Deriving the cosmic ray spectrum from gamma-ray observations

TL;DR

This paper refines the cosmic ray spectrum derived from gamma-ray observations of molecular clouds, aligning it with recent high-precision data and modeling a spectral break related to diffusion changes.

Contribution

It provides a more accurate determination of the Galactic cosmic ray spectrum using improved gamma-ray data and photon production modeling, confirming consistency with recent proton measurements.

Findings

CR proton spectral shape is consistent with PAMELA data

Inclusion of a spectral break improves fit to gamma-ray observations

Enhanced modeling aligns cosmic ray spectra with observational data

Abstract

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. Adding a break of the CR flux at 3 GeV, caused by a corresponding change of the diffusion coefficient, improves further the agreement in the energy range 0.2-3 GeV.