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Cosmic sources of gamma-rays of energy in excess of 0.25 TeV are now well-established using the ground-based atmospheric Cherenkov technique. Recently high resolution cameras (arrays of phototubes) on large optical reflectors have achieved significant improvements in flux sensitivity. Observations with the Compton Gamma Ray Observatory have shown that many sources have significant fluxes at the highest useful energy of EGRET (30 GeV); at least three of these sources have been detected in the TeV energy range. The Crab Nebula is the best established source; its spectrum extends beyond 10 TeV and is well understood in terms of a Compton-synchrotron model. PSR1706-044 is one of five pulsars seen by EGRET but at TeV energies it is seen as a steady (unpulsed) source. Markarian 421 is closest AGN seen by EGRET; at TeV energies it is a strong and variable source. The failure to detect emission from other stronger, but more distant, AGN's has led to speculation that the intergalactic infrared background is responsible for significant absorption causing photon-photon pair production. It is thus possible to derive an upper limit to the intergalactic infrared background. New telescopes are under development which will reduce the energy threshold of the atmospheric Cherenkov technique and bridge the gap to space gamma-ray telescopes.
Supported in part by the U.S.~Dept.~of Energy.
