Introduction to Ground-Based Gamma-Ray Astrophysics

The origin of cosmic rays, specifically the questions of how and where charged nuclei are accelerated to extreme energies, remain open. The spectral distribution of cosmic rays arriving to Earth, well-known to depart from a pure power-law near ”the knee” and ”the ankle”, is believed to indicate a transition from an origin in predominantly Galactic sources at energies below ∼ 1017.5 eV , to extragalactic sources above this value. Further insight on potential sources is given by studies of the transport of cosmic rays in the Galaxy and by the Hillas condition that astrophysical accelerators need to satisfy. The production of gamma rays, either through leptonic processes like inverse Compton scattering or in hadronic scenarios, is reviewed in terms of the observed populations of TeV gamma-ray sources, as high-energy gamma rays provide the most direct link to astrophysical particle acceleration. And while the interaction of TeV gamma rays with the intervening radiation fields impedes testing ultra-high-energy photon emission in extragalactic sources, it also can also be used to test Galactic and intergalactic media.