The Diffuse Galactic Gamma-Ray Emission
Observed with EGRET
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.