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 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.

arXiv commentarXiv admin note: substantial text overlap with arXiv:1707.02744

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• Inference of the Local Interstellar Spectra of Cosmic Ray Nuclei Z<=28 with the GalProp-HelMod Framework
• The Origin of Galactic Cosmic Rays as Revealed by their Composition
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• The TeV Cosmic Ray Bump: a Message from Epsilon Indi or Epsilon Eridani Star?
• On the origin of observed cosmic ray spectrum below 100 TV
• Testing the universality of cosmic-ray nuclei from protons to oxygen with AMS-02
• The Origin of Cosmic Rays: How Their Composition Defines Their Sources and Sites, and the Processes of Their Mixing, Injection and Acceleration
• Interpretation of the excess of antiparticles within a modified paradigm of galactic cosmic rays
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• Intermediate mass and heavy Galactic cosmic-ray nuclei: the case of new AMS-02 measurements
• Steepening of Cosmic Ray Spectra in Shocks with Varying Magnetic Field Direction
• Solar Energetic Particle Charge States and Abundances with Nonthermal Electrons
• Electron Energization in Quasi-Parallel Shocks: Test-Particle-Electrons in a Proton Driven Turbulence
• Direct measurements of cosmic rays and their possible interpretations
• $\overline{\text{D}}$arkRayNet: Emulation of cosmic-ray antideuteron fluxes from dark matter
• Acceleration of Heavy Ions at Non-Relativistic Collisionless Shocks
• Flat Spectra of Energetic Particles in Interplanetary Shock Precursors