A review of Cosmic-ray electrons and fermi-LAT

Abstract

The study of Galactic Cosmic-ray electrons (CREs) saw important developments in recent years, with the assumption of positron production only in interaction of hadronic Cosmic-rays with interstellar matter challenged by new measurements of CRE spectrum and related quantities. Indeed, all recent experiments seem to confirm an hardening in the positrons, a feature that is totally in contrast with the all-secondaries hypothesis, even if significant disagreements are present about the CRE spectral behavior and the possible presence of spectral features. Together with insufficient precision of current measurements, these disagreements prevent the identification of the primary positron source, with models involving Dark matter or astrophysical sources like Super Nova Remnants and Pulsar Wind Nebula all able to explain current data. The fermi-LAT contribution to the CRE study was fundamental, with the 2009 measurement of the positron + electron spectrum extended to the 7 MeV - 1TeV range with a statistics already exceeding previous results by many order of magnitude; since then, the last statistic has largely increased, while the LAT event reconstruction was significantly improved. In this article the reader will find an extensive and historical review of the CRE science, a summary of the history of gamma astronomy before fermi, an accurate description of the LAT and of its data analysis, a review of the present knowledge about CRE spectrum and on the theories that try to explain it, and finally a description of the changes and improvements introduced in the LAT event reconstruction process at the beginning of 2015.