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.
