Is X‐Ray Absorption from Quasar Absorption‐Line Systems Detectable?

Abstract

Using the known properties and statistics of intervening quasar systems, we calculate the cumulative out to high redshifts at soft X-ray (E < 1 keV) energies. Although photoelectric in quasar systems provides a significant source of opacity at the lowest X-ray energies, the imprint of this 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 valley at energies E 100 eV caused by cumulative in the singly ionized helium component of the Lyman forest is effectively hidden from view. Detectable at higher energies E 200 eV in of the Galactic can only arise along sight lines that intercept exceptionally dense or multiply damped Lyα 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 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.