The formation of X-ray spectral features due to iron in a relatively cool cloud of gas with a Thomson depth greater than 1 surrounding a compact source of continuum X-rays is described. Coupled equations are solved for the ionization structure of the cloud and for the radiative transfer of the X-rays. Photoionization suppresses the strength of emission lines and absorption edges. Comptonization of the radiation broadens emission lines, fills in absorption edges, and produces a high-energy cutoff. In order to describe multiple scattering, a Fokker-Planck equation is derived which includes an important modification of the Kompaneets equation. Narrow resonance lines are treated with an escape probability formalism.
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Ross et al. (1978) studied this question.