Cosmic ray propagation time scales: Lessons from radioactive nuclei and positron data

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Abstract

We take a fresh look at high energy radioactive nuclei data reported in the 90's and at the positron data recently reported by PAMELA. Our aim is to study the model independent implications of these data for the propagation time scales of cosmic rays in the Galaxy. Considering radioactive nuclei, using decaying charge to decayed charge ratios - the only directly relevant data available at relativistic energies - we show that a rigidity independent residence time is consistent with observations. The data for all nuclei can be described by f s,i = (ti/100Myr)0.7, where fs,i is the suppression of the flux due to decay and ti is the observer frame lifetime for nucleus specie i. Considering positron measurements, we argue that the positron flux is consistent with a secondary origin. Comparing the positron data with radioactive nuclei at the same energy range, we derive an upper bound on the mean electromagnetic energy density traversed by the positrons, ŪT < 1.25 eV/cm3 at a rigidity of R = 40 GV. Charge ratio measurements within easy reach of the AMS-02 experiment, most notably a determination of the Cl/Ar ratio extending up to R ∼ 100 GV, will constrain the energy dependence of the positron cooling time. Such constraints can be used to distinguish between different propagation scenarios, as well as to test the secondary origin hypothesis for the positrons in detail.

Original languageEnglish
Article number037
JournalJournal of Cosmology and Astroparticle Physics
Volume2011
Issue number11
DOIs
StatePublished - Nov 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics

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