Is X-ray Absorption from Quasar Absorption Line Systems Detectable?

Using the known properties and statistics of intervening quasar absorption systems, we calculate the cumulative absorption out to high redshifts at soft X-ray (E < 1 keV) energies. Although photoelectric absorption in quasar absorption systems provides a signiicant source of opacity at the lowest X-ray energies, the imprint of this absorption is not readily observable since it is generally masked by the much stronger Galactic foreground absorption. In particular, the intense HeII equivalent of thèLyman valley' at energies E < 100 eV caused by cumulative absorption in the singly ionized helium component of the Lyman forest is eeectively hidden from view. Detectable absorption at higher energies E > 200 eV in excess of the galactic absorption can only arise along sight lines that intercept exceptionally dense or multiple damped Lyman alpha systems. However, the available statistics suggest that there are too few such high column density systems for this to occur in more than a few percent of all z 3 quasars. Our ndings therefore support the notion that thèexcess absorption' detected in the soft X-ray spectra of a number of high redshift quasars is most likely of intrinsic or circum-source origin.