Signature of Energy Losses on the Cosmic Ray Electron Spectrum

We show that the fine structure of the electron spectrum in cosmic rays, especially the excess claimed by AMS-02 at energies $\gtrsim 42\mathrm{GeV}$, is fully accounted for in terms of inverse Compton losses in the photon background dominated by ultraviolet, infrared, and cosmic microwave background photons, plus the standard synchrotron losses in the Galactic magnetic field. The transition to the Klein-Nishina regime on the ultraviolet background causes the feature. Hence, contrary to previous statements, observations do not require the overlap of different components. We stress that the feature observed by AMS-02 at energies $\gtrsim 42\mathrm{GeV}$ is not related to the positron excess, which instead requires the existence of positron sources, such as pulsars. Because energy losses are the physical explanation of this feature, we indirectly confirm that the transport of leptons in the Galaxy is loss dominated down to energies of the order of tens of GeV. This finding imposes strong constraints on the feasibility of alternative theories of cosmic transport in which the grammage is accumulated in cocoons concentrated around sources, requiring that electrons and positrons become loss dominated only at very high energies.

Figure 1

Energy loss timescale as a function of the energy of CR electrons during their propagation in the Galaxy. The timescales are multiplied by $E$ to give prominence to the deviations from the standard $b\propto E^2$ regime. The dashed line represents the total timescale, while the solid lines refer to the single contributions by the magnetic field (magenta line) or ISRF components. The shadow region marks out the escape timescale from the Galaxy due to diffusion.

Figure 2

The spectrum of cosmic ray electrons (left) and positrons (right) resulting from the sum of all sources. We also show the prediction for secondary positrons (blue dashed line) and positrons from PWNe (green dashed line). In the left-hand panel, the sum of primary electrons from SNRs and secondaries (dashed orange line) and the total flux obtained neglecting the KN effect (dashed red line) are also shown.