Searches for Cosmic-Ray Electron Anisotropies with the Fermi Large Area Telescope

TL;DR

This study used the Fermi-LAT to analyze cosmic-ray electron arrival directions for anisotropies across various energies and angular scales, finding no significant anisotropy but setting upper limits.

Contribution

It provides the first comprehensive search for anisotropies in cosmic-ray electrons above 60 GeV using Fermi-LAT data, establishing upper limits on anisotropy levels.

Findings

No anisotropy detected within the sensitivity limits.

Upper limits on dipole anisotropy range from 0.5% to 10%.

Results constrain models of cosmic-ray origin and propagation.

Abstract

The Large Area Telescope on board the \textit{Fermi} satellite (\textit{Fermi}-LAT) detected more than 1.6 million cosmic-ray electrons/positrons with energies above 60 GeV during its first year of operation. The arrival directions of these events were searched for anisotropies of angular scale extending from $\sim$ 10 $^\circ$ up to 90$^\circ$, and of minimum energy extending from 60 GeV up to 480 GeV. Two independent techniques were used to search for anisotropies, both resulting in null results. Upper limits on the degree of the anisotropy were set that depended on the analyzed energy range and on the anisotropy's angular scale. The upper limits for a dipole anisotropy ranged from $\sim0.5%$ to $\sim10%$.