From E. Fermi to Fermi-LAT: watching particle acceleration in supernova remnants

TL;DR

This paper reviews how Fermi-LAT observations of supernova remnants have advanced understanding of cosmic ray acceleration, highlighting the role of magnetic field amplification and kinetic simulations in explaining particle energization.

Contribution

It introduces non-linear diffusive shock acceleration models incorporating magnetic field amplification, successfully applied to Tycho's SNR, and discusses kinetic simulations of particle-field coupling.

Findings

Fermi-LAT confirms hadronic gamma-ray emission in some SNRs.

CR spectra are steeper than standard diffusive shock acceleration predictions.

Magnetic field amplification is key to reconciling models with observations.

Abstract

Supernova remnants (SNRs) have been regarded for many decades as the sources of Galactic cosmic rays (CRs) up to a few PeV. However, only with the advent of Fermi-LAT it has been possible to detect - at least in some SNRs - \gamma-rays whose origin is unequivocally hadronic, namely due to the decay of neutral pions produced by collisions between relativistic nuclei and the background plasma. When coupled with observations in other bands (from radio to TeV \gamma-rays), Fermi-LAT data present evidence for CR spectra significantly steeper than the standard prediction of diffusive shock acceleration, forcing us to rethink our theoretical understanding of efficient particle energization at strong shocks. We outline how, by including the effects of CR-triggered magnetic field amplification, it is possible to reconcile non-linear models of diffusive shock acceleration with \gamma-ray observations, in particular providing a successful application of such a theory to Tycho's SNR. Finally, we show how kinetic simulations can investigate the microphysics of the non-linear coupling of accelerated particles and magnetic fields, probing from first principles the efficiency of the Fermi mechanism at strong shocks