X-ray absorption lines in FUV-detected quasars: I. Sample and analysis of the <i>XMM–Newton</i> and <i>Chandra</i> data

ABSTRACT This paper presents initial results of a systematic search for resonance X-ray absorption lines from H–like O VIII and $He$–like O vii caused by the intervening warm–hot intergalactic medium (WHIM). The search is based on far ultra–violet redshift priors from O vi and H i broad Lyman $\alpha$ lines that were previously detected by HST and FUSE in a sample of 51 sources with either XMM–Newton or Chandra data, for a total X-ray redshift path of $\Delta z=10.9$. Of the 1224 absorption–line systems with FUV priors that were analysed, 33 systems feature an absorption–line feature detected with $\ge 99$ % confidence at the same redshift of the FUV prior, some coincident with previously reported absorption line detections. The ultimate goal of this search is to test the hypothesis that X-ray absorbing WHIM gas is the repository of the missing baryons in the local universe. Further results and the cosmological implications of this analysis are presented in a companion paper.