On the transition from galactic to extragalactic cosmic-rays: Spectral and composition features from two opposite scenarios

We study the phenomenology of cosmic-rays (CRs) at the galactic/extragalactic transition, focusing on two opposite models for the composition of the extragalactic (EG) component. Model A assumes a mixed source composition, with nuclear abundances similar to that of the low-energy CRs, while model B assumes that EG sources accelerate only protons. We study the limits within which both scenarios can reproduce the observed high-energy CR spectrum and composition. The ankle in model A is interpreted as the GCR/EGCR transition, while in model B it is the pair-production dip. Model A has a source spectrum ∝E−x with x ∼ 2.2-2.3, while model B requires x ∼ 2.6-2.7. We compare the predictions of both models with the available data on the energy evolution of the high-energy CR composition using the two main composition-related observables: Xmax and 〈lnA〉. We conclude that model A is currently favoured. Uncertainties are discussed and distinctive features of the two models are identified, which should allow one to distinguish between the models in the near future when more precise measurements are available with higher statistics experiments.