Origin of the ankle in the ultra-high energy cosmic ray spectrum and of the extragalactic protons below it

Abstract

The sharp change in slope of the ultrahigh energy cosmic ray (UHECR) spectrum around 10^18.6 eV (the ankle), combined with evidence of a light but extragalactic component near and below the ankle and intermediate composition above, has proved exceedingly challenging to understand theoretically, without fine-tuning. We propose a mechanism whereby photo-disintegration of ultrahigh energy nuclei in the region surrounding a UHECR accelerator accounts for the observed spectrum and inferred composition at Earth. For suitable source conditions, the model reproduces the spectrum and the composition over the entire extragalactic cosmic ray energy range, i.e. above 10^17.5 eV. Predictions for the spectrum and flavors of neutrinos resulting from this process are also presented.

extended discussion of source parameters, accepted for publication in PRD