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 similar to 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 K alpha transitions from neon, carbon, and nitrogen. From the three O VII lines we derive a 2 sigma Doppler parameter constraint of 24 km s(-1) < b < 55 km s(-1). The FUSE spectrum shows strong Galactic low-velocity O VI lambda 1032 absorption and a possible weak O vi high-velocity component (HVC). The Doppler parameter of the low-velocity O vi measured with FUSE is similar to 3 sigma 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 ratio, implying that the HVC is probably not related to the O VII. In addition, the O VIK alpha line detected by Chandra implies a column density similar to 4 times higher than the lambda 1032 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.