Klein-Nishina effect and the cosmic ray electron spectrum

TL;DR

This paper investigates the Klein-Nishina effect on cosmic ray electron spectra, finding that the transition from the Thomson to KN regime is smooth and cannot solely explain observed spectral hardening, implying additional sources are needed.

Contribution

It provides a detailed analysis of the Klein-Nishina effect's impact on electron energy losses and offers a parameterized formula for accurate ICS energy-loss calculations.

Findings

The transition from Thomson to KN regime is smooth.

The KN effect alone cannot explain the spectral hardening.

Additional sources are required to account for observed spectra.

Abstract

Radiative energy losses are very important in regulating the cosmic ray electron and/or positron (CRE) spectrum during their propagation in the Milky Way. Particularly, the Klein-Nishina (KN) effect of the inverse Compton scattering (ICS) results in less efficient energy losses of high-energy electrons, which is expected to leave imprints on the propagated electron spectrum. It has been proposed that the hardening of CRE spectra around 50 GeV observed by Fermi-LAT, AMS-02, and DAMPE could be due to the KN effect. We show in this work that the transition from the Thomson regime to the KN regime of the ICS is actually quite smooth compared with the approximate treatment adopted in some previous works. As a result, the observed spectral hardening of CREs cannot be explained by the KN effect. It means that an additional hardening of the primary electrons spectrum is needed. We also provide a parameterized form for the accurate calculation of the ICS energy-loss rate in a wide energy range.