Origin of the Positron Excess with Pulsar Wind Nebulae

Abstract

AMS-02 has been measured with unprecedented precision data for the positron flux. These data show that there is an excess of cosmic positrons detected for energies above 10 GeV with respect to the well known secondary production which is due to the collision of cosmic rays with the atoms of the interstellar medium. In this paper we have investigated the possibility that the positron excess is due to the acceleration of positrons and electrons from pulsar wind nebulae. We have first made the assumption that only one pulsar, Geminga, dominates the production of positrons detected at Earth. We find that a spectral index for the injection spectrum of e + is needed to fit the data but an efficiency for the conversion of pulsar energy into positron of about 100% is required. This is unphysical since also electrons should be accelerated with the same amount. We have done the more realistic case where all the pulsars reported in the ATNF catalogue produce positrons. We have demonstrated that this case fits well the data with efficiency values of the order of 10-20%. These results imply that pulsar wind nebulae in the Galaxy are likely the main contributors to the positron excess.