Effects of Neutral Particles on Modified Shocks at Supernova Remnants

TL;DR

This paper explores how neutral particles and pickup ions influence the structure and particle acceleration in supernova remnant shocks, revealing effects on compression ratios and cosmic ray spectra.

Contribution

It introduces the role of pickup ions in modified shocks and their impact on shock compression and cosmic ray acceleration in supernova remnants.

Findings

Pickup ions reduce subshock compression when CR pressure exceeds 20%.

Total compression ratio can be less than 4 even with CR-modified shocks.

CR spectra can be harder in quasi-perpendicular shocks with multiple ion reflections.

Abstract

H-alpha emission from supernova remnants (SNRs) implies the existence of neutral hydrogens in the ambient medium. In the precursor of an SNR shock modified by cosmic rays (CRs), upstream plasmas are pushed by the CR pressure, but neutral particles are not, so that the relative velocity appears and some neutral particles become pickup ions by the charge exchange process in the precursor. We investigate how the pickup ions generated in the precursor affect the shock structure and the particle acceleration. If the CR pressure is larger than 20% of the shock ram pressure, the compression of the subshock becomes smaller than that without pickup ions because of the pressure of the pickup ions. Moreover, even if the shock is modified by CRs, the total compression ratio can be smaller than 4. In addition, the pickup ions play an important role for the injection into the particle acceleration. If the shock is a quasi-perpendicular shock and if the the multiply reflected ion acceleration occurs, the CR spectrum can be harder than that of the test particle diffusive shock acceleration below GeV.