On the mechanism for breaks in the cosmic ray spectrum

TL;DR

This paper explains the observed spectral breaks in cosmic rays by proposing that ion-neutral collisions in supernova remnants cause a steepening of the particle energy spectrum, supported by gamma-ray observations.

Contribution

It introduces a mechanism where ion-neutral collisions cause a spectral steepening in cosmic rays, aligning theoretical predictions with recent gamma-ray observations.

Findings

Spectral break at ~7 GeV in SNR W44 matches model predictions.

Ion-neutral collisions cause a steepening of the cosmic ray spectrum by exactly one power.

Proton spectrum transitions from E^{-2} to E^{-3} at the break energy.

Abstract

The proof of cosmic ray (CR) origin in supernova remnants (SNR) must hinge on full consistency of the CR acceleration theory with the observations; direct proof is impossible because of the orbit stochasticity of CR particles. Recent observations of a number of galactic SNR strongly support the SNR-CR connection in general and the Fermi mechanism of CR acceleration, in particular. However, many SNR expand into weakly ionized dense gases, and so a significant revision of the mechanism is required to fit the data. We argue that strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by \emph{exactly one power}. The spectral break is caused by a partial evanescence of Alfven waves that confine particles to the accelerator. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is also calculated. Using the recent Fermi spacecraft observation of the SNR W44 as an example, we demonstrate that the parent proton spectrum is a classical test particle power law $\propto E^{-2}$, steepening to $E^{-3}$ at $E_{br}\approx7GeV$.