Very high energy (VHE)
-ray
astronomy (defined here as observations at energies above 300 GeV and
below 100 TeV) became viable with the development of the atmospheric
Cerenkov imaging technique, which crossed the vital detection threshold
with the detection of the Crab Nebula 10 years ago
(Weekes et al. 1989).
Although at the periphery of the observable electromagnetic spectrum,
the VHE band must now be considered a legitimate astronomical discipline
with established sources - both steady and variable, both Galactic and
extragalactic - a growing number of observatories, and the promise of
significant advances in detection techniques in the next few years. The
confirmation of the detection of the Crab Nebula by more than eight
groups over the past decade has given credibility to the existence of
sources of TeV
-rays and
led to a rapid improvement in the sensitivity of ground-based
-ray
detection techniques.
The challenge therefore in this short review, which is aimed at the
general astronomical community, is to convince the reader that there are
viable methods of detecting
-rays of
energy 300 GeV and above from the ground, that a population of credible
sources of various classes have been detected, and that these detections
make a significant contribution to the astrophysics of high-energy
sources. Although, as we shall see later, there are plans to extend the
ground-based detection techniques down to energies of 20 GeV, and there
are viable air shower experiments that operate at energies of 50 TeV and
above, this review will focus on observations in the 300 GeV-30 TeV
energy range; this has been the most studied energy band because it is
easily accessible to the atmospheric Cerenkov imaging technique.