Ultra high energy cosmic rays: subluminal and superluminal shocks

TL;DR

This paper investigates how relativistic shocks in astrophysical environments accelerate protons to ultra-high energies, comparing subluminal and superluminal shocks, and suggests AGN as primary sources of observed ultra-high energy cosmic rays.

Contribution

It provides a numerical study of proton acceleration efficiency in relativistic shocks, emphasizing the impact of scattering models, Lorentz factors, and magnetic field angles, and develops a cosmic ray model fitting observations.

Findings

AGN spectra fit observed UHECR flux better than GRBs.

Flat spectra in GRB observations support high Lorentz factor shock models.

AGN are likely primary sources of ultra-high energy cosmic rays.

Abstract

Diffusive shock acceleration is invoked to explain non-thermal particle acceleration in Supernova Remnants, Active Galactic Nuclei (AGN) Jets, Gamma ray Bursts (GRBs) and various large scale cosmic structures. The importance of achieving the highest observed particle energies by such a mechanism in a given astrophysical situation is a recurring theme. In this work, shock acceleration in relativistic shocks is discussed, mostly focusing on a numerical study concerning proton acceleration efficiency by subluminal and superluminal shocks, emphasising on the dependence of the scattering model, bulk Lorentz factor and the angle between the magnetic field and the shock flow. We developed a diffuse cosmic ray model based on the study of different shock boost factors, which shows that spectra from AGN fit current observations of ultra high energy cosmic rays, above 5.7 x 10^10 GeV, much better than GRBs, indicating that AGN are the primary candidates to explain the UHECR flux. Recent Fermi observations of GRB090816c indicate very flat spectra which are expected within our model predictions and support evidence that GRB particle spectra can be flat, when the shock Lorentz factor is of order ~1000.