UHECRs from FR 0 Radio Galaxies - Acceleration. Loss. Escape.

Lukas Merten


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 classes of FR 1 and FR 2 radio galaxies. The latter classes have been proposed as sources of ultra-high energy cosmic rays (UHECRs) with energies reaching up to 100 EeV. Based on this conjecture, the possibility of UHECR acceleration and survival in an FR 0 source environment is discussed in this talk.

Based on the FR0CAT an average spectral energy distribution (SED) is compiled. The resulting photon fields are used as targets for the UHECRs, which suffer losses from electromagnetic pair production, photodisintegration, photo-meson production, and synchrotron radiation. To assess the acceleration probability, the two most promising scenarios – pure Fermi-I order and a hybrid scenario including additionally gradual shear acceleration – are presented in tandem with possible particle escape from the source region.

It will be shown that within the hybrid scenario FR 0 galaxies can contribute to the observed UHECR flux if their jet Lorentz factor is large enough Γ>1.6, where shear acceleration starts to dominate over escape. Even in less optimistic scenarios, FR 0s are likely contributing 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.