Galactic and extragalactic sources of very high energy gamma rays

Very high energy gamma-rays are one of the most important messengers of the relativistic non-thermal Universe. The major motivation of very high energy y-ray astronomy is to find sources of high energy cosmic rays. Several astrophysical sources are known to accelerate cosmic rays to very high energies under extreme conditions. Very high energy gamma-rays are produced at these astrophysical sites or in their vicinity through interactions of cosmic rays in the surrounding medium close to the sources. Gamma rays, being neutral, travel in a straight line and thus give us valuable information about the cosmic ray sources and their surroundings. Additionally, very high energy y-ray astronomy can probe many fundamental physics questions. Ground-based gamma-ray astronomy began its journey in 1989 when Whipple telescope detected TeV gamma-rays from the Crab, a pulsar wind nebula in our galaxy. In the last 2 decades, technological improvements have facilitated the development of the latest generation of very high energy detectors and telescopes which have delivered exciting new results. Until now, over two hundred very high energy y-ray sources, both galactic and extra-galactic, have been detected. These observations have provided a deeper insight into a large number of important questions in high energy astrophysics and astroparticle physics. This review article is an attempt to enumerate the most important results in the exciting and rapidly developing field of very high energy gamma-ray astronomy.