A Galactic Origin for the Local Ionized X-ray Absorbers

Abstract

Recent Chandra and XMM observations of distant quasars have shown strong local (z~0) X-ray absorption lines from highly ionized gas, primarily He-like oxygen. The nature of these X-ray absorbers, i.e., whether they are part of the hot gas associated with the Milky Way or part of the intragroup medium in the Local Group, remains a puzzle due to the uncertainties in the distance. We present in this paper a survey of 20 AGNs with Chandra and XMM archival data. About 40% of the targets show local OVII He-alpha absorption with column densities around 10^16 cm^-2; in particular, OVII absorption is present in all the high quality spectra. We estimate that the sky covering fraction of this OVII-absorbing gas is at least 63%, at 90% confidence, and likely to be unity given enough high-quality spectra. Based on (1) the expected number of absorbers along sight lines toward distant AGNs, (2) joint analysis with X-ray emission measurements, and (3) mass estimation, we argue that the observed X-ray absorbers are part of the hot gas associated with our Galaxy. Future observations will significantly improve our understanding of the covering fraction and provide robust test of this result.