Probing the hard X-ray properties of high-redshift radio-quiet quasars with ASCA

This paper reports the X-ray spectral analysis of five high-redshift (z gre ater than or equal to 2), radio-quiet quasars observed by ASCA. A simple po wer-law continuum plus cold Galactic absorption model well fits all the spe ctra (typically between similar to 2 and 30 keV in the sources frame). Neit her the X-ray spectral hardening, which is attributed to a reflection compo nent and observed in Seyfert galaxies, nor the excess absorption previously detected in high-redshift, radio-loud quasars have been revealed. Only mar ginal evidence of a neutral or mildly ionized FeK alpha line is found in on e of the quasars. The average spectral slope in the observed 0.7-10 keV ene rgy range, [Gamma] = 1.67 +/- 0.11 (dispersion sigma similar to 0.07), appe ars to be flatter than that of low-z, radio-quiet quasars (Gamma similar or equal to 1.9-2) and slightly steeper but consistent with Gamma = 1.61 +/- 0.04 (sigma similar to 0.10) of high-z, radio-loud quasars.