Acceleration of Cosmic Rays in Supernova Shocks: elemental selectivity of the injection mechanism

TL;DR

This paper demonstrates through hybrid simulations that supernova remnant shocks can naturally produce the observed elemental selectivity in cosmic ray acceleration, explaining the proton-helium spectral differences without complex environmental assumptions.

Contribution

It provides a self-consistent simulation-based explanation for cosmic ray elemental selectivity, confirming the role of shock Mach number in injection efficiency without extraneous factors.

Findings

Simulations show SNR shocks can preferentially accelerate elements from a homogeneous plasma.

Injection efficiency depends on shock Mach number and charge-to-mass ratio.

The integrated proton-helium spectrum matches AMS-02 and PAMELA observations.

Abstract

Precise measurements of galactic cosmic rays revealed a significant difference between the rigidity spectral indices of protons and helium ions. This finding is a notable contrast to the commonly accepted theoretical prediction that supernova remnant (SNR) shocks accelerate protons and helium ions with the same rigidity alike. Most of the earlier explanations for the "paradox" appealed to SNR environmental factors, such as inhomogeneous $p$/He mixes in the shock upstream medium, variable ionization states of He, or a multi-SNR origin of the observed spectra. The newest observations, however, are in tension with most of them. In this paper, we show by self-consistent hybrid simulations that such special conditions are not vital for the explanation of the cosmic ray rigidity spectra. In particular, our simulations prove that an SNR shock can modify the chemical composition of accelerated cosmic rays by preferentially extracting them from a homogeneous background plasma without additional, largely untestable assumptions. Our results confirm the earlier theoretical predictions of how the efficiency of injection depends on the shock Mach number $M.$ Its increase with the charge-to-mass ratio saturates at a level that grows with $M.$ We have convolved the time-dependent injection rates of protons and helium ions, obtained from the simulations, with a decreasing shock strength over the active life of SNRs. The integrated SNR rigidity spectrum for $p$/He ratio compares well with the AMS-02 and PAMELA data.