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Register a new userScrutinizing FR 0 Radio Galaxies as Ultra-High-Energy Cosmic Ray Source Candidates
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Authors
Lukas Merten
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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.
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The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starburst galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-lobed radio galaxies such as Centaurus A and Fornax A can explain the data.
We explore the possibility that the recently detected dipole anisotropy in the arrival directions of~$>8$~EeV ultra-high energy cosmic-rays (UHECRs) arises due to the large-scale structure (LSS). We assume that the cosmic ray sources follow the matter distribution and calculate the flux-weighted UHECRs RMS dipole amplitude taking into account the diffusive transport in the intergalactic magnetic field (IGMF). We find that the flux-weighted RMS dipole amplitude is $sim8$% before entering the Galaxy. The amplitude in the [4-8] EeV is only slightly lower $sim 5$%. The required IGMF is of the order of {5-30 nG}, and the UHECR sources must be relatively nearby, within $sim$300 Mpc. The absence of statistically significant signal in the lower energy bin can be explained if the same nuclei specie dominates the composition in both energy bins and diffusion in the Galactic magnetic field (GMF) reduces the dipole of these lower rigidity particles. Photodisintegration of higher energy UHECRs could also reduce somewhat the lower energy dipole.
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The status of the Greisen-Zatsepin-Kuzmin (GZK) cutoff and pair-production dip in Ultra High Energy Cosmic Rays (UHECR) is discussed.They are the features in the spectrum of protons propagating through CMB radiation in extragalactic space, and discovery of these features implies that primary particles are mostly extragalactic protons. The spectra measured by AGASA, Yakutsk, HiRes and Auger detectors are in good agreement with the pair-production dip, and HiRes data have strong evidences for the GZK cutoff. The Auger spectrum,as presented at the 30th ICRC 2007, agrees with the GZK cutoff, too. The AGASA data agree well with the beginning of the GZK cutoff at E leq 80 EeV, but show the excess of events at higher energies, the origin of which is not understood. The difference in the absolute fluxes measured by different detectors disappears after energy shift within the systematic errors of each experiment.
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