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 alpha(E)'s. In contrast, the radio-loud quasars at high redshift have substantial absorption and similar alpha(E)'s to low redshift quasars. Implications for the interpretation of the evolution of the luminosity function 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 emitted power law spectrum has evolved with redshift. We argue that this favors models where quasars are numerous and short-lived, rather than rare and long-lived.