Active Galactic Nuclei

X-ray observations of active galactic nuclei (AGN) are of wide astrophysical interest. The amount and efficiency of the energy release within the immediate neighbourhood of black holes belong to the most extreme physical processes observed to date. The most probable explanation for the huge amount of energy output (and for other observational parameters, such as the width of the optical emission lines and the strength of the radio emission) is given by the transformation of potential energy into radiation by accretion of matter onto a supermassive central black hole [43]. The velocities of the accreting matter reach values of about one third of the velocity of light, deduced by relativistically broadened line profiles (centred at about 6.4 keV), [49]. The emission from the matter around the black hole may vary on very short time scales of only a few hundred seconds. The corresponding changes in luminosity reach values of about 10{su10} solar luminosities. All this is further suggestive for the presence of supermassive black holes. Energy production processes and radiation mechanisms for the innermost regions of AGN are an important research field in X-ray astronomy and open a unique possibility to study matter under extreme gravity. Other astrophysical important aspects include the detection and study of binary black holes, expected to lead to strong gravitational wave emission and tidal disruption events of stars in the dense core around the central black holes. At larger distances from the black hole, the emission from optically thin plasma can be studied. Imprinted absorption and emission lines give information on the chemical composition of the gas and of infall and outflow velocities. The log N-log S distribution of AGN and resulting luminosity functions allow to study the density and luminosity evolution of AGN in dependence of redshift. The first compact obscured objects in the universe can be best studied with X-rays.