Diffuse galactic gamma-ray emission:

Cosmic rays interacting with the interstellar medium (ISM) give rise to the emission of gamma-rays over a wide range of energies. The main processes are bremsstrahlung and inverse Compton scattering of electrons, and $\pi^0$ decay subsequent to hadronic interactions of protons and nuclei. Thus, observations of the diffuse galactic gamma-ray emission, together with measurements of the arriving cosmic-ray flux, provide a unique probe of the distribution of both the ISM and the cosmic rays within the Galaxy. Observations with EGRET below 1GeV agree well with models of the distribution of interstellar matter ([Hunter et al. 1997]). At higher energies, the observed gamma-ray intensity exceeds the model predictions by $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$ }}}\hbox{$>$ }}}$50%. This excess could be due to unresolved point sources below the EGRET detection threshold, or it may reflect limitations in our understanding of cosmic rays, such as a local cosmic-ray spectrum which is not representative of the galactic average. VERITAS will measure the differences in the diffuse emission spectrum from different regions of the galaxy with excellent accuracy up to the TeV range, providing the first measurement of the cosmic-ray distribution in our galaxy at such high energies. Together with results from GLAST, a comprehensive and consistent picture of particle interactions with the ISM should evolve.