Very-High Energy Gamma-Ray Astronomy: A 23-Year Success Story in Astroparticle Physics

Very high energy (VHE) gamma quanta contribute only to 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. Before describing the important experiments that paved the way towards the current observations, a short review of gamma rays as cosmic messengers and the general concept of detecting and filtering them from the vast majority of charged cosmic rays will be given in the introduction. The main part of the chapter on VHE gamma-ray astronomy highlights the various experiments that have pushed the field to the current success. At the end, a short overview of the challenging physics questions will be given summarizing the goals of the experiments in order to give a coherent picture of the underlying reasons behind the experiments. In coherence with the energy range of cosmic rays observable from Earth, we will set the lower energy limit to about 30 GeV above which ground-based observations with the most powerful instruments are now possible. Below this energy range observations can only be performed with balloon or satellite-borne detectors. Their detection range is below the classical cosmic-ray energy range.