Measurement of separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope

TL;DR

This study uses the Fermi Large Area Telescope to measure separate cosmic-ray electron and positron spectra by exploiting Earth's magnetic shadow, confirming the positron fraction increases with energy up to 200 GeV.

Contribution

First measurement of separate electron and positron spectra with Fermi-LAT using Earth's magnetic shadow for charge discrimination.

Findings

Positron fraction rises with energy between 20-100 GeV.

Electron and positron spectra are consistent with a continuing rise in positron fraction up to 200 GeV.

Background proton contamination was effectively estimated and subtracted.

Abstract

We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting the Earth's shadow, which is offset in opposite directions for opposite charges due to the Earth's magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 GeV and 200 GeV. We confirm that the fraction rises with energy in the 20-100 GeV range. The three new spectral points between 100 and 200 GeV are consistent with a fraction that is continuing to rise with energy.