Photoionization Models for High Density Gas

TL;DR

This paper develops high-density photoionization models for gas near compact objects, incorporating atomic processes affected by high densities and radiation intensities to improve interpretation of relativistic iron lines.

Contribution

It introduces atomic rate coefficients suitable for high-density environments and applies them to photoionization calculations, advancing modeling accuracy for accretion-powered sources.

Findings

Produced ionization balance curves for high-density gas.

Calculated X-ray emissivities and opacities relevant to near-compact object environments.

Enhanced models account for stimulated processes and ion interactions at high densities.

Abstract

Relativistically broadened and redshifted 6.4 -- 6.9 keV iron K lines are observed from many accretion powered objects, including X-ray binaries and active galactic nuclei (AGN). Existence of gas close to the central engine implies large radiation intensities and correspondingly large gas densities if the gas is to remain partially ionized. Simple estimates indicate that high gas densities are needed to allow survival of iron against ionization. These are high enough that rates for many atomic processes are affected by mechanisms related to interactions with nearby ions and electrons. Radiation intensities are high enough that stimulated processes can be important. Most models currently in use for interpreting relativistic lines use atomic rate coefficients designed for use at low densities and neglect stimulated processes. In our work so far we have presented atomic structure calculations with the goal of providing physically appropriate models at densities consistent with line-emitting gas near compact objects. In this paper we apply these rates to photoionization calculations, and produce ionization balance curves and X-ray emissivities and opacities which are appropriate for high densities and high radiation intensities. The final step in our program will be presented in a subsequent paper: Model atmosphere calculations which incorporate these rates into synthetic spectra.