De-excitation nuclear gamma-ray line emission from low-energy cosmic rays in the inner galaxy

Abstract

Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated, which, added to the standard cosmic-ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV nucleon-1 and several hundred MeV nucleon-1 in the interstellar medium would be a detection of nuclear γ -ray line emission in the range Eγ ∼ 0.1-10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent γ -ray cross section compilation for nuclear collisions, γ -ray line emission spectra are calculated alongside the high-energy γ -ray emission due to π0 decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above Eγ = 0.1 GeV. Our predicted fluxes of strong nuclear γ -ray lines from the inner Galaxy are well below the detection sensitivities of the International Gamma-Ray Astrophysics Laboratory, but a detection, especially of the 4.4MeV line, seems possible with new-generation γ -ray telescopes based on available technology.We also predict strong γ -ray continuum emission in the 1-8 MeV range, which, in a large part of our model space for low-energy cosmic rays, considerably exceeds the estimated instrument sensitivities of future telescopes