Gamma-ray Emitting AGN and Unified Schemes

Abstract

Active Galactic Nuclei (AGN) are now known to be strong gamma-ray emitters. After briefly describing the different classes of AGN and the basic tenets of unified schemes, I discuss the role of blazars (that is BL Lacs and flat-spectrum radio quasars) as gamma-ray sources. The main properties of blazars and their connection with relativistic beaming are then summarized. Finally, I address the question of why blazars, despite being extreme and very rare objects, are the only AGN detected at very high (E > 100 MeV) energies, and touch upon the relevance of TeV astronomy for AGN research.

10 pages, 2 figures, LaTeX file, uses moriond.sty (included) and psfig.tex. Invited review talk at the XXXII Moriond Conference "Very High Energy Phenomena in the Universe", Les Arcs, France, January 1997, to be published in the proceedings. TeX and postscript files also available at http://itovf2.roma2.infn.it/padovani/moriond.html