Status of Agn Unification Models

Abstract

Unification of Active Galactic Nuclei is obviously multidimensional. Dimensions include the mass and angular momentum of the postulated central black hole, the mass accretion rate through the accretion disk surrounding the black hole, and the orientation of this central engine plus associated dust with respect to the line of sight (e.g., Blandford 1990). As for orientation related unification, considerable progress has been made over the past decade. This review will deal with this geometric unification.Mechanisms in AGN include:1.relativistic beaming in radio jets (as manifested by rapid variability from radio to ϒ-rays, superluminal motion, synchrotron self-Compton2.emission by an optically thick, non-spherical body (e.g., hot accretion disk);3.external collimation and shadowing by an anisotropic dust distribution (as manifested by the existence of hidden continua, observed through polarization or photon deficit in extended emission line regions, and hidden broad line regions, observed through polarization or in the less opaque near infrared part of the spectrum).These anisotropic mechanisms introduce selection biases, they lead to incorrect inferred luminosities and make us overestimate the AGN diversity. They furthermore imply unification at some level.