CNO Line Radiation and the X-ray Bowen Fluorescence Mechanism in Optically-Thick, Highly-Ionized Media

Abstract

Radiative transfer effects due to overlapping X-ray lines in a high-temperature, optically-thick, highly-ionized medium are investigated. One particular example, where the O VIII Ly-alpha doublet coincide in frequency with the N VII Ly-zeta lines is studied in detail to illustrate the effects on the properties of the emergent line spectrum. We solve the radiative transfer equation to study the energy transport of resonance line radiation in a static, infinite, plane-parallel geometry, which is used to compute the destruction/escape probabilities for each of the lines for various total optical thicknesses of the medium, as well as destruction probabilities by sources of underlying photoelectric opacity. It is found that a large fraction of the O VIII Ly-alpha line radiation can be destroyed by N VII, which can result in an reversal of the O VIII Ly-alpha/N VII Ly-alpha line intensity ratio similar to what may be seen under non-solar abundances. Photoelectric absorption by ionized carbon and nitrogen can also subsequently increase the emission line intensities of these ions. We show that line ratios, which are directly proportional to the abundance ratios in optically thin plasmas, are not good indicators of the true CNO abundances. Conversely, global spectral modeling that assumes optically thin conditions may yield incorrect abundance estimates when compared to observations, especially if the optical depth is large. Other potentially important overlapping lines and continua in the X-ray band are also identified and their possible relevance to recent high resolution spectroscopic observations with Chandra and XMM-Newton are briefly discussed.