Knee structure in high-energy inverse Compton scattering of CMB photons

TL;DR

This paper extends the analysis of inverse Compton scattering of CMB photons by high-energy electrons to higher photon energies using Klein-Nishina formula, revealing a knee structure in the spectral intensity.

Contribution

It provides an analytic expression for the spectral intensity function at high energies, incorporating Klein-Nishina effects, and predicts a knee feature at PeV energies.

Findings

Identified a knee structure at PeV energies in the spectral intensity.

Derived analytical formulas applicable from Thomson to Klein-Nishina regimes.

Predicted observability of the knee with gamma-ray telescopes like Fermi-LAT.

Abstract

We study the inverse Compton scattering of the CMB photons off nonthermal high-energy electrons. In the previous study, assuming the power-law distribution for electrons, we derived the analytic expression for the spectral intensity function $I(\omega)$ in the Thomson approximation, which was applicable up to the photon energies of $\omega <$ O(GeV). In the present paper, we extend the previous work to higher photon energies of $\omega >$ O(GeV) by taking into account the terms dropped in the Thomson approximation, i.e., the Klein-Nishina formula. The analytic expression for $I(\omega)$ is derived with the Klein-Nishina formula. It is shown that $I(\omega)$ has a "knee" structure at $\omega =$ O(PeV). The knee, if exists, should be accessible with gamma-ray observatories such as Fermi-LAT. We propose simple analytical formulae for $I(\omega)$ which are applicable to wide photon energies from Thomson region to extreme Klein-Nishina region.