GeV-TeV Gamma-ray Astronomy

The history of gamma-ray astronomy began in the 1950’s when Hayakawa [1] and Morrison [2] predicted that gamma-rays would be produced via neutral pion decays generated in collisions of cosmic-rays with interstellar matter. Cherenkov observation of air showers was pioneered by Jelly [3] and Chudakov applied this technique to search for gamma-ray signals from celestial sources [4]. Since this early time the Crab nebula was the most promising source due to its high activity [5, 6]. Point sources were found by satellites such as SAS-II and COS B in the 1970’s and early 1980’s, but the launch of the Compton gamma-ray observatory was the beginning of a new era with its discovery of more than 200 sources [7, 8]. On the other hand, ground-based Cherenkov telescopes with imaging capability started to produce reliable detections of sources in the late 1980’s [9]. A distinctive feature of gamma-ray astronomy is the inherent non-thermal origin of radiation at these energies, regardless of its detailed mechanism such as bremsstrahlung or inverse Compton scattering of high-energy electrons in matter or in radiation fields and decay of neutral pions generated in collisions of high-energy protons in interstellar matter. In other words, gamma-ray astronomy can reveal violent aspects of the Universe, far apart from the thermal Universe where most processes are in equilibrium.