E. Parizot

Implications of the cosmic ray spectrum for the mass composition at the highest energies

The significant attenuation of the cosmic ray flux above ~5 × 1019 eV suggests that the observed high energy spectrum is shaped by the so-called GZK effect (GZK: Greisen–Zatsepin–Kuzmin). This interaction of ultra-high energy cosmic rays (UHECRs) with the ambient radiation fields also affects their composition. We review the effect of photodissociation interactions on different nuclear species and analyze the phenomenology of secondary-proton production as a function of energy. We show that, by itself, the UHECR spectrum does not constrain the composition of cosmic rays at their extragalactic sources. While the propagated composition (i.e., as observed at Earth) cannot contain significant amounts of intermediate mass nuclei (say between He and Si), whatever the source composition, and while it is vastly proton dominated when protons are able to reach energies above 1020 eV at the source, we show that the propagated composition can be dominated by Fe and sub-Fe nuclei at the highest energies, either if the sources are very strongly enriched in Fe nuclei (a rather improbable situation), or if the accelerated protons have a maximum energy of a few 1019 eV at the sources. We also show that in the latter cases, the expected flux above 3 × 1020 eV is very much reduced as compared to the case when protons dominate in this energy range, both at the sources and at Earth.

Propagation of high-energy cosmic rays in extragalactic turbulent magnetic fields: resulting energy spectrum and composition

We extend previous studies of mixed-composition extragalactic cosmic-ray source models, investigating the influence of a nonnegligible extragalactic magnetic field on the propagated cosmic-ray spectrum and composition. We study the transport of charged particles in turbulent fields and the transition from a ballistic to a diffusive propagation regime. We introduce a method allowing a fast integration of the particle trajectories, which allows us to calculate extragalactic cosmic-ray spectra in the general case, without using either the diffusive or the rectilinear approximation. We find that the main features of the mixed-composition models – regarding the interpretation of the ankle and the non-monotonous evolution of the average cosmic-ray mass – remain essentially unchanged as long as the magnetic field intensity does not exceed a few nG.

Signatures of the extragalactic cosmic-ray source composition from spectrum and shower depth measurements

We discuss the differences induced by the assumed composition of extragalactic sources on the predicted UHECR spectrum and the energy evolution of

Anisotropy expectations for ultra-high-energy cosmic rays with future high-statistics experiments

, i.e. the mean value of the atmospheric depth at the cosmic-ray air shower maximum. We show that different assumptions for the source power evolution do not modify our earlier finding that in the case of a mixed composition the ankle can be interpreted as the end of the transition from galactic to extragalactic cosmic rays. We show the characteristic features in the shape of

Detecting gamma-ray bursts with the pierre auger observatory using the single particle technique

(E) that are associated with this transition for each cosmic-ray composition model. These characteristic features are present whatever the hadronic model used for the calculation. In the mixed composition cases, a signature of the interactions of nuclei with the photon backgrounds is also expected above

Enhancing the relative Fe-to-proton abundance in ultra-high-energy cosmic rays

10^{19}

Extended clustering analyses to constrain the deflection angular scale and source density of the ultra-high-energy cosmic rays

eV. The comparisons with Stereo HiRes and Flys Eye data favour an extragalactic mixed composition and the corresponding interpretation of the ankle. Confrontation of model predictions with future data at the highest energies will allow a better determination of the transition features and of the cosmic-ray source composition, independently of hadronic models. We also emphasize that in the pure proton case, a combined analysis of the spectrum and composition below the ankle could lead to constraints on the source power evolution with redshift.

The EUSO@Turlab Project: Results from Phase II

We study the effect of random extra-galactic magnetic fields on the propagation of protons of energy larger than