Where is the End of the Cosmic-Ray Electron Spectrum?

TL;DR

This paper explores the potential reasons for the unknown cutoff in the cosmic-ray electron spectrum, analyzing various astrophysical and observational factors, and proposes strategies to improve understanding and detection.

Contribution

It provides a comprehensive framework identifying multiple factors that could cause the CR electron spectrum cutoff and suggests observational strategies to address these uncertainties.

Findings

Multiple steps could cause the spectrum cutoff, including acceleration limits, escape, propagation, and detector sensitivity.

Range of possibilities for each step is estimated, highlighting key uncertainties.

Strategies for future observations and multi-messenger approaches are outlined.

Abstract

Detecting the end of the cosmic-ray (CR) electron spectrum would provide important new insights. While we know that Milky Way sources can accelerate electrons up to at least $\sim$1PeV, the observed CR electron spectrum at Earth extends only up to 5TeV (possibly 20TeV), a large discrepancy. The question of the end of the CR electron spectrum has received relatively little attention, despite its importance. We take a comprehensive approach, showing that there are multiple steps at which the observed CR electron spectrum could be cut off. At the highest energies, the accelerators may not have sufficient luminosity, or the sources may not allow sufficient escape, or propagation to Earth may not be sufficiently effective, or present detectors may not have sufficient sensitivity. For each step, we calculate a rough range of possibilities. Although all of the inputs are uncertain, a clear vista of exciting opportunities emerges. We outline strategies for progress based on CR electron observations and auxiliary multi-messenger observations. In addition to advancing our understanding of CRs in the Milky Way, progress will also sharpen sensitivity to dark matter annihilation or decay.