Active Galactic Nuclei

The study of Active Galactic Nuclei (AGN) has become a major astrophysical topic in the last decade, both observationally and theoretically. AGN, in particular quasars, are the most luminous objects in the Universe and can thus be seen to the highest redshifts. Thus, they represent ideal probes for testing directly the physical conditions at large look back times. Although a general qualitative understanding exists about the nature of the central machines, namely accretion onto supermassive black holes, a detailed knowledge about the mechanisms of the actual physical emission processes and about the cosmological evolution of the objects is still missing.

AGN generally come in two "flavours": radio-quiet or radio-loud, depending on whether the ratio of the fluxes emitted in the radio and in the optical band is less or greater than one, respectively. Radio-loud objects constitute the smaller fraction of AGN but they have been studied in great detail and seem to form a more extreme population. Radio observations reveal directed outflows (jets), often with apparent superluminal velocities, and nearly all recently detected gamma-ray AGN are radio-loud objects.

The ROSAT instrument provided, for the first time, the opportunity to study a very large number of AGN in X-rays and more than 20000 AGN are expected to be detected in the Survey. Using recently available large scale sensitive radio surveys for a correlation with the RASS source catalogue, we obtained a list of several thousand radio/X-ray objects of which more than two thirds are currently optically unidentified.

Considerable effort has been put into the construction of these large AGN - samples, into the discussion of their class - specific statistical properties, and into the optical identification of hundreds of these sources. For example, after ROSAT, the number of known BL Lac objects has been doubled.

In parallel, we have performed detailed X-ray studies, in ROSAT follow-up pointed observations and with other available X-ray instruments, of specific objects appearing to be outstanding in some of their properties (like high-redshift quasars) or distinct classes of objects (like Broad Absorption Line quasars) which appear to be ideal tools for a study of the various source populations as a whole.

The new X-ray missions XMM-Newton and Chandra with their greatly improved sensitivity and spectral resolution provide new opportunities for a new, very active era of AGN research.

Recent Publications and Reviews