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

TL;DR

This paper reviews cosmic-ray sources, their acceleration mechanisms, and how laboratory plasma experiments can shed light on the complex non-linear processes involved in particle acceleration at astrophysical sites.

Contribution

It highlights the role of laboratory plasma experiments in understanding non-linear particle acceleration processes in cosmic-ray sources, complementing astrophysical observations.

Findings

Cosmic-ray spectrum shows features like the knee and ankle, with uncertain origins.

Galactic sources likely dominate cosmic rays up to 10^17 eV.

Laboratory experiments can help understand shock-related acceleration processes.

Abstract

Laboratory experiments to explore plasma conditions and stimulated particle acceleration can illuminate aspects of the cosmic particle acceleration process. Here we discuss the cosmic-ray candidate source object variety, and what has been learned about their particle-acceleration characteristics. We identify open issues as discussed among astrophysicists. -- The cosmic ray differential intensity spectrum is a rather smooth power-law spectrum, with two kinks at the "knee" (~10^15 eV) and at the "ankle" (~3 10^18 eV). It is unclear if these kinks are related to boundaries between different dominating sources, or rather related to characteristics of cosmic-ray propagation. We believe that Galactic sources dominate up to 10^17 eV or even above, and the extragalactic origin of cosmic rays at highest energies merges rather smoothly with Galactic contributions throughout the 10^15--10^18 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. Acceleration processes are related to shocks from violent ejections of matter from energetic sources such as supernova explosions or matter accretion onto black holes. Details of such acceleration are difficult, 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.