ABSORPTION IN X-RAY-SPECTRA OF HIGH-REDSHIFT QUASARS

We present evidence that X-ray absorption is common in high-redshift q uasars. We have studied six high-redshift (z similar to 3) quasars wit h the ROSAT PSPC of which four are in directions of low Galactic N-H. Three out of these four show excess absorption, while only three in si milar to 50 z less than or similar to 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z > 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126- 158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value a t better than 99.999% confidence level. If the absorber is at the reds hift of the quasar, then values of N-H = [0.86(+0.49, -0.28)] x 10(22) atoms cm(-2) for PKS 0438-436, and N-H = [1.45(+1.20, -0.64)] x 10(22 ) atoms cm(-2) for PKS 2126-158, are implied, assuming solar abundance s. The spectrum of S4 0636+680 also suggests the presence of a similar ly large absorption column density at the 98% confidence level. This a bsorption reverses the trend for the most luminous active galactic nuc lei (AGN) to have the least X-ray absorption, so a new mechanism is li kely to be responsible. Intervening absorption due to damped L gamma a lpha systems is a plausible cause. We also suggest, as an intrinsic mo del, that intracluster material, e.g., a cooling flow, around the quas ar could account for both the X-ray spectrum and other properties of t hese quasars. All the quasars are radio-loud and three are gigahertz p eaked (two of the three showing absorption). No excess absorption abov e the Galactic value is seen toward Q0420-388. This quasar has two dam ped L gamma alpha systems at z=3.08. The limit on the X-ray column den sity implies a low ionization fraction, N(H I)/N(H)greater than or sim ilar to 4 x 10(-3) (3 sigma), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band similar to 0.5-10 keV, which h as been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than Delta alpha(E) > 0.3 (99% confidence) with either redshift or luminosity, for radio-loud quasars.