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Session 37 - Rossi Prize Lecture.
Invited session, Thursday, January 08
International Ballroom Center,
High energy astrophysics, in particular high energy gamma-ray astronomy, has generally been considered as in the domain of space astronomy. Somewhat surprisingly, very high energy gamma ray astronomy can be pursued using ground-based techniques. The air showers generated by the interaction of gamma rays in the atmosphere can be detected using rather simple optical detectors to register the Cherenkov radiation in the atmosphere. In recent years the atmospheric Cherenkov imaging technique, which has been mostly developed at the Whipple Observatory, has been shown to be extremely sensitive to gamma-ray fluxes in the 200 GeV to 10 TeV energy region. The major breakthrough came with the detection of the Crab Nebula which has now been established as the standard candle for the new discipline. Although other supernova remnants have been detected, it appears that in all cases the gamma rays come from Compton scattering by electrons in the synchrotron nebula surrounding the pulsar. TeV gamma-ray astronomy has not yet provided the "smoking gun" for shock acceleration of hadrons in supernova remnants, the canonical picture of cosmic ray production in the Galaxy. BL Lac objects are also powerful sources of TeV gamma rays; in this case the observed gamma rays are almost certainly associated with the relativistic particles in the jets. Doubling times as short as 15 minutes have been observed in Markarian 421. In 1997 Markarian 501 has been in a state of high emission making it the brightest TeV source in the sky. A new generation of telescopes (e.g. VERITAS) is now at an advanced state of planning and will complement the next generation of high energy gamma-ray space telescopes (e.g. GLAST).
