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Astron. Astrophys. 339, 587-590 (1998)

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

Martin Pohl

Danish Space Research Institute, Juliane Maries Vej 30, DK-2100 Copenhagen O, Denmark

Received 26 June 1998 / Accepted 31 August 1998


RXTE, GINGA, and OSSE observations have revealed an intense low-energy [FORMULA]-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 [FORMULA]-ray continuum through inverse bremsstrahlung.

A flux of low-energy cosmic rays strong enough to produce the observed spectrum of [FORMULA]-rays implies substantial [FORMULA]-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 [FORMULA]-decay [FORMULA]-ray emission at higher energies, are in serious conflict with an inverse bremsstrahlung origin of the Galactic soft [FORMULA]-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 low-energy protons the implied [FORMULA]-ray line flux and [FORMULA]-decay continuum intensity are larger than the existing limits by at least a factor of 2.

Key words: acceleration of particles – radiation mechanisms: non-thermal – cosmic rays – gamma rays: theory

Send offprint requests to: mkp@dsri.dk

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998