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 . 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), . 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 1010 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.
The early rocket flights in the sixties detected about 50 X-ray sources, including three AGN. The first AGN discovered was M 87 in the Virgo cluster , the other two were the nearby active galaxy NGC 1275 and the quasar 3C 273. Ariel V established the Seyfert 1 galaxies as a class of X-ray emitters . HEAO-1 provided luminosity functions of AGN  and numerous broad band spectra of AGN [35,57]. Further progress in the field was made with EXOSAT and Ginga. Observations with the Einstein satellite further improved our knowledge on accreting sources and enabled the study of distant quasars, previously detected in other wavebands. The first
X-ray all sky survey with an imaging X-ray telescope carried out with ROSAT provided large complete samples of AGN. The ROSAT pointed observations lead to many discoveries and answered in particular one of the oldest questions of X-ray astronomy by resolving the X-ray background into discrete sources, mainly AGN. Presently the advanced telescopes on Chandra, XMM-Newton and Suzaku contribute many new results to the field.
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