Letter to the Editor

UHE nuclei propagation and the interpretation of the ankle in the cosmic-ray spectrum

¹ Institut de Physique Nucléaire d'Orsay, IN2P3-CNRS/Université Paris-Sud, 91406 Orsay Cedex, France
² Kavli Institute of Cosmological Physics, University of Chicago, 5640 S. Ellis, Chicago, IL 60637, USA
³ Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis, Chicago, IL 60637, USA e-mail: denis@oddjob.uchicago.edu
⁴ Institut d'Astronomie et d'Astrophysique, ULB, CP226, 1050 Brussels, Belgium

Received: 27 May 2005
Accepted: 5 October 2005

Abstract

We consider the stochastic propagation of high-energy protons and nuclei in the cosmological microwave and infrared backgrounds, using revised photonuclear cross-sections and following primary and secondary nuclei in the full 2D nuclear chart. We confirm earlier results showing that the high-energy data can be fit with a pure proton extragalactic cosmic ray (EGCR) component if the source spectrum is E^-2.6. In this case the ankle in the CR spectrum may be interpreted as a pair-production dip associated with the propagation. We show that when heavier nuclei are included in the source with a composition similar to that of Galactic cosmic-rays (GCRs), the pair-production dip is not present unless the proton fraction is higher than 85%. In the mixed composition case, the ankle recovers the past interpretation as the transition from GCRs to EGCRs and the highest energy data can be explained by a harder source spectrum E^-2.2–E^-2.3, reminiscent of relativistic shock acceleration predictions, and in good agreement with the GCR data at low-energy and holistic scenarios.