Canonical X-Ray Fluorescence Line Intensities as Column Density Indicators in X-Ray Binaries Roi Rahin and Ehud Behar Department of Physics, Technion, Haifa 32000, Israel 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. Conclusions Several conclusion follow from our analysis: • Kα intensity ratios between elements follow similar trends in most sources, except for a few cases in which the lower-Z lines (Mg, Si) are reduced by orders of magnitude. • For the most part, the relative Kα intensities follow a simple plane-parallel approximation of a dense, near-neutral optically thick medium. • The reduced Kα intensities in the low-Z elements is explained satisfactorily and self-consistently by excess column density along the line of sight. This excess column is corroborated by independent measurements • Kα intensity ratios thus can be used to measure ambient column density in the X-ray binary system Excess column density and canonical values X-ray Fluorescence Relative Line Intensities The relative intensities of X-ray fluorescence lines are derivable from fundamental physics under a few simple assumptions. Using a simple plane-parallel approximation, we derive line intensities using only the source spectrum I, the elemental abundances AZ, the fluorescence yield ωKα, and the reflector column density. By assuming an optically thick reflecting medium, we reduce the dependency to be on the source spectrum, abundances, and fluorescence yield. The resulting expression is: We compare the relative intensities derived from this model to observations. The figures clearly show a discrepancy that is more pronounced for low-Z elements. Notice especially GX 301-2, where disparity of two orders of magnitude is visible for Si. Another discrepancy is between different Vela X-1 phases, where it is clear the abundances cannot change, and the source spectrum is unlikely to change drastically. The model gives a good fit for NGC 1068 and MRK 3, implying a missing ingredient in the model for the other sources. We find excess line-of-sight column density as a possible solution. Top: relative intensities of Kα fluorescence lines originating from different column densities of the fluorescing medium. Bottom: relative intensities for different power-law indices for an optically thick medium Relative intensities of Kα fluorescence lines for an optically thick medium compared to actual observations from various sources. Relative flux of Kα fluorescence lines compared to theoretical values after excess column density correction. The green line in the bottom figure corresponds to a finite reflector medium with 𝑁𝐻= 2 × 1023cm−2 We propose a simple explanation for the relatively weak low-Z lines. Since the deficiency is Z- dependent, excess line-of-sight column density beyond the Galactic column could explain the observed ratios. If this explanation is correct, a single column density value must account for the quenching of a