X-RAY SPECTRAL EVOLUTION OF HIGH-REDSHIFT QUASARS

At z approximate to 3, the x-ray spectra of radio-loud and radio-quiet quasars are different. High-redshift radio-quiet quasars either have large absorbing columns, N-H, and steeper power law spectral indices, alpha(E), than low redshift quasars, or no absorption and similar alph a(E)'s. In contrast, the radio-loud quasars at high redshift have subs tantial absorption and similar alpha(E)'s to low redshift quasars. Imp lications for the interpretation of the evolution of the luminosity fu nction of quasars are discussed. If the absorption arises outside the central engine for both radio-loud and radio-quiet quasars, then radio -quiet quasars differ from the radio-loud quasars in that their emitte d power law spectrum has evolved with redshift. We argue that this fav ors models where quasars are numerous and short-lived, rather than rar e and long-lived.