γ-rays and neutrinos

Abstract

Cosmic rays are particles which originate from space and impinge on the earth's atmosphere with energies spanning over fourteen orders of magnitude. Despite first discovered almost hundred years ago, the origin of cosmic rays is still not completely unveiled. At very high energies (above 1015 eV) the composition and the spectral behavior are not fully assessed; the sources are not yet unambiguously identified as well and unclear are the underlying acceleration mechanisms. Strong indications are however provided by the evidence of non-thermal emission from several objects, which suggest acceleration of particles in plasma shock waves in the proximity of violent astrophysical environments. With the operation of a new generation of imaging Cherenkov telescopes, enormous progress has been achieved in the last years in the discovery of sources of high energy γ-rays. If originated in the decay of neutral pions, the observed γ-rays can represent the fingerprint of cosmic ray sources. In most cases, however, the observed electromagnetic emission can be attributed to other mechanisms. The discovery of high energy neutrinos of cosmic origin would instead provide an unambiguous signature for acceleration of hadrons, but no indication of cosmic neutrino signals has been collected so far. Here we review the success achieved during the latest years in observing the γ-ray imprint of the universe and the perspectives of opening a new observational window with the establishment of neutrino astrophysics.