ASCA and ROSAT X-ray spectra of high-redshift radio-loud quasars

Results are presented on the X-ray properties of 9 high-redshift (1.2 < z < 3.4) radio-loud quasars (RLQs) observed by the Advanced Satellite for Cosmology and Astrophysics (ASCA; 10 observations) and ROSAT (11 observations, for a subset of six quasars). New ASCA observations of S5 0014+81 (z = 3.38) and S5 0836+71 (z = 2.17) and ROSAT observations of PKS 2126-158 for which results were never presented elsewhere are included. A simple model consisting of a power law plus cold, uniform absorption gives acceptable fits to the spectra of all sources. The ASCA spectra of the six brightest objects show evidence for absorption in excess of the Galactic value at a much greater than 99% confidence level. Comparison with the ROSAT data suggests that absorption has significantly varied (Delta N-H similar to 8 x 10(20) cm(-2)) in the case of S5 0836+71, on a time-scale of approximately 0.8 yr in the quasar frame. For the remaining five sources for which ROSAT spectra were available, the two instruments gave consistent results, and the data were combined yielding unprecedented spectral coverage (typically about 0.4-40 keV in the quasar frame) for high-z quasars. This allows us to put severe limits on several different descriptions of the continuum (e.g., broken power law, bremsstrahlung, reflection component). No Fe K alpha emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe K alpha at the quasar redshift is marginally detected in S5 0014+81. Possible origins for the observed low energy absorption are discussed. In particular, contributions from the molecular clouds and dust present in our Galaxy (usually disregarded) are carefully considered. In light of the new results for S5 0836+71 and S5 0014+81, absorption intrinsic to the quasars is considered and discussed. The average slope obtained from the eight ASCA spectra in the observed similar to 0.5-10 keV energy band is [Gamma(0.5-10) keV] similar or equal to 1.61 +/- 0.04, with a dispersion sigma(0.5-10 keV) similar or equal to 0.10 +/- 0.03, The average photon index in the observed 2-10 keV band, where the effect of absorption is negligible, is [Gamma(2-10 keV)] similar or equal to 1.53 +/- 0.05, with a dispersion sigma(2-10 keV) less than or similar to 0.12. Furthermore, the implications of the present results on the calculations of the contribution of quasars to the cosmic X-ray and gamma-ray backgrounds are briefly discussed.