What can be learnt from UHECR anisotropies observations? Paper I : large-scale anisotropies and composition features

Abstract

We investigate the implications of the current data regarding large scale anisotropies, and examine to what extent they can be used to constrain the origin of UHECRs and the astrophysical parameters of the source scenarios. We discuss the possibility of observing an associated anisotropy of the composition, the potential benefit of the separation of the different nuclear components and the interest of observing the UHECR sky with larger exposure future observatories. We simulate UHECR sky maps for various astrophysical scenarios satisfying the current observational constraints, taking into account the energy losses of the UHE protons and nuclei and their deflections by intervening magnetic fields. We investigate scenarios in which the UHECR source distribution follows that of the galaxies, varying the source composition and spectrum, the source density and the magnetic field models. We apply similar analyses as those used by the Auger collaboration. We find that: i) reproducing the observed dipole anisotropy and its energy evolution is relatively easy within our assumptions; ii) this agreement can be obtained with different sets of assumptions on the astrophysical parameters, and is thus not, at this stage, very constraining for UHECR source scenarios; iii) the actual reconstructed direction of the dipole appears non natural in essentially all scenarios investigated, and challenges their main assumptions on the source distribution or the assumed magnetic field configuration, especially in the Galaxy; iv) except for protons, the energy range in which the GZK horizon strongly reduces is a key target for anisotropy searches for each given nuclear species; v) The composition anisotropy naturally expected in our models is unlikely to account for that recently reported by Auger unless the observed amplitude is a strong positive statistical fluctuation of an intrinsically weaker signal.