Cosmic Ray Acceleration by Relativistic Shocks: Limits and Estimates

TL;DR

This paper investigates the maximum energy cosmic rays can attain through relativistic shocks, concluding that ultra-high energy cosmic rays are unlikely to be accelerated by such shocks, especially for light ions.

Contribution

It provides new estimates on the energy limits of cosmic ray acceleration by relativistic shocks, challenging the idea that these shocks are responsible for ultra-high energy cosmic rays.

Findings

Acceleration of light ions above 100 EeV by relativistic shocks is unlikely.

Relativistic shocks probably do not account for ultra-high energy cosmic rays.

Non-relativistic shocks may be responsible for ultra-high energy cosmic ray acceleration.

Abstract

We examine limits to the energy to which cosmic rays can be accelerated by relativistic shocks, showing that acceleration of light ions as high as 100 EeV is unlikely. The implication of our estimates is that if ultra-high energy cosmic rays are accelerated by shocks, then those shocks are probably not relativistic.