Case for Centaurus A as the main source of ultrahigh-energy cosmic rays

Abstract

We discuss the possibility that a dominant fraction of the cosmic rays above the ankle (i.e., above 5 EeV)is due to a single nearby source, considering in particular the radio galaxy Centaurus A. We focus on theproperties of the source spectrum and composition required to reproduce the observations, showing thatthe nuclei are strongly suppressed for E > 10Z EeV, either by a rigidity dependent source cutoff or by thephotodisintegration interactions with the cosmic microwave background at the giant dipole resonance. Thevery mild attenuation effects at lower energies imply that the secondary nuclei from this source, producedin photodisintegration processes during propagation, only provide a small contribution. Given the moderateanisotropies observed, the deflections in extragalactic and Galactic magnetic fields should play a crucialrole in determining the cosmic ray arrival direction distribution. The diffusion in extragalactic fields as wellas the finite source lifetime also significantly affect the shape of the observed spectrum. The cosmic ray fluxat tens of EeV is dominated by the CNO component, and we show that it is actually better reproduced by amixture of C and O nuclei rather than by the usual assumption of a N component effectively describing thismass group. The Si and Fe group components become dominant above 70 EeV, in the energy range inwhich a strong spectral suppression is present. If the localized flux excess appearing above 40 EeV aroundthe Centaurus A direction is attributed to the CNO component, the He nuclei from the source in the energyrange from 10 to 20 EeV could lead to a similar anisotropy unless its contribution is suppressed. Thecosmic ray flux at a few EeV should mostly result from a more isotropic light component associated to apopulation of extragalactic sources. The inclusion of the subdominant contribution of heavy nuclei from theGalactic component helps to reproduce the observations around 1 EeV.