Scrutinizing FR 0 Radio Galaxies as Ultra-High-Energy Cosmic Ray Source Candidates

TL;DR

This study investigates whether Fanaroff-Riley type 0 radio galaxies can accelerate and contribute to ultra-high-energy cosmic rays, analyzing their environments and acceleration mechanisms through spectral energy distributions.

Contribution

It provides the first detailed analysis of FR 0 galaxies as potential sources of UHECRs, including acceleration processes and environmental conditions.

Findings

FR 0 galaxies can contribute to UHECR flux if shear acceleration dominates.

They can account for cosmic rays between the knee and ankle energies.

Results are robust across different turbulence and shock models.

Abstract

Fanaroff-Riley (FR) 0 radio galaxies compose a new class of radio galaxies, which are usually weaker but much more numerous than the well-established class of FR 1 and FR 2 galaxies. The latter classes have been proposed as sources of the ultra-high-energy cosmic rays (UHECRs) with energies reaching up to ${\sim}10^{20}$ eV. Based on this conjecture, the possibility of UHECR acceleration and survival in an FR 0 source environment is examined in this work. In doing so, an average spectral energy distribution (SED) based on data from the FR 0 catalog (FR0CAT) is compiled. The resulting photon fields are used as targets for UHECRs, which suffer from electromagnetic pair production, photo-disintegration, photo-meson production losses, and synchrotron radiation. Multiple mechanisms are discussed to assess the UHECR acceleration probability, including Fermi-I order and gradual shear accelerations, and particle escape from the source region. This work shows that in a hybrid scenario, combining Fermi and shear accelerations, FR 0 galaxies can contribute to the observed UHECR flux, as long as $\Gamma_\mathrm{j}\gtrsim 1.6$, where shear acceleration starts to dominate over escape. Even in less optimistic scenarios, FR 0s can be expected to contribute to the cosmic-ray flux between the knee and the ankle. Our results are relatively robust with respect to the realized magnetic turbulence model and the speed of the accelerating shocks.