X‐Ray Absorption Line Spectroscopy of the Galactic Hot Interstellar Medium

Abstract

We present an X-ray absorption line spectroscopic study of the large-scale hot interstellar medium (HISM) in the Galaxy. We detect Ne IX K$_\\alpha$ absorption lines in the {\\sl Chandra} grating spectra of seven Galactic low-mass X-ray binaries. Three of these sources also show absorption of O VII K$_\\alpha$, O VII K$_\\beta$, and/or O VIII K$_\\alpha$. Both centroid and width of the lines are consistent with a Galactic HISM origin of the absorption. By jointly fitting the multiple lines, accounting for line saturation and assuming the collisional ionization equilibrium, we estimate the average absorbing gas temperature as $\\sim 2.4\\pm0.3\\times10^6$ K (90% confidence errors). We further characterize the spatial density distribution of the gas as ${\\rm 6.4_{-1.7}^{+2.4} exp(-|z|/1.2_{-0.5}^{+1.0} kpc) \\times10^{-3}{\\rm cm^{-3}}} $ (a disk morphology) or ${\\rm 6.2_{-2.1}^{+3.8} [ 1 + (R/2.3_{-1.1}^{+1.6} kpc)^2]^{-1} \\times10^{-3} {\\rm cm^{-3}}}$ (a sphere morphology), where $z$ and $R$ are the distances from the Galactic plane and Galactic center (GC) respectively. Since nearly all the sight-lines with significant absorption lines detected are somewhat toward GC and at low Galactic latitudes, these results could be severely biased. More observations toward off-GC sight-lines and at high latitudes are urgently needed to further the study. Nevertheless, the results demonstrate the excellent potential of X-ray absorption line spectroscopy in the study of the HISM.