The Diffuse Galactic Gamma-Ray Emission

What is the Origin and Distribution of Galactic Cosmic-Rays?

• The Galactic diffuse gamma-ray emission, the dominant feature of the gamma-ray sky, is produced primarily by cosmic ray electron and proton interactions with the matter (via electromagnetic and nuclear interactions) and photons (via inverse Compton scattering) in the interstellar medium.

• The spatial distribution of the Galactic diffuse emission observed with EGRET traces, and can be fairly accurately explained in terms of the distribution of atomic and molecular gas and photons in the interstellar medium using reasonable assumptions about the distribution of cosmic rays in the Galaxy.

• The spectrum of the Galactic diffuse emission, however, is not completely explained in terms of the theoretical prediction of cosmic rays interacting with matter and photons in the Galaxy.

• The mechanism which accelerates cosmic ray protons to high energies (recently indicated by X-rays to be shock acceleration due to supernovae) could be verified from the energy spectra of gamma-rays produced by accelerated cosmic-rays interacting with the molecular clouds near supernova remnants.

• A future high-energy gamma-ray telescope with better angular resolution will permit more detailed searches for cosmic ray gradients including variations in the electron to proton ratio, cosmic ray contrast between the Galactic arm/inter-arm regions, and evidence for regions in which the cosmic ray spectrum differs from the local observed spectrum indicating particle acceleration.