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 K α absorption lines in the Chandra grating spectra of seven Galactic low-mass X-ray bi-naries. Three of these sources also show absorption of O VII K α , O VII K β , and/or O VIII K α. 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 ∼ 2.4 ± 0.3 × 10 6 K (90% confidence errors). We further characterize the spatial density distribution of the gas as 6.4