Astroparticle Physics

635 Acceleration Mechanisms“Physics also solves puzzles. However,these puzzles are not posed by mankind, butrather by nature.”Maria Goeppert-MayerThe origin of cosmic rays is one of the major unsolved astrophysicalproblems. The highest-energy cosmic rays possessmacroscopic energies and their origin is likely to beassociated with the most energetic processes in the universe.When discussing cosmic-ray origin, one must in principledistinguish between the power source and the accelerationmechanism. Cosmic rays can be produced by particle interactionsat the sites of acceleration like in pulsars. Theacceleration mechanism can, of course, also be based onconventional physics using electromagnetic or gravitationalpotentials such as in supernova remnants or active galacticnuclei. One generally assumes that in most cases cosmicrayparticles are not only produced in the sources but alsoaccelerated to high energies in or near the source. Candidatesites for cosmic-ray production and acceleration are supernovaexplosions, highly magnetized spinning neutron stars,i.e., pulsars, accreting black holes, and the centers of activegalactic nuclei. However, it is also possible that cosmicrayparticles powered by some source experience accelerationduring the propagation in the interstellar or intergalacticmedium by interactions with extensive gas clouds. Thesegas clouds are created by magnetic-field irregularities andcharged particles can gain energy while they scatter off theconstituents of these ‘magnetic clouds’.In top–down scenarios energetic cosmic rays can also beproduced by the decay of topological defects, domain walls,or cosmic strings, which could be relics of the Big Bang.There is a large number of models for cosmic-ray acceleration.This appears to indicate that the actual accelerationmechanisms are not completely understood and identified.On the other hand, it is also possible that different mechanismsare at work for different energies. In the following themost plausible ideas about cosmic-ray-particle accelerationwill be presented.origin of cosmic raysacceleration mechanismssupernovaepulsarsblack holesactive galactic nucleitop–down scenario