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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 ∼1020 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 Γj≳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.
Highlights
The idea of radio galaxies (RGs) contributing to the overall flux of ultra-high-energy cosmic rays (UHECRs), as measured on Earth, is several decades old and has been revisited many times in the past (e.g., [1,2,3,4,5])
Radio galaxies are among those source populations that are considered capable of accelerating charged nuclei to energies up to ∼ 1020 eV, and they fulfill the famous Hillas criterion [6] as well as energetics criteria [7,8] — as do their beamed counterparts, blazars
We refrain from detailed modeling of cosmic-ray energy spectra and secondary spectral energy distributions (SEDs) in the source region but rather attempt to answer whether FR 0 can potentially reach the required energies to contribute to the UHECR flux
Summary
The idea of radio galaxies (RGs) contributing to the overall flux of ultra-high-energy cosmic rays (UHECRs), as measured on Earth, is several decades old and has been revisited many times in the past (e.g., [1,2,3,4,5]). Radio galaxies are among those source populations that are considered capable of accelerating charged nuclei to energies up to ∼ 1020 eV, and they fulfill the famous Hillas criterion [6] as well as energetics criteria [7,8] — as do their beamed counterparts, blazars In this latter case, a few powerful sources are considered as the origin of detected UHECRs, and. We scrutinize this class of RGs as a contributor to the observed UHECR flux with a particular focus on their environment For this purpose, we first (Section 2) study various acceleration mechanisms for CR nuclei — Fermi-I/II, gradual shear, and relativistic blast wave acceleration — in tandem with all relevant particle losses in the radiative environment of AGN jets — Bethe-Heitler pair production, photo-disintegration, photo-meson production synchrotron losses, and diffusive particle escape.
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