Ultra high energy cosmic rays: implications of Auger data for source spectra and chemical composition

Abstract

Abstract. We use a kinetic-equation approach to describe the propagation of ultra high energy cosmic ray protons and nuclei and calculate the expected spectra and mass composition at the Earth for different assumptions on the source injection spectra and chemical abundances. When compared with the spectrum, the elongation rate X max (E) and dispersion σ(X max ) as observed with the Pierre Auger Observatory, several important consequences can be drawn: a) the injection spectra of nuclei must be very hard, ∼ E −γ with γ ∼ 1 − 1.6; b) the maximum energy of nuclei of charge Z in the sources must be ∼ 5Z × 10 18 eV, thereby not requiring acceleration to extremely high energies; c) the fit to the Auger spectrum can be obtained only at the price of adding an ad hoc light extragalactic component with a steep injection spectrum (∼ E −2.7 ). In this sense, at the ankle (E A ≈ 5 × 10 18 eV) all the components are of extragalactic origin, thereby suggesting that the transition from Galactic to extragalactic cosmic rays occurs below the ankle. Interestingly, the additional light extragalactic component postulated above compares well, in terms of spectrum and normalization, with the one recently measured by KASCADE-Grande.

The Physics of UHECRs: Spectra, Composition and the Transition Galactic-Extragalactic

“…Nevertheless, the anisotropy expected for a galactic light component extending up to 10 18 eV exceeds by more than one order of magnitude the upper limit measured by Auger [25]. The possible origin of the additional (light and extra-galactic) component can be modelled essentially in two ways: (i) assuming the presence of different classes of sources: one injecting also heavy nuclei with hard spectrum and the other only proton and helium nuclei with soft spectrum [26,27] or (ii) identifying a peculiar class of sources that could provide at the same time a steep light component and a flat heavy one [28][29][30].…”

Ultra High Energy Cosmic Rays and Neutrinos

“…As discussed above, the effect of EBL on proton propagation has an important role for the production of secondary neutrinos, but it negligibly affects the expected proton flux. Figure 1 shows the fluxes of UHECR as measured by TA (purple filled squares), Auger (green filled circles) Figure 3: Flux of UHECR in the case of the model with two classes of sources as in [23,30]. Experimental data as in figure 1.…”

“…The ankle feature will be related to a change in the source family contributing to the flux [23,30,31] or to specific dynamic at the source [32,33].…”

“…Taking the model presented in [23,30] in figure 3 we plot the flux of UHECR together with the experimental data of Kascade-Grande, Auger and TA, in figure 4 we plot the corresponding flux of secondary neutrinos. The production of secondary neutrinos in this case is restricted only to PeV energies and comes from the photopion production process on the EBL radiation field suffered by protons of the low energy tail of UHECR.…”

“…Tacking into account the contribution of protons, gammas and neutrinos produced by SHDM decay, in figure 7 we plot the flux of UHECR summing the contribution of the mixed composition model of [23,30] to that coming from SHDM. In figure 7 we also plot the experimental data of Auger (red points) and TA (black points) and the expected sensitivities of ARA (thick solid blu line), a future neutrino observatory based on the Askarian effect [40], and JEM-EUSO (thick solid red line), an UHECR detector planned onboard the international space station [41].…”