The `excess' of primary cosmic ray electrons

With the accurate cosmic ray (CR) electron and positron spectra (denoted as $Φ_{\rm e^{-}}$ and $Φ_{\rm e^{+}}$, respectively) measured by AMS-02 collaboration, the difference between the electron and positron fluxes (i.e., $ΔΦ=Φ_{\rm e^{-}}-Φ_{\rm e^{+}}$), dominated by the propagated primary electrons, can be reliably inferred. In the standard model, the spectrum of propagated primary CR electrons at energies $\geq 30$ GeV softens with the increase of energy. The absence of any evidence for such a continuous spectral softening in $ΔΦ$ strongly suggests a significant `excess' of primary CR electrons and at energies of $100-400$ GeV the identified excess component has a flux comparable to that of the observed positron excess. Middle-age but `nearby' supernova remnants (e.g., Monogem and Geminga) are favored sources for such an excess.

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