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Issue A&A
Volume 443, Number 3, December I 2005
Page(s) L29 - L32
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500199



A&A 443, L29-L32 (2005)
DOI: 10.1051/0004-6361:200500199

Letter

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

D. Allard1, 2, 3, E. Parizot1, A. V. Olinto2, 3, E. Khan1 and S. Goriely4

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 $\propto$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 $\propto$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.


Key words: cosmic rays -- abundances -- nuclear reactions




© ESO 2005


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