Kinetic Simulations of Cosmic-Ray-Modified Shocks II: Particle Spectra

TL;DR

This study uses hybrid simulations to demonstrate that cosmic-ray-modified shocks produce steeper particle spectra due to CR advection in magnetic turbulence, challenging previous predictions of flatter spectra.

Contribution

It provides the first self-consistent simulation evidence that CR-modified shocks generate steeper spectra, highlighting the role of the postcursor in spectral shaping.

Findings

CR-modified shocks produce steeper spectra

Enhanced CR advection in the postcursor causes steepening

Results align with observations of supernova remnants

Abstract

Diffusive shock acceleration is a prominent mechanism for producing energetic particles in space and in astrophysical systems. Such energetic particles have long been predicted to affect the hydrodynamic structure of the shock, in turn leading to CR spectra flatter than the test-particle prediction. However, in this work along with a companion paper, C. C. Haggerty and D. Caprioli, 2020, arXiv:2008.12308 [astro-ph.HE], we use self-consistent hybrid (kinetic ions-fluid electrons) simulations to show for the first time how CR-modified shocks actually produce steeper spectra. The steepening is driven by the enhanced advection of CRs embedded in magnetic turbulence downstream of the shock, in what we call the "postcursor". These results are consistent with multi-wavelength observations of supernovae and supernova remnants and have significant phenomenological implications for space/astrophysical shocks in general.