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

Abstract

Recently, the PAMELA, FERMI, HESS, and ATIC instruments have discovered interesting spectral features in the positron to total electron ratio and in the total electronic component of cosmic rays at high energy. These observations are studied with analytical models for the propagation of cosmic rays emanating from a discrete set of sources or from a smooth distribution of sources extending over the Galaxy. The contrast between the theoretically expected spectra from these two source distributions is seen to play a crucial role in the interpretation of the recent findings. It is shown that the positron to electron ratio, observed by PAMELA, may be fit over the entire energy range in the nested leaky-box model for cosmic ray propagation and that the ratio is expected to reach an asymptotic value of $\sim0.6$ at high energies in all conventional models of cosmic ray propagation. We also derive the spectral shape of the electrons and positrons expected from the annihilation of dark matter in the Galaxy and show that the spectral shape of the peak will provide important information, not only regarding the mass of the dark matter particles, but also regarding their spatial distribution. The spectrum of secondary positrons and electrons, calculated with the nested leaky-box model, is subtracted from the spectrum of the electronic component to determine the spectrum of primary electrons emerging from the cosmic ray sources. This spectrum is analyzed in terms of contributions from a set of discrete sources sprinkled across the Galactic volume. Our analysis suggests the possible presence of electrons, accelerated in high Mach number shocks, in the cosmic rays.