Cosmic ray protons and electrons from supernova remnants

TL;DR

This paper models the spectra of cosmic ray protons and electrons from supernova remnants, considering magnetic field amplification and particle escape, and finds differences at high energies that could explain observations.

Contribution

It provides a detailed calculation of cosmic ray spectra from supernova remnants including magnetic field amplification and particle escape mechanisms, highlighting differences between electrons and protons at various energies.

Findings

Electron and proton spectra differ mainly above 100 GeV.

Low-energy spectral differences imply substantial late-stage magnetic amplification.

Additional acceleration effects or energy losses may influence observed spectra.

Abstract

The spectrum of cosmic ray protons and electrons released by supernova remnants throughout their evolution is poorly known, because of the difficulty in accounting for particle escape and confinement in the downstream of a shock front, where both adiabatic and radiative losses are present. Here we calculate the spectrum of cosmic ray protons released during the evolution of supernovae of different types, accounting for the escape from upstream and for adiabatic losses of particles advected downstream of the shock and liberated at later times. The same calculation is carried out for electrons. The magnetic field in the post-shock region is calculated by using an analytic treatment of the magnetic field amplification due to non--resonant and resonant streaming instability and their saturation. We find that when the field is the result of the growth of the cosmic-ray--driven non--resonant instability alone, the spectrum of electrons and protons released by a supernova remnant are indeed different, but such a difference becomes appreciable only at energies $\gtrsim 100-1000$ GeV, while observations of the electron spectrum require such a difference to be present at energies as low as $\sim 10$ GeV. An effect at such low energies requires substantial magnetic field amplification in the late stages of the supernova remnant evolution (shock velocity $\ll 1000$ km/s), perhaps not due to streaming instability but hydrodynamical processes. We comment on the feasibility of such conditions and speculate on the possibility that the difference in spectral shape between electrons and protons may reflect either some unknown acceleration effect, or additional energy losses in cocoons around the sources.