Canonical X-Ray Fluorescence Line Intensities as Column Density Indicators

Abstract

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 K$\alpha$ line at $\lambda =1.937$Å(6.4\,keV). This paper presents a survey of all well-measured Chandra/HETG grating spectra featuring several K$\alpha$ fluorescence lines from elements between Mg and Ni. Despite the variety of sources and physical conditions, we identify a common trend that dictates the K$\alpha$ line intensity ratios between elements. For the most part, the line intensities are well described by a simple, plane-parallel approximation of a near-neutral, solar-abundance, high column density ($N_{\text{H}}>10^{24}$ cm${}^{-2}$) medium. This approximation gives canonical photon-intensity line ratios for the K$\alpha$ fluorescence of all elements, e.g., 0.104:\,0.069:\,1.0:\,0.043 for Si:\,S:\,Fe:\,Ni, respectively. Deviations from these ratios are shown to be primarily due to excess column density along the line of sight beyond the Galactic column. Therefore, measured fluorescence line ratios provide an independent estimate of $N_{\text{H}}$ and insight into the environment of accretion sources. Residual discrepancies with the canonical ratios could be due to a variety of effects such as a fluorescing medium with $N_{\text{H}}<10^{24}$\,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.

arXiv commentSubmitted to ApJ, comments welcome

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