X-ray fluorescence lines as probes of the gaseous environment in accretion sources

X-ray line fluorescence is ubiquitous around powerful accretion sources, namely active galactic nuclei and X-ray binaries. The brightest and best-studied line is the Fe Kα line at 6.4 keV, but the Kα lines of other elements hold essential information about the source. We present a survey of well-measured Chandra/HETG grating spectra featuring several Kα fluorescence lines from elements between Mg and Ni. Despite the variety of sources and physical conditions, we identify a common trend that dictates the Kα line intensity ratios between elements. The line intensities are well described by a simple, plane-parallel approximation of a near-neutral, solar-abundance, high column density (> 10²⁴ cm⁻²) medium. This approximation gives canonical photon-intensity line ratios for the Kα fluorescence of all elements, e.g., 0.104: 0.069: 1.0: 0.043 for Si: S: Fe: Ni, respectively

We show that deviations from these intensity ratios are due to excess column density along the line of sight beyond the Galactic column. Therefore, measured fluorescence line ratios provide an independent estimate of column density and insight into the environment of accretion sources. The presentation will demonstrate this power of line ratio diagnostics to investigate the phase-dependent emission properties of Vela X-1 and GX 301-2. This analysis of fluorescence lines implies their source is at a different location than the highly ionized plasma in these X-ray binaries. Residual discrepancies with the canonical ratios could be due to a variety of effects such as a fluorescing medium with NH<10²⁴ cm⁻², a non-neutral medium, variations in the illuminating spectrum, non-solar abundances, or an irregular source geometry. However, evidently and perhaps surprisingly, these are uncommon, and their effect remains minor