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X-ray line fluorescence is ubiquitous around powerful accretion sources, such as 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. 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. We interpret deviations from these intensity ratios as due to excess column density along the line of sight beyond the Galactic column. Specifically for Vela X-1 and GX 301-2, our method shows phase-dependent estimates of the excess column, which sheds light on the varying environment of the binaries as the system rotates. In GX 301-2, we can differentiate between the excess obstruction on the line of sight towards the ambient ionized gas and the neutral fluorescing medium.

Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity ($L_{\mathrm{X}}$) per star formation rate (SFR) with redshift due to the decrease of metallicity ($Z$) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 $z\sim2$ star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for $\sim1500$ galaxies in the CANDELS fields at $1.37, Comment: 22 pages, 8 figures, 2 tables