Fermi Gamma-ray Space Telescope

Measurement of the Cosmic-ray Proton Spectrum with the Fermi Large Area Telescope

David Green
(Elizabeth A. Hays)


We present the measurement of the cosmic-ray proton spectrum using 10 years of Pass 8 flight data from the Fermi Large Area Telescope (LAT). We developed a dedicated proton event selection with an acceptance of 0.25 m^2 sr. Our event selection yields a large dataset with a statistical uncertainty under 5% and residual contamination less than 5% across the entire energy range. The systematic errors associated with the acceptance, energy measurement and GEANT4 Monte Carlo simulations are an order of magnitude larger than the statistical uncertainty. The resulting event selection and spectral measurement create the opportunity for additional proton analyses with the LAT, such as a dedicated proton anisotropy search.