[ITAL]Chandra[/ITAL] Detection of Local O [CSC]vii[/CSC] H[CLC]e[/CLC]α Absorption along the Sight Line toward 3C 273

With the Chandra X-ray Telescope we have detected a zero-redshift OVIIHe-alpha absorption line along the sight line toward 3C 273. This linedetection is highly significant, with a signal-to-noise ratio (S/N) of 6.4. Weexplore two models, which associate this line with (1) the intragroup gas inthe Local Group, and (2) the hot halo gas in the vicinity of our Milky Way. Inthe first model, we find that for a standard beta-model of the gas distributionin the Local Group, the temperature is constrained to 2.3e5 < T < 1.2e6 K andthe baryon overdensity is delta_b ~ 100; both results are consistent with theproperties of the so-called warm-hot intergalactic medium (WHIM) predicted bycosmological simulations. We also find the core radius of the gas distributionshould be > 100 kpc. In the second model we discuss several possible Galacticorigins for the absorption, and we comment on the possibility that the OVII isassociated with the OVI absorption observed in this direction by the FarUltraviolet Spectroscopic Explorer (FUSE). We find that there is a strongindication that collisional ionization is the dominant ionization source forthe observed absorption.Comment: 17 pages, 3 figures. Accepted for publication in ApJ Letter