[Paper Review] Extragalactic Propagation of Ultra-High Energy Cosmic Rays

D. Kuempel|arXiv (Cornell University)|Sep 10, 2014

Astrophysics and Cosmic Phenomena5 references3 citations

TL;DR

This paper reviews the propagation of ultra-high energy cosmic rays (UHECRs) through intergalactic space, focusing on interactions with cosmic microwave background photons and deflections in extragalactic magnetic fields. Using advanced simulation codes, it demonstrates that the GZK suppression and composition data from Pierre Auger and Telescope Array constrain source spectra and chemical composition, favoring hard injection indices and limiting proton fractions to ~10%, implying a need for multi-messenger analysis to resolve astrophysical origins.

ABSTRACT

More than 100 years after the discovery of cosmic rays and various experimental efforts, the origin of ultra-high energy cosmic rays (E > 100 PeV) remains unclear. The understanding of production and propagation effects of these highest energetic particles in the universe is one of the most intense research fields of high-energy astrophysics. With the advent of advanced simulation engines developed during the last couple of years, and the increase of experimental data, we are now in a unique position to model source and propagation parameters in an unprecedented precision and compare it to measured data from large scale observatories. In this paper we revisit the most important propagation effects of cosmic rays through photon backgrounds and magnetic fields and introduce recent developments of propagation codes. Finally, by comparing the results to experimental data, possible implications on astrophysical parameters are given.

Motivation & Objective

• To understand how ultra-high energy cosmic rays (UHECRs) propagate through cosmic photon backgrounds and magnetic fields, affecting their energy spectra and arrival directions.
• To assess the impact of photo-pion production and energy loss mechanisms such as the GZK effect on UHECR flux suppression.
• To evaluate the role of magnetic field deflections in altering the sky distribution of UHECRs and influencing composition interpretation.
• To compare simulated UHECR propagation with data from large-scale observatories like Pierre Auger and Telescope Array to constrain astrophysical models.
• To advocate for a multi-messenger approach combining spectrum, composition, arrival directions, and secondary messengers (neutrinos, photons) for improved model constraints.

Proposed method

• Uses the interaction length formula λ⁻¹(E) = ∫ n(ε)σ_avg(ε) dε to model energy loss due to photon backgrounds, with n(ε) from CMB, IRB, and URB spectra.
• Applies the threshold energy for photo-pion production: E_thres^N,π ≈ 6.8×10¹⁹ eV × (10⁻³ eV / ε)⁻¹, accounting for Lorentz-boosted photon energies in the nucleus rest frame.
• Models nuclear interactions via the approximation that pion production in nuclei scales with the number of nucleons, assuming similar dynamics to proton interactions.
• Employs 3D propagation simulations on computer clusters to model cosmic ray transport through magnetic fields and photon backgrounds.
• Compares simulated spectra, X_max (shower maximum depth), and σ(X_max) (fluctuations) to data from Pierre Auger Observatory, using interaction models like DPMJET and QGSP.
• Utilizes event-shape observables and energy-energy correlations to probe turbulent and coherent magnetic fields, and cross-references secondary particle production (photons, neutrinos) with observational limits.

Experimental results

Research questions

• RQ1 What is the impact of cosmic microwave background interactions on the energy spectrum and composition of ultra-high energy cosmic rays?
• RQ2 How do extragalactic magnetic fields affect the arrival direction distribution and deflection angles of UHECRs?
• RQ3 To what extent can the observed GZK suppression and spectrum shape constrain the intrinsic source spectral index and maximum energy?
• RQ4 Can the observed X_max and σ(X_max) values from air shower data be reproduced by simulations assuming different source compositions and injection models?
• RQ5 How can arrival direction patterns and secondary messengers (neutrinos, photons) improve constraints on UHECR astrophysics beyond spectrum and composition alone?

Key findings

• The observed UHECR spectrum above ~5×10¹⁸ eV requires a hard source spectral index of β ≈ -1 to be reproduced, indicating a non-thermal, high-energy acceleration mechanism.
• The Pierre Auger data show that a pure proton component with a fraction >10% at E > 10¹⁸ eV is disfavored by anisotropy upper limits, suggesting a significant contribution from heavier nuclei.
• Simulations indicate that the lower-energy part of the UHECR spectrum (below ~5×10¹⁸ eV) cannot be explained by a single population without introducing a second component, such as galactic cosmic rays or light-element extragalactic sources.
• The shape of the simulated X_max and its dispersion σ(X_max) show good agreement with Auger data only when using specific interaction models and assuming a mixed composition, with iron-like nuclei providing better fits than protons at high energies.
• Event-shape observables and energy-energy correlations in arrival directions show sensitivity to coherent and turbulent magnetic fields, enabling indirect measurement of field strength and structure.
• Secondary neutrino and photon fluxes predicted by simulations are consistent with current observational limits, supporting the hadronic origin of UHECRs and enabling future multi-messenger constraints.