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Cosmic-ray propagation and interactions in the Galaxy

Andrew W. Strong, Igor V. Moskalenko, Vladimir S. Ptuskin

TL;DR

Cosmic rays propagate through diffusion, convection, and reacceleration in a magnetized, gas-rich interstellar medium, up to energies around $10^{15}$ eV. The paper surveys the theoretical transport framework, derives the key transport coefficients, and tests predictions against direct CR measurements and indirect gamma-ray and synchrotron data, highlighting the role of the diffusion coefficient, halo size, and local sources. It emphasizes the GALPROP numerical code as a central tool for self-consistent modeling of CRs, gamma rays, and radio emission, and discusses tensions such as the antiproton flux and the GeV gamma-ray excess that motivate refined propagation scenarios. The work underscores the importance of integrating multiwavelength observations to constrain propagation physics, ISM structure, and the CR impact on Galactic dynamics, with implications for future missions and data analysis.

Abstract

We survey the theory and experimental tests for the propagation of cosmic rays in the Galaxy up to energies of 10^15 eV. A guide to the previous reviews and essential literature is given, followed by an exposition of basic principles. The basic ideas of cosmic-ray propagation are described, and the physical origin of its processes are explained. The various techniques for computing the observational consequences of the theory are described and contrasted. These include analytical and numerical techniques. We present the comparison of models with data including direct and indirect - especially gamma-ray - observations, and indicate what we can learn about cosmic-ray propagation. Some particular important topics including electrons and antiparticles are chosen for discussion.

Cosmic-ray propagation and interactions in the Galaxy

TL;DR

Cosmic rays propagate through diffusion, convection, and reacceleration in a magnetized, gas-rich interstellar medium, up to energies around eV. The paper surveys the theoretical transport framework, derives the key transport coefficients, and tests predictions against direct CR measurements and indirect gamma-ray and synchrotron data, highlighting the role of the diffusion coefficient, halo size, and local sources. It emphasizes the GALPROP numerical code as a central tool for self-consistent modeling of CRs, gamma rays, and radio emission, and discusses tensions such as the antiproton flux and the GeV gamma-ray excess that motivate refined propagation scenarios. The work underscores the importance of integrating multiwavelength observations to constrain propagation physics, ISM structure, and the CR impact on Galactic dynamics, with implications for future missions and data analysis.

Abstract

We survey the theory and experimental tests for the propagation of cosmic rays in the Galaxy up to energies of 10^15 eV. A guide to the previous reviews and essential literature is given, followed by an exposition of basic principles. The basic ideas of cosmic-ray propagation are described, and the physical origin of its processes are explained. The various techniques for computing the observational consequences of the theory are described and contrasted. These include analytical and numerical techniques. We present the comparison of models with data including direct and indirect - especially gamma-ray - observations, and indicate what we can learn about cosmic-ray propagation. Some particular important topics including electrons and antiparticles are chosen for discussion.

Paper Structure

This paper contains 29 sections, 1 equation, 16 figures.

Table of Contents

  1. Introduction
  2. Cosmic-ray Propagation: Theory
  3. Basics and Approaches
  4. Propagation equation
  5. Diffusion
  6. Convection
  7. Reacceleration
  8. Galactic structure
  9. Interactions
  10. Weighted Slabs and Leaky Boxes
  11. Explicit models
  12. GALPROP
  13. Numerical versus analytical
  14. Self-consistent models
  15. Confrontation of Theory with Data
  16. ...and 14 more sections

Figures (16)

  • Figure 1: Compilation of spectral data $10^{10} - 10^{17}$ eV for p, He, Fe, combining balloon, satellite and ground-based measurements. From 2005astro.ph..8014H and G. Hörandel, private communication.
  • Figure 2: Preliminary spectra of p, He from ATIC-2, compared with AMS01, BESS-TeV and CAPRICE98. Plot from ATIC collaboration Panov2006. The ATIC-2 data indicate a slighly harder spectrum for He above 1 TeV.
  • Figure 3: Compilation of spectral data for element groups CNO, NeMgSi, Fe 2005ApJ...628L..41D from HEAO-3, SANRIKU, CRN, SOKOL, JACEE and RUNJOB (upper) and of separate even-Z elements from preliminary ATIC-2, HEAO-3, CRN and TRACER (lower). Plot from ATIC collaboration Panov2006. The ATIC-2 data suggest a hardening above 10 TeV.
  • Figure 4: Measurements of the electron spectrum, including AMS01, CAPRICE94, HEAT and SANRIKU, compared with possible contributions of distant sources and local supernova remnants, from 2004ApJ...601..340K.
  • Figure 5: Measurements of the positron spectrum, including data from MASS91, AMS01, CAPRICE94 and HEAT, from 2002AA...392..287G. Propagation calculations for interstellar (upper curves) and modulated (lower curves) are shown. Dotted, dot-dashed: GALPROP 1998ApJ...493..694M; dashed, solid: 2001AdSpR..27..687S.
  • ...and 11 more figures