IS X-RAY-ABSORPTION FROM QUASAR ABSORPTION-LINE SYSTEMS DETECTABLE

Using the known properties and statistics of intervening quasar absorp tion systems, we calculate the cumulative absorption out to high redsh ifts at soft X-ray (E < 1 keV) energies. Although photoelectric absorp tion in quasar absorption systems provides a significant source of opa city at the lowest X-ray energies, the imprint of this absorption is n ot readily observable, since it is generally masked by the much strong er Galactic foreground absorption. In particular, the intense He II eq uivalent of the ''Lyman valley'' at energies E less than or similar to 100 eV caused by cumulative absorption in the singly ionized helium c omponent of the Lyman forest is effectively hidden from view. Detectab le absorption at higher energies E greater than or similar to 200 eV i n excess of the Galactic absorption can only arise along sight lines t hat 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 perc ent of all z similar to 3 quasars. Our findings therefore support the notion that the ''excess absorption'' detected in the soft X-ray spect ra of a number of high-redshift quasars is most likely of intrinsic or circum-source origin.