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 E _γ ~ 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 E _γ = 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 4.4 MeV 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.