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Acceleration of Heavy Ions at Non-Relativistic Collisionless Shocks

Damiano Caprioli, Luca Orusa, Miha Cernetic, Colby C. Haggerty, Bricker Ostler

TL;DR

This paper shows that in non-relativistic, collisionless shocks, partially ionized heavy ions (notably He) are preferentially accelerated via DSA, especially at high Mach numbers. Using 2D hybrid simulations with solar-abundance heavy ions, the authors demonstrate that He can dominate the non-thermal energy budget, enhance magnetic-field amplification, and raise the maximum achievable energy, thereby boosting hadronic gamma-ray and neutrino production. They find the injection enhancement scales strongly with the Mach number, with $K_{sp}$ for He and CNO increasing markedly as shocks become stronger, and the downstream spectra steepen due to the amplified turbulent fields. These results imply that heavy-ion physics must be included in models of strong astrophysical shocks, such as supernova remnants, to accurately predict CR spectra and multi-messenger emissions, particularly in metal-rich environments. The study also highlights limitations to be addressed in future work, including oblique and fully 3D configurations.

Abstract

We investigate the process of Diffusive Shock Acceleration (DSA) of particles with mass number to charge number ratios $A/Q > 1$, e.g., partially-ionized heavy ions. To this end, we introduce helium- and carbon-like ions at solar abundances into two-dimensional hybrid (kinetic ions-fluid electrons) simulations of non-relativistic collisionless shocks. This study yields three main results: 1) Heavy ions are preferentially accelerated compared to hydrogen. For typical solar abundances, the energy transferred to accelerated helium ions is comparable to, or even exceeds, that of hydrogen, thereby enhancing the overall shock acceleration efficiency. 2) Accelerated helium ions contribute to magnetic field amplification, which increases the maximum attainable particle energy and steepen the spectra of accelerated particles. 3) The efficient acceleration of helium significantly enhances the production of hadronic gamma rays and neutrinos, likely dominating the one due to hydrogen. These effects should be taken into account, especially when modeling strong space and astrophysical shocks.

Acceleration of Heavy Ions at Non-Relativistic Collisionless Shocks

TL;DR

This paper shows that in non-relativistic, collisionless shocks, partially ionized heavy ions (notably He) are preferentially accelerated via DSA, especially at high Mach numbers. Using 2D hybrid simulations with solar-abundance heavy ions, the authors demonstrate that He can dominate the non-thermal energy budget, enhance magnetic-field amplification, and raise the maximum achievable energy, thereby boosting hadronic gamma-ray and neutrino production. They find the injection enhancement scales strongly with the Mach number, with for He and CNO increasing markedly as shocks become stronger, and the downstream spectra steepen due to the amplified turbulent fields. These results imply that heavy-ion physics must be included in models of strong astrophysical shocks, such as supernova remnants, to accurately predict CR spectra and multi-messenger emissions, particularly in metal-rich environments. The study also highlights limitations to be addressed in future work, including oblique and fully 3D configurations.

Abstract

We investigate the process of Diffusive Shock Acceleration (DSA) of particles with mass number to charge number ratios , e.g., partially-ionized heavy ions. To this end, we introduce helium- and carbon-like ions at solar abundances into two-dimensional hybrid (kinetic ions-fluid electrons) simulations of non-relativistic collisionless shocks. This study yields three main results: 1) Heavy ions are preferentially accelerated compared to hydrogen. For typical solar abundances, the energy transferred to accelerated helium ions is comparable to, or even exceeds, that of hydrogen, thereby enhancing the overall shock acceleration efficiency. 2) Accelerated helium ions contribute to magnetic field amplification, which increases the maximum attainable particle energy and steepen the spectra of accelerated particles. 3) The efficient acceleration of helium significantly enhances the production of hadronic gamma rays and neutrinos, likely dominating the one due to hydrogen. These effects should be taken into account, especially when modeling strong space and astrophysical shocks.

Paper Structure

This paper contains 8 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Simulations
  3. High-$M$ regime
  4. The scaling with $M$
  5. Observables
  6. High-energy $\gamma$ rays and neutrinos
  7. Overall spectra of He and heavy ions
  8. Conclusion

Figures (3)

  • Figure 1: Panel a: Downstream energy spectra from $M=40$ shock simulations with and without heavy ions at time $t=500\,\omega_c^{-1}$. Panels b and c: Injection efficiency $\varepsilon_{s}$ and total injection efficiency $\varepsilon_{s, \rm tot}$ as a function of time.
  • Figure 2: Enhancement $K_{sp}$ of He and CNO with respect to H as a function of $M$. For stronger shocks, heavy ions are preferentially injected into DSA.
  • Figure 3: As a function of Mach number we show the individual injection efficiency $\varepsilon_{s}$ (panel a), the total injection efficiency $\varepsilon_{s,\mathrm{tot}}$ (panel b), and the slope of the non-thermal tail for different species (panel c) at $t = 500\,\omega_c^{-1}$ (the superscript $sat$ refers to the measurement done at this specific time, when the efficiencies have saturared).