HIGH ENERGY GAMMA RAYS Mathieu de Naurois

The Very High Energy Gamma Ray Astronomy (VHE) is a rapidly evolving branch of modern astronomy, which covers the range from about 50 GeV to several tens of TeV from the ground. In the past years, the second generation instruments firmly established a growing and varied list of sources including plerions, supernova remnants and active galactic nuclei, and started to study some fundamental questions such as the origin of cosmic rays or the emission mechanisms of the active galactic nuclei. New results now include the first VHE unidentified sources as well as more puzzling sources such as the Galactic center. The arrival of new generation instruments (HESS, CANGAROO III, VERITAS, MAGIC) already gives a impressive look at the near future. Here we attempt to summarize the current status of the field. We briefly describe the instruments and analysis techniques, and give an outlook on the sources detected sofar. 1 1 The universe in gamma-rays 1.1 A little bit of history Since their discovery by Victor Hess in 1912, one of the most puzzling (and still not completely solved) problems of astronomy is the origin of Cosmic Rays. These high energy particles mostly consist of charged nuclei and are spread over more than 10 orders of magnitude in energy up to 10 20 eV (and possibly above). The search for the corresponding cosmic accelerators motivated the developement of gamma-ray astronomy, first from space and then from the ground. In contrast to the charged component of the cosmic rays, the gamma-rays are not deviated by the Galactic and extra-galactic magnetic fields and thus point back to their emission source. They are emitted by particle physics processes (non-thermal synchrotron radiation of accelerated electrons, inverse Compton scattering of cosmic rays off ambient photons, pion decay,.. .) occuring in high electromagnetic field acceleration regions, shocks in astrophysical plasmas or interaction of the cosmic rays with the interstellar medium. In the active galactic nuclei or other compact objects, they are thought to be emitted by the very central engine (instead of being secondary products) and can therefore probably provide the most valuable constraints on the emission models. Finally, they could also possibly originate from annihilation of neutralinos in dark matter clumps.