Chandra and Far Ultraviolet Spectroscopic Explorer Observations of z ~ 0 Warm-Hot Gas toward PKS 2155–304

Abstract

The X-ray-bright z = 0.116 quasar PKS 2155-304 is frequently observed as a Chandra calibration source, with a total of 483 ks of Low Energy Transmission Grating (LETG) exposure time accumulated through May 2006. Highly ionized metal absorption lines, including numerous lines at z = 0 and a putative O VIII Kα line at z = 0.055, have been reported in past Chandra studies of this source. Using all available Chandra LETG spectra and analysis techniques developed for such z = 0 X-ray absorption along other sight lines, we revisit these previous detections. We detect four absorption lines at >3 σ significance (O VII Kα/β, O VIII Kα, and Ne IX Kα), with O VII Kα being a 7.3 σ detection. The 1 σ range of z = 0 O VII column density and Doppler parameter are consistent with that derived for Mrk 421 and within 2 σ of the Mrk 279 absorption. Temperatures and densities inferred from the relative O VII and other ionic column densities are found to be consistent with either the local warm-hot intergalactic medium or a Galactic corona. Unlike the local X-ray absorbers seen in other sight lines, a link with the low- or high-velocity far-ultraviolet O VI absorption lines cannot be ruled out. The z = 0.055 O VIII absorption reported by Fang et al. is seen with 3.5 σ confidence in the ACIS/LETG spectrum, but no other absorption lines are found at the same redshift.