Anisotropies of ultrahigh energy cosmic ray nuclei diffusing from extragalactic sources

Abstract

We obtain the dipolar anisotropies in the arrival directions of ultrahigh energy cosmic ray nuclei diffusing from nearby extragalactic sources. We consider mixed-composition scenarios in which different cosmic ray nuclei are accelerated up to the same maximum rigidity, so that $E<Z{E}_{\mathrm{max}}^{p}$, with $Z$ the atomic number and ${E}_{\mathrm{max}}^{p}$ the maximum proton energy. We adopt ${E}_{\mathrm{max}}^{p}\ensuremath{\simeq}6\text{ }\text{ }\mathrm{EeV}$ so as to account for an increasingly heavy composition above the ankle. We obtain the anisotropies through Monte Carlo simulations that implement the cosmic ray diffusion in extragalactic turbulent fields as well as the effects of photodisintegrations and other energy losses. Dipolar anisotropies at the level of 5% to 10% at energies $\ensuremath{\sim}10\text{ }\text{ }\mathrm{EeV}$ are predicted for plausible values of the source density and magnetic fields.