Abstract
In this paper I discuss the status of observational studies aiming at probing the cosmological evolution of the central engine in high-luminosity, high-accretion rate Active Galactic Nuclei (AGN). X-ray spectroscopic surveys, supported by extensive multi-wavelength coverage, indicate a remarkable invariance of the accretion disk plus corona system, and of their coupling up to redshifts z≈6. Furthermore, hard X-ray (<em>E</em> &gt;10 keV) surveys show that nearby Seyfert Galaxies share the same central engine notwithstanding their optical classication. These results suggest that the high-luminosity, high accretion rate quasar phase of AGN evolution is homogeneous over cosmological times.
Highlights
Thanks to its leap in sensitivity by over two orders of magnitude, the Medium Sensitivity Survey carried out by the Einstein observatory (EMSS, Maccacaro et al 1981) collected for the first a statistically sizable sample of extragalactic X-ray sources
Hard X-ray (E ∼>10 keV) surveys show that nearby Seyfert Galaxies share the same central engine notwithstanding their optical classification
These results suggest that the high-luminosity, high accretion rate quasar phase of Active Galactic Nuclei (AGN) evolution is homogeneous over cosmological times
Summary
Thanks to its leap in sensitivity by over two orders of magnitude, the Medium Sensitivity Survey carried out by the Einstein observatory (EMSS, Maccacaro et al 1981) collected for the first a statistically sizable sample of extragalactic X-ray sources. Studying a sample of 190 extragalactic sources, Giovannelli & Polcaro (1986) concluded that the observed linear relation between Xray luminosity and redshift over several orders of magnitude made evident a physical continuity between the different classes of extragalactic X-ray objects. Time is ripe to address some fundamental questions on the nature of accretion onto super-massive black holes, that the small size of the EMSS extragalactic sample left open. In this contribution I will focus on two of them:. The contribution by M.Elitzur to these proceedings discusses a central engine unification scheme encompassing lowluminosity, low-accretion rate systems
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