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In contrast to the 'normal' galaxies, active galaxies are characterized by the presence of an extremely bright nucleus, which dominates over the emission of the surrounding stars. The luminosity of an active galactic nucleus (AGN) ranges from 1041 to 1047 erg /s, reaching even 1048 erg /s for the brightest quasars. Since their rapid variability implies that the majority of emission originates in a very small region, the most plausible explanation of this unusual amount of energy powering AGN is the accretion of gas onto a supermassive black hole. 

* Two classes of Seyfert galaxies

Characteristic feature of the AGN spectra is the presence of various emission lines. Among these the two distinct groups are observed: broad, permitted emission lines (FWHM in the range 1500 - 30000 km /s) and narrow emission lines (FWHM < 900 km /s), which may be either permitted or forbidden. The lines are produced in two separate regions, Narrow Line Region (NLR) and Broad Line Region (BLR), and as it is assumed in the currently accepted unification scheme of Syfert 1 and Seyfert 2 galaxies, BLR lies much closer to the central engine, being in Seyfert 2 galaxies obscured by a thick, molecular torus. Thus the orientation effects are the cause of the observed differences between these two types of Seyferts.
* Spectra of AGN

The spectra of AGN extend over several decades of energy, from radio to X-ray and Gamma-ray bands. The picture (Laor et al., 1997, ApJ, 477, 93) shows the composite spectral energy distribution of a sample of quasars. The bolometric luminosity is dominated however by the ultraviolet emission, which has a form of the so called Big Blue Bump. This component comes from the accretion disk. Above 1 keV the X-ray spectrum has a power law shape, which in logarithmic plot has a form of a straight line. This hard X-ray tail is formed due to Compton upscattering of soft photons in a hot plasma outside the accretion disk. Many AGN exhibit also the reflection component and fluorescent iron line, which both originate form the 'reflection' of hard X-rays from a cold 'mirror' - accretion disk. Additionally, in most sources the observations reveal an excess of emission above the extrapolated power law in the soft X-ray band. The origin of this 'Soft Excess' is not yet confirmed, mainly because any interpretation relies on extrapolating the spectrum through the XUV band, which is not observable due to the Galactic absorption.