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.