Eur. Phys. J. D 55, 509-518 (2009)
https://doi.org/10.1140/epjd/e2009-00183-8

Particle acceleration in cosmic sites

Astrophysics issues in our understanding of cosmic rays

Max Planck Institut für extraterrestrische Physik, 85748 Garching, Germany

Corresponding author: ^a rod@mpe.mpg.de

Received: 12 December 2008
Revised: 24 February 2009
Published online: 9 July 2009

Abstract

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 stimulate particle acceleration carry significant potential to illuminate some aspects of the cosmic particle acceleration process. Here we summarize our understanding of cosmic particle acceleration, as derived from 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 10¹⁰ eV to 10²¹ eV reveals a rather smooth power-law spectrum. Two kinks occur at the “knee” (≃10¹⁵ eV) and at the “ankle” (≃ 3×10¹⁸ eV). It is unclear if these kinks are related to boundaries between different dominating sources, or rather related to characteristics of cosmic-ray propagation. Currently we believe that galactic sources dominate up to 10¹⁷ eV or even above, and the extragalactic origin of cosmic rays at highest energies merges rather smoothly with galactic contributions throughout the 10¹⁵–10¹⁸ eV 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.