The X-Ray Spectral Properties of Photoionized Plasmas and Transient Plasmas

Abstract

I present the fundamental concepts of X-ray spectral formation in X-ray photoionized plasmas and transient plasmas, emphasizing the role of atomic kinetics in determining the attendant spectral characteristics.

Objects which harbor compact sites of hard X-ray production — active galactic nuclei, X-ray binaries, and cataclysmic variables — are the domains of photoionized plasmas. With a focus on the basic elements of discrete spectroscopy, I discuss the unique properties of X-ray spectra in such environments. As a prelude, an introduction to the theory of C,omptonization is provided, including a full derivation of the Kompaneets Equation, and its application to heating and cooling in X-ray nebulae. In the discussions on line spectroscopy, the mechanisms behind various plasma diagnostics are described, including recombination continua, Δn = 0 dielectronic recombination, density diagnostics, and Kα fluorescence.

The second topic of this chapter, the effect on X-ray spectra of time-dependent ionization conditions, is restricted to ionizing plasmas dominated by electron-ion impact processes. This scenario is motivated by impulsively heated plasmas, such as occur in supernova remnants and solar flares. Treated in this way, transient ionization is an extension of collisional ionization equilibrium. I present the fundamentals of ionization dynamics, followed by a numerical solution of the ionization equations for oxygen, a case study intended to illustrate the spectroscopic consequences of rapid ionization.