New ASCA perspectives on high-redshift quasars Original Research Article

In this paper the results of the X-ray spectral analysis of a sample of high-redshift (z≥2) radio-quiet quasars (RQQs) are presented. There is an indication that high-z RQQs may be characterized by a flatter X-ray spectrum with respect to low-z quasars. Since no spectral flattening ascribed to a reflection component seems to come up by the present observations (in the ∼ 2–30 keV energy range, source rest frame), the flatter X-ray slope may indicate either a different emission mechanism or X-ray spectral evolution of RQQs.