Chandra detection of local O VII Heα absorption along the sight line toward 3C 273

We detect a zero-redshift O VII Healpha absorption line with a signal-to-noise ratio of 6.4. We explore two models, which associate this line with (1) the intragroup gas in the Local Group and (2) the hot halo gas in the vicinity of our Milky Way. In model 1, we find for a standard beta-model of the gas distribution that the temperature is 2.3 x 10(5) K < T < 1.2 x 10(6) K and the baryon overdensity is delta(b) similar to 100; both results are consistent with the properties of the so-called warm-hot intergalactic medium predicted by cosmological simulations. We also find the core radius of the gas distribution should be greater than 100 kpc. In model 2, we explore several possible Galactic origins for the absorption, including the possibility that the O VII is associated with the O vi absorption observed in this direction by the Far Ultraviolet Spectroscopic Explorer. We find a strong indication that collisional ionization is the dominant ionization source for the observed absorption.