A 3D time-dependent model for Galactic cosmic rays and gamma rays

In studies of cosmic-ray (CR) propagation and diffuse continuum gamma-ray emission from the Galaxy it has usually been assumed that the source function can be taken as smooth and time-independent. However, especially for electrons at high energies where energy losses are rapid, the effect of the stochastic nature of the sources becomes apparent and indeed has been invoked to explain the GeV excess in the diffuse emission observed by EGRET. In order to address this problem in detail a model with explicit time-dependence and a stochastic SNR population has been developed, which follows the propagation in three dimensions. The results indicate that although the inhomogeneities are large they are insufficient to easily explain the GeV excess. However the fluctuations should show up in the gamma-ray distribution at high energies and this should be observable with GLAST. Estimates of the TeV continuum emission from the plane are consistent with the Whipple upper limit.