Probing the Hard X‐Ray Properties of High‐Redshift Radio‐Quiet Quasars withASCA

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.