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 significant 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 He II equivalent of the ''Lyman valley'' at energies E less than or similar to 100 eV caused by cumulative absorption in the singly ionized helium component of the Lyman forest is effectively hidden from view. Detectable absorption at higher energies E greater than or similar to 200 eV in excess of the Galactic absorption can only arise along sight lines that intercept exceptionally dense or multiply damped Ly 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 similar to 3 quasars. Our findings therefore support the notion that the ''excess absorption'' detected in the soft X-ray spectra of a number of high-redshift quasars is most likely of intrinsic or circum-source origin.