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Production and propagation of cosmic-ray positrons and electrons

I. V. Moskalenko, A. W. Strong

TL;DR

This paper develops a self-consistent three-dimensional diffusive-halo model of Galactic cosmic-ray propagation to predict secondary positrons and primary electrons. It evolves primaries and secondaries with momentum-space diffusion, reacceleration, and energy losses, and constrains parameters using the Boron/Carbon ratio and interstellar gas/radiation fields, including $D(\rho)$ and $v_A$. The results show good agreement with measured positron fractions up to $\sim$10 GeV, with a potential excess at higher energies that can be explained either by a harder interstellar nucleon spectrum or by a primary positron component. The study links gamma-ray observations with local CR measurements, suggesting that the Galactic interstellar proton/Helium spectra may be harder than the heliospheric ones and has implications for diffuse Galactic gamma-ray emission models.

Abstract

We have made a new calculation of the cosmic-ray secondary positron spectrum using a diffusive halo model for Galactic cosmic-ray propagation. The code computes self-consistently the spectra of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. The models are first adjusted to agree with the observed cosmic-ray Boron/Carbon ratio, and the interstellar proton and Helium spectra are then computed; these spectra are used to obtain the source function for the secondary positrons/electrons which are finally propagated with the same model parameters. The primary electron spectrum is evaluated, again using the same model. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. Our study includes a critical re-evaluation of the secondary decay calculation for positrons. The predicted positron fraction is in good agreement with the measurements up to 10 GeV, beyond which the observed flux is higher than that calculated. Since the positron fraction is now accurately measured in the 1-10 GeV range our primary electron spectrum should be a good estimate of the true interstellar spectrum in this range, of interest for gamma ray and solar modulation studies. We further show that a harder interstellar nucleon spectrum, similar to that suggested to explain EGRET diffuse Galactic gamma ray observations above 1 GeV, can reproduce the positron observations above 10 GeV without requiring a primary positron component.

Production and propagation of cosmic-ray positrons and electrons

TL;DR

This paper develops a self-consistent three-dimensional diffusive-halo model of Galactic cosmic-ray propagation to predict secondary positrons and primary electrons. It evolves primaries and secondaries with momentum-space diffusion, reacceleration, and energy losses, and constrains parameters using the Boron/Carbon ratio and interstellar gas/radiation fields, including and . The results show good agreement with measured positron fractions up to 10 GeV, with a potential excess at higher energies that can be explained either by a harder interstellar nucleon spectrum or by a primary positron component. The study links gamma-ray observations with local CR measurements, suggesting that the Galactic interstellar proton/Helium spectra may be harder than the heliospheric ones and has implications for diffuse Galactic gamma-ray emission models.

Abstract

We have made a new calculation of the cosmic-ray secondary positron spectrum using a diffusive halo model for Galactic cosmic-ray propagation. The code computes self-consistently the spectra of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. The models are first adjusted to agree with the observed cosmic-ray Boron/Carbon ratio, and the interstellar proton and Helium spectra are then computed; these spectra are used to obtain the source function for the secondary positrons/electrons which are finally propagated with the same model parameters. The primary electron spectrum is evaluated, again using the same model. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. Our study includes a critical re-evaluation of the secondary decay calculation for positrons. The predicted positron fraction is in good agreement with the measurements up to 10 GeV, beyond which the observed flux is higher than that calculated. Since the positron fraction is now accurately measured in the 1-10 GeV range our primary electron spectrum should be a good estimate of the true interstellar spectrum in this range, of interest for gamma ray and solar modulation studies. We further show that a harder interstellar nucleon spectrum, similar to that suggested to explain EGRET diffuse Galactic gamma ray observations above 1 GeV, can reproduce the positron observations above 10 GeV without requiring a primary positron component.

Paper Structure

This paper contains 13 sections, 23 equations, 8 figures.

Table of Contents

  1. Introduction
  2. Description of the models
  3. The model parameters
  4. Production of secondary positrons and electrons
  5. Interstellar positron and electron spectra
  6. A harder interstellar nucleon spectrum ?
  7. Conclusions
  8. Spectra of secondaries from $pp$-collisions
  9. Pion production in $pp$-collisions
  10. Stecker's model
  11. The scaling model
  12. Pion decay
  13. Kaon decay

Figures (8)

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  • ...and 3 more figures