With the Chandra X-ray Telescope we have detected a zero-redshift O vii He \alpha absorption line along the sight line toward 3C 273 .
This line detection is highly significant , with a signal-to-noise ratio ( S/N ) 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 the first model , we find that for a standard \beta -model of the gas distribution in the Local Group , the temperature is constrained to 2.3 \times 10 ^ { 5 } < T < 1.2 \times 10 ^ { 6 } K and the baryon overdensity is \delta _ { b } \sim 100 ; both results are consistent with the properties of the so-called warm-hot intergalactic medium ( WHIM ) predicted by cosmological simulations .
We also find the core radius of the gas distribution should be > 100 kpc .
In the second model we discuss several possible Galactic origins for the absorption , and we comment on the possibility that the O vii is associated with the O vi absorption observed in this direction by the Far Ultraviolet Spectroscopic Explorer ( \sl FUSE ) .
We find that there is a strong indication that collisional ionization is the dominant ionization source for the observed absorption .