Constraining Sources of Ultra High Energy Cosmic Rays Using High Energy Observations with the Fermi Satellite

TL;DR

This study uses multi-wavelength photon data from satellites to constrain the astrophysical sources capable of accelerating particles to ultra-high energies, specifically examining active galactic nuclei and their structures.

Contribution

It introduces a novel method leveraging high-energy to low-energy photon luminosity ratios to evaluate the potential of AGN regions to produce UHECRs, excluding core regions of radio-loud AGN.

Findings

Core regions of nearby radio-loud AGN are excluded as UHECR sources.

Giant radio lobes are viable acceleration sites for UHECR protons.

Proton acceleration to ~10^{20} eV in Cen A occurs at >~ 100 kpc from the core.

Abstract

We analyze the conditions that enable acceleration of particles to ultra-high energies, ~10^{20} eV (UHECRs). We show that broad band photon data recently provided by WMAP, ISOCAM, Swift and Fermi satellites, yield constraints on the ability of active galactic nuclei (AGN) to produce UHECRs. The high energy (MeV - GeV) photons are produced by Compton scattering of the emitted low energy photons and the cosmic microwave background or extra-galactic background light. The ratio of the luminosities at high and low photon energies can therefore be used as a probe of the physical conditions in the acceleration site. We find that existing data excludes core regions of nearby radio-loud AGN as possible acceleration sites of UHECR protons. However, we show that giant radio lobes are not excluded. We apply our method to Cen A, and show that acceleration of protons to ~10^{20} eV can only occur at distances >~ 100 kpc from the core.