Cosmic Ray Spectrum in Supernova Remnant Shocks

TL;DR

This study uses kinetic simulations to explore how supernova remnant shocks accelerate cosmic rays, highlighting the influence of the interstellar medium's temperature and magnetic field on acceleration efficiency and spectrum shape.

Contribution

It provides new insights into the dependence of cosmic ray acceleration on ISM conditions and the effects of Alfvénic drift on the resulting energy spectrum.

Findings

Hot ISM leads to inefficient CR acceleration with less than 10% energy transfer.

CR spectra can be steeper than E^{-2} due to Alfvénic drift effects.

Particles can reach knee energies but with a softened spectrum.

Abstract

We performed kinetic simulations of diffusive shock acceleration in Type Ia supernova remnants (SNRs) expanding into a uniform interstellar medium (ISM). The preshock gas temperature is the primary parameter that governs the cosmic ray (CR) acceleration, while magnetic field strength and CR injection rate are secondary parameters. SNRs in the hot ISM, with an injection fraction smaller than 10^{-4}, are inefficient accelerators with less than 10 % energy getting converted to CRs. The shock structure is almost test-particle like and the ensuing CR spectrum can be steeper than E^{-2}. Although the particles can be accelerated to the knee energy of 10^{15.5}Z eV with amplified magnetic fields in the precursor, Alfv'enic drift of scattering centers softens the source spectrum as steep as E^{-2.1} and reduces the CR acceleration efficiency.