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Active Galactic Nuclei

Over the past decade and half, one of the most exciting advances in high energy astrophysics has been the detection of astronomical objects at TeV energies with ground-based gamma-ray facilities, following the ground-breaking discovery of a population of sources at MeV--GeV energies with the EGRET instrument aboard the Compton Gamma Ray Observatory. Among the sources detected, blazars are arguably the most intriguing. Blazars belong to a sub-class of AGN that are radio-loud. They have a flat spectral energy distribution (SED), which is characterized by highly variable and primarily non-thermal emission at most wavelengths. The observed rapid variability and radio properties of these objects imply that they have relativistic jets whose axes make small angles with respect to the line of sight. The gamma-ray emission is also most naturally explained as beamed radiation from the jet that is Doppler boosted in energy and intensity, although the beaming hypothesis still needs to be confirmed observationally. Recent studies of extended quasar jets suggest that X-ray emission might originate in Compton scattering of the cosmic microwave background photons, which would require strong beaming. The results might be extrapolated to radio-loud AGN in general, under the unification scheme. It is interesting to note that blazars are the only type of AGN that have been detected at TeV energies.

Detailed models have been developed to explain the observed SEDs of blazars, which invariably show two peaks, with one located at optical-X-ray energies and the other at GeV-TeV energies. A popular class of models, collectively referred to as leptonic models, attribute the first SED peak to synchrotron emission from relativistic electrons in the jets and the second one to inverse-Compton scattering of soft photons by the electrons. The seed photons being up-scattered could either be due to the synchrotron emission of the jet itself, or to external radiation from the accretion disk or the broad-line regions. An alternative class of models, often referred to as hadronic models, assumes that protons are accelerated at the base of the jet up to highly relativistic energies, exceeding the threshold for photo-pair and photo-pion production. The gamma-ray emission is then produced through photomeson production, proton synchrotron radiation, muon and pion synchrotron radiation, and subsequent synchrotron-pair cascading. In hadronic models, emission associated with the lower SED peak is also due to relativistic electrons in the jet, as in leptonic models. Therefore, observations at GeV-TeV energies probably hold the key to distinguishing the two classes of models.