Signature of Energy Losses on the Cosmic Ray Electron Spectrum

TL;DR

The paper explains the fine structure in the cosmic ray electron spectrum, especially the AMS-02 excess at 42 GeV, as a result of inverse Compton and synchrotron energy losses, confirming loss-dominated lepton transport in the Galaxy.

Contribution

It demonstrates that the spectral feature is due to energy losses in the photon background, challenging previous interpretations and constraining cosmic ray transport models.

Findings

The 42 GeV feature is caused by Klein-Nishina effects on UV background photons.

Energy losses dominate lepton transport down to tens of GeV.

The positron excess is unrelated to the spectral feature, implying separate sources.

Abstract

We show that the fine structure of the electron spectrum in cosmic rays, especially the excess claimed by AMS-02 at energies $\sim$42 GeV, is fully accounted for in terms of inverse Compton losses in the photon background dominated by ultraviolet, infrared and CMB 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 $\sim$42 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.