The effects of X-ray absorption on the spectra of distant objects

Abstract

We have calculated in detail the X-ray absorption spectrum above 0.1 keV that would be introduced into the continuous X-ray spectrum of a quasar by an intervening uniform, hot (T> or =10/sup 6/ K), intergalactic gas with a small admixture of atoms of C, N, O, Ne, Mg, Si, S and Fe. This work is relevant to the well-known search for cosmologically distributed ''missing mass''. Our results indicate that soft X-ray absorption can be appreciable (i.e. tau> or approx. =1) for all quasar X-ray sources, from the most distant (e.g., Q0420-388 at z=3.12) to the very nearby (e.g., 3C 273 at z=0.158), for a significant range of IGM temperatures, densities, heavy-element abundances, and observed photon energies. A brief comparison with the preliminary results of the Einstein Observatory quasar sample is made. A table of minimum detectable densities and abundances for each T> or =10/sup 6/ K and for certain T's for each ion is also calculated for a representative broad-band soft X-ray measurement (between 0.1 and 6 keV) with resolution E/..delta..Eapprox.1--4, of a quasar at z=3, in order to facilitate either a detection or the placing of upper limits.