Broad-band diffuse gamma ray emission of the galactic disk

The contributions of different radiation mechanisms to the diffuse γ-ray emission of the galactic disk are studied in a broad energy region from 104 to 1014 eV. Our analysis shows that at energies between 1 and 100 MeV the radiation is dominated by the bremsstrahlung of relatively low energy, typically less than 1 GeV electrons, but with a non-negligible contribution from the inverse Compton (IC) scattering of higher energy electrons. Also, a significant fraction of the radiation observed at energies around 1 MeV could be contributed by mildly relativistic positrons annihilating “in flight” with the ambient thermal electrons. At energies from 100 MeV to 100 GeV the γ-ray flux is dominated by interactions of cosmic ray protons and nuclei with the ambient gas through production and subsequent decay of secondary π0-mesons. The interpretation of the GeV γ-ray emission of the inner Galaxy as a truly diffuse radiation requires a substantially harder spectrum of relativistic protons and nuclei in the interstellar medium compared with the local cosmic ray spectrum measured directly in the solar neighborhood. In the very high energy domain, Eγ ≥ 100GeV, the contribution of the IC component of radiation may become comparable with, or even could exceed the fluxes of π0-decay component if the energy spectrum of electrons injected into the interstellar medium extends well beyond 1 TeV. Another signature of multi-TeV electrons is the synchrotron radiation which could account for a significant fraction of the diffuse hard X-ray flux of the galactic ridge. The future detailed studies of spatial and spectral characteristics of γ-ray emission of the galactic disk, especially at very high energies around 100 GeV by GLAST, and hopefully also at TeV energies by planned ground-based instruments should provide important insight into the understanding of the sites and mechanisms of acceleration of galactic cosmic rays and the character of their propagation in the interstellar magnetic fields.