A search for the missing baryons with X--ray absorption lines towards the blazar 1ES 1553+113

This paper presents an analysis of XMM X--ray spectra of the quasar 1ES 1553+113, in search for absorption lines from the intervening warm--hot intergalactic medium. A search for OVII, OVIII and NeIX resonance absorption lines was performed at eight fixed redshifts that feature OVI or HI broad Lyman--$α$ absorption lines that were previously detected from HST data. The search yielded one possible detection of OVII at a redshift z=0.1877 with an OVI prior, with a statistical significance that is equivalent to a 2.6-$σ$ confidence level. The spectra were also stacked at the wavelengths of the expected redshifted OVII and OVIII lines, but the analysis did not reveal evidence for the presence of additional X--ray absorbing WHIM. Moreover, the spectra were used to investigate two putative OVII absorption lines that were detected serendipitously in an earlier analysis of the same data by F. Nicastro and collaborators. The paper also presents a comprehensive statistical framework for cosmological inferences from the analysis of absorption lines, which makes use of cosmological simulations for the joint probability distributions of FUV and X--ray ions. Accordingly, we conclude that the new possible OVII absorption at z=0.1877 is consistent with a contribution from the hot WHIM to the baryon density in an amount of $Ω_{WHIM,X}/Ω_b = 44\pm22$\%. However, there are large systematic uncertainties associated with the temperature and abundances of the absorbers, and only a larger sample of X-ray sources can provide an accurate determination of the cosmological density of the WHIM.