1. 1 Schematic Picture of AGN Some galaxies are known to emit radiation with extremely high luminosities from a rather small volume in the ??ray, X-ray and UV continuum. Such active cores are the so-called Active Galactic Nuclei (AGN) and the radiation is commonly believed to be a result of gravitational energy released by matter spiraling around 9 a supermassive central black hole of about 10 M (see Fig. 1). Though the central engine which produces the enormous observed activity cannot be resolved observationally, a standard picture of an AGN has gradually emerged to explain the richness of the radiation spectra: - an accretion disk with radius from about 2 to 100 gravitational radii, R , g feeding the central black hole and emitting mainly in the UV and soft X-rays; - the broad line optically emitting clouds (BLR), which seem to be absent in 3 some sources (e. g. FRI, see hereafter) and extend up to a few 10 R from g the center.
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