Acceleration of Cosmic Rays in Supernova Shocks: Elemental Selectivity of the Injection Mechanism

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 past models. In this paper, we show by self-consistent hybrid simulations that such special conditions are not vital for explaining 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 lives of SNRs. The integrated SNR rigidity spectrum for p /He ratio compares well with the AMS-02 and PAMELA data.