Cosmic Ray Spectrum in Supernova Remnant Shocks

TL;DR

This study uses kinetic simulations to explore how supernova remnant shocks accelerate cosmic rays, revealing the influence of environmental conditions and magnetic fields on the energy spectrum and acceleration efficiency.

Contribution

It introduces detailed kinetic modeling of DSA in SNRs, highlighting the roles of preshock temperature, injection rate, and magnetic field amplification in shaping CR spectra.

Findings

Higher injection fractions lead to efficient acceleration and flatter spectra.

SNRs in hot ISM are less efficient, producing steeper spectra.

Magnetic field amplification can steepen the CR spectrum to match observations.

Abstract

We perform kinetic simulations of diffusive shock acceleration (DSA) in Type Ia supernova remnants (SNRs) expanding into a uniform interstellar medium (ISM). Bohm-like diffusion assumed, and simple models for Alfvenic drift and dissipation are adopted. Phenomenological models for thermal leakage injection are considered as well. We find that the preshock gas temperature is the primary parameter that governs the cosmic ray (CR) acceleration efficiency and energy spectrum, while the CR injection rate is a secondary parameter. For SNRs in the warm ISM, if the injection fraction is larger than 10^{-4}, the DSA is efficient enough to convert more than 20 % of the SN explosion energy into CRs and the accelerated CR spectrum exhibits a concave curvature flattening to E^{-1.6}. Such a flat source spectrum near the knee energy, however, may not be reconciled with the CR spectrum observed at Earth. On the other hand, SNRs in the hot ISM, with an injection fraction smaller than 10^{-4}, are inefficient accelerators with less than 10 % of the explosion energy getting converted to CRs. Also the shock structure is almost test-particle like and the ensuing CR spectrum can be steeper than E^{-2}. With amplified magnetic field strength of order of 30 microG, Alfven waves generated by the streaming instability may drift upstream fast enough to make the modified test-particle power-law as steep as E^{-2.3}, which is more consistent with the observed CR spectrum.