Origin of the Positron Excess in Cosmic Rays

We show that the positron excess measured by the PAMELA experiment in the region between 10 and 100 GeV may well be a natural consequence of the standard scenario for the origin of Galactic cosmic rays. The “excess” arises because of positrons created as secondary products of hadronic interactions inside the sources, but the crucial physical ingredient which leads to a natural explanation of the positron flux is the fact that the secondary production takes place in the same region where cosmic rays are being accelerated. Therefore secondary positrons (and electrons) participate in the acceleration process and turn out to have a very flat spectrum, which is responsible, after propagation in the Galaxy, for the observed positron excess. This effect cannot be avoided though its strength depends on the values of the environmental parameters during the late stages of evolution of supernova remnants.

Figure 1

Positron fraction as a function of energy. The data points are the results of the PAMELA measurement.

Figure 2

Fluxes of $e^{-}$ and $e^{+}$ at Earth for $E_{\max }=100\mathrm{TeV}$. The dotted line refers to primary electrons; the dashed lines are the fluxes of positrons (upper curve) and electrons (lower curve) from interactions of cosmic rays in the Galaxy. The dash-dotted lines are the fluxes of positrons (upper curve) and electrons (lower curve) from production in the sources. The thick solid line is the total flux. The data points are from Fermi/LAT [18].