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Implications of the cosmic ray spectrum for the mass composition at the highest energies
Allard D., Busca N.G., Decerprit G., Olinto A., Parizot E.
Journal of Cosmology and Astroparticle Physics 10 (2008) 033 - http://hal.in2p3.fr/in2p3-00285473
Physique/Astrophysique/Cosmologie et astrophysique extra-galactique
Planète et Univers/Astrophysique/Cosmologie et astrophysique extra-galactique
Implications of the cosmic ray spectrum for the mass composition at the highest energies
D. Allard1, N.G. Busca1, G. Decerprit1, A. Olinto1, 2, E. Parizot1
1 :  APC - UMR 7164 - AstroParticule et Cosmologie
CNRS : UMR7164 – IN2P3 – Observatoire de Paris – Université Paris VII - Paris Diderot – CEA : DSM/IRFU
APC - UMR 7164, Université Paris Diderot, 10 rue Alice Domon et Léonie Duquet, case postale 7020, F-75205 Paris Cedex 13
2 :  University of Chicago
University of Chicago
Edward H. Levi Hall 5801 South Ellis Avenue Chicago, Illinois 60637
The significant attenuation of the cosmic ray flux above ~5 × 10^19 eV suggests that the observed high energy spectrum is shaped by the so-called GZK effect (GZK: Greisen-Zatsepin-Kuzmin). This interaction of ultra-high energy cosmic rays (UHECRs) with the ambient radiation fields also affects their composition. We review the effect of photodissociation interactions on different nuclear species and analyze the phenomenology of secondary-proton production as a function of energy. We show that, by itself, the UHECR spectrum does not constrain the composition of cosmic rays at their extragalactic sources. While the propagated composition (i.e., as observed at Earth) cannot contain significant amounts of intermediate mass nuclei (say between He and Si), whatever the source composition, and while it is vastly proton dominated when protons are able to reach energies above 10^20 eV at the source, we show that the propagated composition can be dominated by Fe and sub-Fe nuclei at the highest energies, either if the sources are very strongly enriched in Fe nuclei (a rather improbable situation), or if the accelerated protons have a maximum energy of a few 10^19 eV at the sources. We also show that in the latter cases, the expected flux above 3 × 10^20 eV is very much reduced as compared to the case when protons dominate in this energy range, both at the sources and at Earth.

Articles dans des revues avec comité de lecture
Journal of Cosmology and Astroparticle Physics
Publisher Institute of Physics (IOP)
ISSN 1475-7508 (eISSN : 1475-7516)

16 pages, 7 figures
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