Limits for an inverse bremsstrahlung origin of the diffuse Galactic soft gamma-ray emission

RXTE, GINGA, and OSSE observations have revealed an intense low-energy γ-ray continuum emission from the Galactic plane, which is commonly interpreted as evidence for the possible existence of a strong flux of low-energy cosmic ray electrons. In this Paper I discuss the scenario of a hadronic origin of the soft Galactic γ-ray continuum through inverse bremsstrahlung. A flux of low-energy cosmic rays strong enough to produce the observed spectrum of γ-rays implies substantial γ-ray emission at a few MeV through nuclear de-excitation. It is shown that the existing limits on excess 3-7 MeV emission from the Galactic plane, in concert with the constraints from π0-decay γ-ray emission at higher energies, are in serious conflict with an inverse bremsstrahlung origin of the Galactic soft γ-ray emission for any physically plausible low-energy cosmic ray spectrum. While in case of energetic heavy nuclei the limits are violated by about an order of magnitude, for a large population of lowenergy protons the implied γ-ray line flux and π0-decay continuum intensity are larger than the existing limits by at least a factor of 2.