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
