Probing the Local Group Medium toward Markarian 421 with Chandra and the Far Ultraviolet Spectroscopic Explorer

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-1 < b < 55 km s-1. 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 VIHVC/O VII ratio, implying that the HVC is probably not related to the O VII. In addition, the O VIKα line 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.