2005ApJ...631..856W

We report the detection of highly ionized gas at z∼0 seen in resonant UV and X-ray absorption lines toward the z=0.03 blazar Mrk 421. A total of 13 X-ray and three UV lines were measured (or upper limits derived), including three lines in the O VII K series and Kα transitions from neon, carbon, and nitrogen. From the three O VII lines we derive a 2 σ Doppler parameter constraint of 24 km/s<b<55 km/s. The FUSE spectrum shows strong Galactic low-velocity O VI λ1032 absorption and a possible weak O VI high-velocity component (HVC). The Doppler parameter of the low-velocity O VI measured with FUSE is ∼3 σ higher than that derived from the O VII line ratios, indicating that the O VII and Galactic O VI arise in different phases. This velocity dispersion along with limits on the gas temperature and density from the X-ray line ratios (assuming a single phase with collisional ionization equilibrium plus photoionization) are all consistent with an extragalactic absorber. However, the O VII Doppler parameter is inconsistent with the high temperature required to produce the observed O VI_HVC/O VII ratio, implying that the HVC is probably not related to the O VII. In addition, the O VI_Kαline detected by Chandra implies a column density ∼4 times higher than the λ1032 absorption. Although an extragalactic absorber is fully consistent with the measured column density ratios, a Galactic origin cannot be ruled out given the uncertainties in the available data.