Cosmic-ray Acceleration and Propagation

TL;DR

This paper reviews recent advances in understanding cosmic-ray acceleration and propagation, highlighting kinetic simulations of shocks, observational evidence from SNRs, and open questions about CR spectra and Galactic propagation.

Contribution

It presents state-of-the-art particle-in-cell simulations of shock acceleration, compares them with observations, and discusses open questions linking acceleration mechanisms to Galactic CR propagation.

Findings

Ion and electron acceleration efficiencies depend on shock parameters.

Magnetic field amplification occurs at non-relativistic shocks.

Open questions include the origin of the CR spectral knee.

Abstract

The origin of cosmic rays (CRs) has puzzled scientists since the pioneering discovery by Victor Hess in 1912. In the last decade, however, modern supercomputers have opened a new window on the processes regulating astrophysical collisionless plasmas, allowing the study of CR acceleration via first-principles kinetic simulations. At the same time, a new-generation of X-ray and $\gamma$-ray telescopes has been collecting evidence that Galactic CRs are accelerated in the blast waves of supernova remnants (SNRs). I present state-of-the-art particle-in-cells simulations of non-relativistic shocks, in which ion and electron acceleration efficiency and magnetic field amplification are studied in detail as a function of the shock parameters. I then discuss the theoretical and observational counterparts of these findings, comparing them with predictions of diffusive shock acceleration theory and with multi-wavelength observations of young SNRs. I especially outline some major open questions, such as the possible causes of the steep CR spectra inferred from $\gamma$-ray observations of SNRs and the origin of the knee in the Galactic CR spectrum. Finally, I put such a theoretical understanding in relation with CR propagation in the Galaxy in order to bridge the gap between acceleration in sources and measurements of CRs at Earth.