Kavli Affiliate: Claude Canizares
| First 5 Authors: Taotao Fang, Christopher McKee, Claude Canizares, Mark Wolfire,
| Summary:
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.
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