High resolution X-ray spectroscopy of the multiphase interstellar medium toward Cyg X-2

TL;DR

This study uses high-resolution X-ray spectroscopy to analyze the interstellar medium along the line of sight to Cyg X-2, revealing detailed ionization states, abundances, and physical conditions of the gas in different phases.

Contribution

It provides the first high-quality spectrum detection of multiple ionization states of elements in the ISM and estimates their abundances, enhancing understanding of the multiphase interstellar medium.

Findings

Detected multiple ionization states of O, Ne, Mg in the ISM.

Estimated elemental abundances and depletion levels in cool and hot phases.

Attributed OVI absorption to interfaces between cool and hot gas.

Abstract

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 OI, OII, NeI, NeII, and NeIII, and the high ionization ones of OVI, OVII, OVIII, NeIX, and MgXI, 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 interstellar medium 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 HI and optical Halpha 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}_{-0.10}\times10^{-4}, O/H=3.83^{+0.48}_{-0.43}\times10^{-4}, and Mg/H=0.35^{+0.09}_{-0.11}\times10^{-4}, and O and Mg in the hot phase to O/H=5.81^{+1.30}_{-1.34}\times10^{-4} and Mg/H=0.33^{+0.09}_{-0.09}\times10^{-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 OVI likely originates from the conductive interfaces between the cool and hot gases, from which a significant amount of NV and CIV emission is predicted.