On the Contribution of Pulsars to the Positron Fraction in Cosmic Rays

Several cosmic ray experiments have measured the positron fraction up to few hundred GeV. Their data have revealed an excess of positrons above 10 GeV that is not consistent with the secondary production of these particles in the interstellar medium. A primary source like dark matter or astrophysical sources (e.g pulsars and their nebulae) were considered to account for such an excess. In this paper we analyse the possibility of a primary positron production due to pulsars. Under the assumption of equal initial spectra at the source for positrons, electrons, and gamma-rays we study the propagation of particle spectra using a diffusion model in the Galaxy. We focused our analysis on the Vela and Crab pulsars and their associated nebulae, which are well observed in gamma-rays. Comparison with experimental data is reported. The propagated positron and electron spectra generated from these sources result in a positron ratio, which is largely inconsistent with the excess observed by PAMELA and AMS