Fast Plasma Processes in Active Galactic Nuclei

The physics of active galactic nuclei (AGN) and related objects is one of the most actively pursued areas in astrophysics. Their large luminosities over the entire electromagnetic spectrum combined with extremely short-time variability has stimulated a large number of astrophysicists to propose rather unconventional ideas. After a simplistic review of the early work on AGN, a case is made for the important role of some of the nonlinear plasma processes in the generation of the nonthermal continuum as well as its interaction with the plasma surrounding the central source in the form of accretion disks and emission line regions. These plasma processes are fast, as they operate on characteristic time scales much shorter than the single particle processes do. Due to their collective nature. the efficiencies of scattering, absorption and emission increase many fold. In particular, it is shown how the parametric instabilities cause anomalous absorption and scattering and in turn heat the plasma to much higher temperatures than could be achieved through particle collisions. A combination of the stimulated Raman and Compton scattering processes is shown to be able to account for the major part of the spectrum of 3C273, as an example. An attempt is made to address simultaneously the questions of acceleration of particles and their radiation through the concerted action of a sequence of plasma processes. This review ends with some reflections on future research directions.