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

TL;DR

This paper analyzes cosmic ray radioactive nuclei and positron data to infer propagation time scales in the Galaxy, suggesting a rigidity-independent residence time and supporting a secondary origin for positrons.

Contribution

It provides a model-independent analysis of cosmic ray propagation time scales using radioactive nuclei and positron data, highlighting a rigidity-independent residence time and constraining positron energy losses.

Findings

Radioactive nuclei data support a rigidity-independent residence time.

Positron flux is consistent with a secondary origin.

Upper bound on electromagnetic energy density traversed by positrons is 1.25 eV/cm^3.

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}=(t_i/100 Myr)^{0.7}, where f_{s,i} is the suppression of the flux due to decay and t_i 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, \bar U_T<1.25 eV/cm^3 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\sim100 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.