2009ApJ...696.1418Y

High-resolution X-ray absorption spectroscopy is a powerful diagnostic tool for probing chemical and physical properties of the interstellar medium (ISM) at various phases. We present detections of K transition absorption lines from the low-ionization ions of O I, O II, Ne I, Ne II, and Ne III, and the high-ionization ones of O VI, O VII, O VIII, Ne IX, and Mg XI, as well as details of neutral absorption edges from Mg, Ne, and O in an unprecedented high-quality spectrum of the low-mass X-ray binary Cyg X-2. These absorption features trace the intervening ISM which is indicated by the unshifted line centroids with respect to the rest-frame wavelengths of the corresponding atomic transitions. We have measured the column densities of each ion. We complement these measurements with the radio H I and optical Hα observations toward the same sight line and estimate the mean abundances of Ne, O, and Mg in the cool phase to Ne/H = 0.84^+0.13_–0.10x10^–4, O/H = 3.83^+0.48 _–0.43x10^–4, and Mg/H = 0.35^+0.09_–0.11x 10^–4, and O and Mg in the hot phase to O/H = 5.81^+1.30_–1.34x10^–4 and Mg/H = 0.33^+0.09 _–0.09x10^–4, respectively. These results indicate a mild depletion of oxygen into dust grains in the cool phase and little or no depletion of magnesium. We also find that absorption from highly ionized ions in the hot Galactic disk gas can account for most of the absorption observed toward the extragalactic sight lines like Mrk 421. The bulk of the observed O VI likely originates from the conductive interfaces between the cool and hot gases, from which a significant amount of N V and C IV emission is predicted.