1999ApJ...516..582V

This paper reports the X-ray spectral analysis of five high-redshift (z≥2), radio-quiet quasars observed by ASCA. A simple power-law continuum plus cold Galactic absorption model well fits all the spectra (typically between ∼2 and 30 keV in the sources frame). Neither the X-ray spectral hardening, which is attributed to a reflection component and observed in Seyfert galaxies, nor the excess absorption previously detected in high-redshift, radio-loud quasars have been revealed. Only marginal evidence of a neutral or mildly ionized FeKα line is found in one of the quasars. The average spectral slope in the observed 0.7-10 keV energy range, Γ=1.67±0.11 (dispersion σ∼0.07), appears to be flatter than that of low-z, radio-quiet quasars (Γ≃1.9-2) and slightly steeper but consistent with Γ=1.61±0.04 (σ∼0.10) of high-z, radio-loud quasars.