Particle Acceleration in Cosmic Sites Astrophysics Issues in our Understanding of Cosmic Rays

Particles are accelerated in cosmic sites probably under conditions very different from those at terrestrial particle accelerator laboratories. Nevertheless, specific experiments which explore plasma conditions and stimulated particle acceleration carry significant potential to illuminate some aspects of the cosmic particle acceleration process. Here we summarize what we have learned about cosmic particle acceleration, through observations of the properties of cosmic ray particles, and through astronomical signatures caused by these near their sources or throughout their journey in interstellar space. We discuss the candidate source object variety, and what has been learned about their particle-acceleration characteristics. We conclude identifying open issues as they are discussed among astrophysicists. – The cosmic ray differential intensity spectrum across energies from 10eV to 10eV reveals a rather smooth power-law spectrum. Two kinks occur at the “knee” ('10eV) and at the “ankle” ('3 10eV). It is unclear if these kinks are related to boundaries between different dominating sources, or rather related to characteristics of cosmicray propagation. Currently we believe that Galactic sources dominate up to 10eV or even above, and the extragalactic origin of cosmic rays at highest energies merges rather smoothly with Galactic contributions throughout the 10–10eV range. Pulsars and supernova remnants are among the prime candidates for Galactic cosmic-ray production, while nuclei of active galaxies are considered best candidates to produce ultrahigh-energy cosmic rays of extragalactic origin. The acceleration processes are probably related to shocks formed when matter is ejected into surrounding space from energetic sources such as supernova explosions or matter accreting onto black holes. Details of shock acceleration are complex, as relativistic particles modify the structure of the shock, and simple approximations or perturbation calculations are unsatisfactory. This is where laboratory plasma experiments are expected to contribute, to enlighten the non-linear processes which occur under such conditions. PACS. 96.40 cosmic rays – 95.30 astrophysical plasma