[Paper Review] Acceleration and Interaction of Ultra High Energy Cosmic Rays

R. J. Protheroe|arXiv (Cornell University)|Dec 3, 1998

Astrophysics and Cosmic Phenomena2 references16 citations

TL;DR

This paper investigates shock acceleration in Fanaroff-Riley Class II radio galaxies as the most plausible source for ultra-high-energy cosmic rays (UHECRs), showing that such sources can naturally produce particles up to 300 EeV. It argues that standard astrophysical mechanisms—without exotic models like topological defects—can explain the observed spectrum, composition, and anisotropy, especially when accounting for energy losses and magnetic field effects during propagation through the intergalactic medium.

ABSTRACT

In this chapter I give an overview of shock acceleration, including a discussion of the maximum energies possible and the shape of the spectrum near cut-off, interactions of high energy cosmic rays with, and propagation through, the background radiation, and the resulting electron-photon cascade. Possible sources of the highest energy cosmic rays are discussed including active galaxies, gamma ray bursts and topological defects. I argue that while the origin of the highest energy cosmic rays is still uncertain, it is not necessary to invoke exotic models such as emission by topological defects to explain the existing data. It seems likely that shock acceleration at Fanaroff-Riley Class II radio galaxies can account for the existing data. However, new cosmic ray data, as well as better estimates of the extragalactic radiation fields and magnetic fields will be necessary before we will be certain of the origin of the highest energy particles occurring in nature.

Motivation & Objective

• To assess the viability of conventional astrophysical sources—particularly Fanaroff-Riley Class II radio galaxies—for accelerating cosmic rays to ultra-high energies.
• To evaluate the role of interactions with cosmic background radiation and magnetic fields in shaping the observed energy spectrum and composition of UHECRs.
• To determine whether the observed GZK cut-off and spectrum above 10 EeV can be explained by standard propagation and acceleration mechanisms.
• To test whether exotic models such as topological defects are necessary to explain the data, or if conventional physics suffices.
• To guide future observational efforts by identifying key observables—such as neutrino fluxes and arrival direction patterns—for distinguishing between competing models.

Proposed method

• Modeling shock acceleration in extragalactic sources, particularly FR II radio galaxies, using standard diffusive shock acceleration theory.
• Calculating energy losses via Bethe-Heitler pair production and pion photoproduction on cosmic microwave and infrared background radiation.
• Simulating electron-photon cascades initiated by UHECR interactions with background radiation fields.
• Using magnetic field models (e.g., 0.1 μG in superclusters) to assess deflection and anisotropy in UHECR arrival directions.
• Comparing simulated spectra and arrival patterns with observational data from AGASA and other experiments.
• Evaluating the impact of source distribution, magnetic field structure, and injection spectrum (e.g., E⁻².⁴) on the final observed flux.

Experimental results

Research questions

• RQ1 Can shock acceleration in Fanaroff-Riley Class II radio galaxies produce cosmic rays with energies up to 300 EeV?
• RQ2 To what extent do interactions with cosmic background radiation and magnetic fields alter the energy spectrum, composition, and arrival direction of UHECRs?
• RQ3 Is the observed GZK cut-off and the spectrum above 10 EeV consistent with standard propagation models, or does it require exotic sources?
• RQ4 How do electron-photon cascades initiated by UHECRs affect the observed gamma-ray and neutrino fluxes?
• RQ5 Can the observed anisotropy and lack of correlation with known sources be explained by magnetic field deflection and source distribution?

Key findings

• Shock acceleration in Fanaroff-Riley Class II radio galaxies can produce cosmic rays with energies up to at least 300 EeV, consistent with the highest observed events.
• The observed spectrum above 10 EeV, including the flat component and the GZK cut-off, is well explained by standard propagation and acceleration physics without invoking exotic models.
• Energy losses via pion photoproduction and pair production limit maximum energies to ~10¹¹ GeV for sources beyond ~20 Mpc, consistent with the GZK cut-off.
• Cosmic rays above 100 EeV are significantly deflected by magnetic fields, with arrival directions spread over ~80° below 100 EeV and ~10° above 200 EeV, explaining the lack of correlation with known sources.
• Electron-photon cascades initiated by UHECRs produce detectable fluxes of high-energy gamma rays and neutrinos, which may dominate the high-energy spectrum.
• The local supercluster environment with ~0.1 μG magnetic fields and a 10 Mpc source distance reproduces the observed spectrum above 10 EeV when combined with a soft injection spectrum (E⁻².⁴).