The galactic electrons are essentially primary with supernovae as sources. A small fraction of electrons, of the order of 10%, are secondary and arise out from the interaction of cosmic rays, mostly protons and helium nuclei, on the interstellar matter. They are the end product of the decay of short-lived particles (mostly pions via the decay π±→μ±→e±) produced in these interactions.
Figure 1 shows the electron energy spectrum measured by PAMELA between 1 GeV and 625 GeV, together with previous experiments.
Figure 2 shows the best fit to the PAMELA data by a model (short-dashed line) in which, to the two electron components above considered, another electron component is added, providing a better agreement with the data. This new component, which dominates the high energy region with a harder spectrum, is of the same order of the positron excess, in agreement with the hypothesis of the existence of a new source of pairs of electrons and positrons.