A Galactic Origin for the Local Ionized X-ray Absorbers

Abstract

Recent Chandra and XMM-Newton 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-Newton archival data. About 40% of the targets show local O VII He α absorption with column densities around 10[superscript 16] cm[superscript -2]; in particular, O VII absorption is present in all the high-quality spectra. We estimate that the sky covering fraction of this O VII-absorbing gas is at least 63%, at 90% confidence, and likely to be unity given enough high-quality spectra. On the basis of (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 tests of this result.