. We also suggest using rapidly-variable iron K$\alpha$ lines to estimate the viscosity parameter of an accretion disk. There is a forbidden region in the line energy versus Eddington ratio plane, at low Eddington ratios, where an accretion disk cannot produce highly-ionized iron K$\alpha$ lines. If such emission is observed in low-Eddington-ratio sources, it is either coming from a highly-ionized outflow, or is a blue-shifted component from fast-moving neutral matter.