Efficiency limits of diffusive shock acceleration

TL;DR

This paper investigates the efficiency limits of diffusive shock acceleration, especially at relativistic speeds, using Monte Carlo simulations to understand cosmic ray spectrum formation.

Contribution

It provides new insights into the efficiency of diffusive acceleration at relativistic shocks through detailed Monte Carlo simulations.

Findings

Efficiency decreases at relativistic shock speeds

Limits on maximum cosmic ray energies from relativistic shocks

Implications for cosmic ray origin models

Abstract

It is well accepted today that diffusive acceleration in shocks results to the cosmic ray spectrum formation. This is in principle true for non-relativistic shocks, since there is a detailed theory covering a large range of their properties and the resulting power-law spectrum, which is nevertheless not as efficient to reach the very high energies observed in the cosmic ray spectrum. On the other hand, the cosmic ray maximum energy and the resulting spectra from relativistic shocks, are still under investigation and debate concerning their contribution to the features of the cosmic ray spectrum and the measured, or implied, cosmic ray radiation from candidate astrophysical sources. Here, we discuss the efficiency of the first order Fermi (diffusive) acceleration mechanism up to relativistic shock speeds, presenting Monte Carlo simulations.