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 ksec 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 OVIII K-alpha 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 sightlines, we revisit these previous detections. We detect 4 absorption lines at >3σsignificance (OVII K-alpha/beta, OVIII K-alpha, and NeIX K-alpha), with OVII K-alpha being a 7.3σdetection. The 1σranges of z=0 OVII column density and Doppler parameter are consistent with those derived for Mrk 421 and within 2σof the Mrk 279 absorption. Temperatures and densities inferred from the relative OVII 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 sightlines, a link with the low- or high-velocity far-ultraviolet OVI absorption lines cannot be ruled out. The z=0.055 OVIII absorption reported by Fang et al. is seen with 3.5σconfidence in the ACIS/LETG spectrum, but no other absorption lines are found at the same redshift.