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

We present an X-ray absorption line spectroscopic study of the large-scale hot interstellar medium (HISM) in the Galaxy. We detect Ne IX Kα absorption lines in the Chandra grating spectra of seven Galactic low-mass X-ray binaries. Three of these sources also show absorption of O VII Kα, O VII Kβ, and/or O VIII Kα. Both the 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 ~(2.4 ± 0.3) × 10⁶ K (90% confidence errors). We further characterize the spatial density distribution of the gas as 6.4 exp(-|z|/1.2 kpc) × 10^-3 cm^-3 (a disk morphology) or 6.2[1 + (R/2.3 kpc)²]^-1 × 10^-3 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.