Cosmic ray acceleration at relativistic shocks, shear layers, .

TL;DR

This review discusses how relativistic shocks influence cosmic ray acceleration, showing that high-energy tails are less common than previously thought, with recent observations and theories indicating steeper spectra and alternative acceleration processes.

Contribution

It provides a comprehensive review of recent theoretical developments and observational evidence, challenging previous assumptions about particle distributions at relativistic shocks.

Findings

Relativistic shocks produce particle distributions with a compressed injected component and limited high-energy tails.

Increasing shock Lorentz factor steepens the high-energy tail of the particle spectrum.

Observations of Cyg A hot spots show spectra deviating from standard models, with flat low-energy and steep high-energy parts.

Abstract

A review of theoretical results on cosmic ray first-order Fermi acceleration at relativistic shock waves is presented, with recent results substantially changing the existing knowledge on these processes. In particular one can not expect such shocks to form particle distributions extending to very high energies. Instead, distributions with the shock compressed injected component followed by a more or less extended high energy tail are usually created. Increasing the shock Lorentz factor leads to steepening of the energetic tail. An observational check of real electron spectra in the Cyg A hot spots provides results clearly deviating from the standard expectations. The spectrum consist of a very flat low energy part (sigma ~1.5)up to electron energies ~1 GeV, and a much steeper part (sigma > 3)at higher energies. We conclude with remarks on the Fermi second-order processes acting in relativistic plasmas, possibly the main accelerating agent for very high energy cosmic rays.