Deriving the cosmic ray spectrum from gamma-ray observations

Understand

A fundamental problem of cosmic ray (CR) physics is the determination of the average properties of Galactic CRs outside the Solar system.

• Starting from COS-B data in the 1980's, gamma-ray observations of molecular clouds in the Gould Belt above the Galactic plane have been used to deduce the Galactic CR energy spectrum.
• We reconsider this problem in view of the improved precision of observational data which in turn require a more precise treatment of photon production in proton-proton scatterings.
• We show that the spectral shape $dN/dp\propto p^{-2.85}$ of CR protons as determined by the PAMELA collaboration in the energy range 80 GeV<pc<230 GeV is consistent with the photon spectra from molecular clouds observed with Fermi-LAT down to photon energies E\sim 1-2 GeV.