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

Abstract

UHECRs have attracted a lot of attention due to their challengingly high energies and their potential value to constrain physical processes and astrophysical parameters in the most energetic sources of the universe. Current detectors have failed to detect significant anisotropies which had been expected to allow source identification. Some indications about the UHECR composition, which may become heavier at the highest energies, has even put into question the possibility that such a goal could be achieved soon. We investigate the potential value of a new-generation detector, with 10 times larger exposure, to overcome the current situation and make significant progress in the detection of anisotropies and thus in the study of UHECRs. We take as an example the expected performances of the JEM-EUSO, assuming a uniform full-sky coverage with a total exposure of 300,000 km2 sr yr. We simulate realistic UHECR sky maps for a wide range of possible astrophysical scenarios allowed by the current constraints, taking into account the energy losses and photo-dissociation of the UHECRs, as well as their deflections by magnetic fields. These sky maps, built for the expected statistics of JEM-EUSO as well as for the current Auger statistics, as a reference, are analyzed from the point of view of their intrinsic anisotropies, using the two-point correlation function. A statistical study of the resulting anisotropies is performed for each astrophysical scenario, varying the UHECR source composition and spectrum as well as the source density. We find that significant anisotropies are expected to be detected by a next-generation UHECR detector, for essentially all the astrophysical scenarios studied, and give precise, quantitative meaning to this statement. Our results show that a gain of one order of magnitude in exposure would make a significant difference compared to the existing detectors.