On the Positron Fraction and the Spectrum of the Electronic Component in Cosmic Rays

Abstract

The recent observations of the positron fraction in cosmic rays by PAMELA indicate that the fraction of positrons to the total electronic component in cosmic rays initially decreases in the energy region 1-10 GeV and increases thereafter. In this paper, we show that it is natural to expect such an increase of the positron fraction within the context of cosmic ray propagation models. It is shown that this ratio should reach an asymptotic value of ~0.6 at very high energies. The specific measurements by PAMELA help us to distinguish amongst various models for cosmic ray propagation, and in particular, they support the nested leaky box model. They also provide, in conjunction with the observations of the total electronic component by HESS, FERMI, ATIC, and other experiments, a way of estimating the spectrum of electrons directly accelerated by discrete sources of cosmic rays in the Galaxy.