CANGAROO Project for High-Energy Gamma-Ray Astrophysics

Astrophyisical motivations, recent status and results from the international CANGAROO project in Woomera, South Australia are described. Since 1990, with ground-based gamma-ray telecopes using the imaging Cherenkov technique, we are exploring the highenergy phenomena in the Universe and providing new insights to active and extreme astrophysical objects by its observational data at the highest end of photon spectrum. CANGAROO is an acronym for Collaboration between Australia and Nippon (Japan) for a Gamma-Ray Observatory in the Outback. We started this international project in 1990 with a 3.8m telescope originally developed for lunar ranging, and we equipped it with a fine-resolution Cherenkov imaging camera. 1) Now an array of four 10m Cherenkov telescopes are to be ready in 2003 for high-sensitive detection of highenergy gamma-rays in Woomera, South Australia. We have discovered new gammaray objects in the southern sky with unprecedented sensitivity as the first imaging atmospheric telescope in the southern hemisphere. Astrophysical motivations and major discoveries by CANGAROO are described in this article. §2. High-energy gamma-ray astronomy The highest energy end of the photon spectrum is the last frontier of astronomy. ⁄⁄) In general, expected number of photons decreases with energy and the statistics is the most crucial problem in gamma-ray astrophysics. The earth atmosphere is not transparent for gamma-rays and satellites are the only possible gamma-ray observatories about a decade ago. The detection area of space-based gamma-ray detectors is limited in size of satellites, 1 m 2 or so, and this determines