Probing the hard X-ray properties of high-redshift Radio-Quiet Quasars with ASCA

This paper reports the X-ray spectral analysis of 5 high-redshift (z \ge 2) radio-quiet quasars observed by ASCA. A simple power law continuum plus cold Galactic absorption model well fits all the spectra (tipically between $\sim$ 2-30 keV in the sources frame). Neither the X-ray spectral hardening, 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 a marginal evidence of a neutral or mildly ionized FeK\alpha line is found in one of the quasars. The average spectral slope in the observed 0.7--10 keV energy range, $<\Gamma> = 1.67 \pm{0.11}$ (dispersion $\sigma \sim 0.07$), appears to be flatter than that of low-z radio-quiet quasars ($\Gamma \simeq 1.9-2$) and slightly steeper, but consistent with $\Gamma$ = 1.61 \pm{0.04} (\sigma \sim 0.10)$ of high-z radio-loud quasars.