[Paper Review] Cosmic ray propagation with CRPropa 3

Rafael Alves Batista, M. Erdmann+8 more|arXiv (Cornell University)|Oct 20, 2014

Astrophysics and Cosmic Phenomena6 references4 citations

TL;DR

CRPropa 3 is a next-generation Monte Carlo simulation code for ultra-high-energy cosmic ray (UHECR) propagation, featuring a modular C++/Python architecture, shared-memory parallelism, and support for time-dependent cosmological evolution and high-resolution galactic magnetic fields. It enables accurate modeling of UHECR energy spectra, composition, and arrival directions, with simulations showing that galactic magnetic fields suppress small-scale anisotropies while preserving large-scale structure in arrival directions.

ABSTRACT

Solving the question of the origin of ultra-high energy cosmic rays (UHECRs) requires the development of detailed simulation tools in order to interpret the experimental data and draw conclusions on the UHECR universe. CRPropa is a public Monte Carlo code for the galactic and extragalactic propagation of cosmic ray nuclei above $\sim 10^{17}$ eV, as well as their photon and neutrino secondaries. In this contribution the new algorithms and features of CRPropa 3, the next major release, are presented. CRPropa 3 introduces time-dependent scenarios to include cosmic evolution in the presence of cosmic ray deflections in magnetic fields. The usage of high resolution magnetic fields is facilitated by shared memory parallelism, modulated fields and fields with heterogeneous resolution. Galactic propagation is enabled through the implementation of galactic magnetic field models, as well as an efficient forward propagation technique through transformation matrices. To make use of the large Python ecosystem in astrophysics CRPropa 3 can be steered and extended in Python.

Motivation & Objective

• To develop a flexible, extensible, and high-performance simulation framework for ultra-high-energy cosmic ray (UHECR) propagation in complex astrophysical environments.
• To enable accurate modeling of UHECR propagation through both extragalactic and galactic magnetic fields, including time-dependent cosmological evolution.
• To support high-resolution magnetic field and matter distributions using shared-memory parallelism and dynamic step-size adjustment.
• To integrate with the Python ecosystem for user-friendly simulation steering, extension, and analysis.
• To reproduce key observables—energy spectra, composition (via $X_{\mathrm{max}}$), and arrival direction anisotropies—using realistic astrophysical scenarios.

Proposed method

• CRPropa 3 uses a modular simulation architecture where each physical process (e.g., deflection, interaction, decay) is encapsulated in independent, composable modules.
• A bidding system dynamically selects the optimal step size for numerical accuracy, based on the smallest bid from all active modules.
• The code is implemented in C++ with Python interface via SWIG, enabling cross-language polymorphism and direct extension of simulations using Python.
• Shared-memory parallelism via OpenMP allows efficient use of multicore processors, particularly for high-resolution magnetic field and matter distributions.
• Galactic propagation is enabled through transformation matrices and a lensing technique that models deflections using a 3D galactic magnetic field model.
• Cosmological evolution is implemented in 4D simulations using time-dependent source and background radiation distributions.

Experimental results

Research questions

• RQ1 How can UHECR propagation simulations be made more flexible and extensible to explore diverse astrophysical scenarios?
• RQ2 To what extent do high-resolution galactic magnetic fields and time-dependent cosmological evolution affect UHECR energy spectra and arrival direction patterns?
• RQ3 Can a modular, high-performance simulation framework like CRPropa 3 accurately reproduce observed UHECR energy spectra and composition via $X_{\mathrm{max}}$?
• RQ4 How do deflections in the galactic magnetic field suppress or preserve small-scale anisotropies in the UHECR sky distribution?
• RQ5 What is the impact of using realistic, structured matter and magnetic field distributions on UHECR propagation and observables?

Key findings

• The simulation reproduces the observed UHECR energy spectrum above $10^{18.7}$ eV reasonably well for a mixed composition of H, He, N, and Fe.
• The $X_{\mathrm{max}}$ moments are not well reproduced in the simulated scenario, indicating a need for refined composition or interaction models.
• Galactic magnetic fields reduce small-scale anisotropies in the arrival direction distribution, as shown in HEALpix maps before and after propagation.
• Despite deflections, large-scale anisotropic features persist in the arrival direction map, consistent with the underlying matter distribution.
• The use of the JF12 galactic magnetic field model enables realistic deflection patterns, with deflection angles dependent on particle rigidity and field strength.
• The modular design and Python interface allow for efficient simulation configuration, extension, and integration with external astrophysical tools.