X-ray spectral evolution of high redshift quasars

At z≈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, α_E, than low redshift quasars, or no absorption and similar α_E's. In contrast, the radio-loud quasars at high redshift have substantial absorption and similar α_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.