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Published online by Cambridge University Press: 12 April 2016
Cosmic rays (CR) are fast or relativistic protons and electrons (and nuclei), detected on the top of the terrestrial atmosphere. Except at the highest energies (>1017eV) the Larmor radii of the particles in the galactic magnetic field are much smaller than the scale height of the interstellar gas, and the arriving particles have completely lost memory of their sources. Therefore, their origin is unknown, at least directly. But recent advances in γ-ray astronomy, and in shock acceleration mechanisms, have shed a radically new light on a old problem.