Very-high energy (VHE) gamma quanta contribute only a minuscule fraction -below one per million -to the flux of cosmic rays. Nevertheless, being neutral particles they are currently the best "messengers" of processes from the relativistic/ultra-relativistic Universe because they can be extrapolated back to their origin. The window of VHE gamma rays was opened only in 1989 by the Whipple collaboration, reporting the observation of TeV gamma rays from the Crab nebula. After a slow start, this new field of research is now rapidly expanding with the discovery of more than 150 VHE gamma-ray emitting sources. Progress is intimately related with the steady improvement of detectors and rapidly increasing computing power. We give an overview of the early attempts before and around 1989 and the progress after the pioneering work of the Whipple collaboration. The main focus of this article is on the development of experimental techniques for Earth-bound gamma-ray detectors; consequently, more emphasis is given to those experiments that made an initial breakthrough rather than to the successors which often had and have a similar (sometimes even higher) scientific output as the pioneering experiments. The considered energy threshold is about 30 GeV. At lower energies, observations can presently only be performed with balloon or satellite-borne detectors. Irrespective of the stormy experimental progress, the success story could not have been called a success story without a broad scientific output. Therefore we conclude this article with a summary of the scientific rationales and main results achieved over the last two decades. arXiv:1207.6003v1 [physics.hist-ph] 25 Jul 2012Very-high energy gamma-ray astronomy 3 b) Decay of neutral pions produced in hadronic interactions:Accelerated protons or heavier nucleons interact with ambient protons, nucleons or photons in stellar environments or cosmic gas clouds. Dominantly, charged and neutral pions are produced. Charged pions decay in a two-step process into electrons and two neutrinos while neutral pions decay with > 99% probability into two gamma quanta. For proton-nucleus interactions one has: p + nucleus → p . . . + π ± + π 0 + . . . and π 0 → 2γ; π → µν µ ; µ → eν µ ν e .Heavier secondary mesons, much rarer, normally decay in a variety of lighter ones and eventually mostly into π ± and π 0 and/or γ.Spectra and morphology of gamma-ray emissions can provide only circumstantial evidence for either leptonic or hadronic origin of the gamma rays, while the observation of neutrinos would be an unambiguous proof for hadronic acceleration processes in the source.
The long road to the discovery of the first VHE-emitting gamma-ray sourceThe main driving force for VHE gamma-ray astronomy was initially the search for the sources of the charged cosmic rays, while now, after the discovery of many sources, the interests have shifted to general astrophysics questions. In earlier times the searches were hampered by a few fundamental questions:1. How large is the fraction of cosmic γ rays of the ...