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

Abstract

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 the EG sources accelerate only protons. We study the limits within which both scenarios can reproduce the high-energy CR data, from the point of view of the energy spectrum, each with its own interpretation of the ankle (as the GCR/EGCR transition for model A; as a pair-production dip for model B) and its own source spectrum (in E?2.3 model A; in E?2.6 or E?2.7 for model B). We then focus on the energy evolution of the high-energy CR composition and compare our predictions for two main composition-related observables (Xmax and hlnAi) with the available data, to 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.

26 pages 9 figures. Calculations updated using a recent IRB density and evolution (see text). Minor quantitative changes, conclusions unchanged

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