The Diffuse Galactic Gamma-Ray Gradient

One of the unsolved problems in cosmic ray (CR) physics is the small radial gradient of the γ-ray intensity compared to the inferred CR source distribution in the Galactic disk. In diffusive CR propagation models the most natural explanation is very efficient spatial mixing du e to MHD turbulence in the interstellar medium. However, even in the most favorable case of a very large diffusive CR halo the γ-ray gradient is still larger than observed. In our view the small γ-ray gradient could be the result of strong advection by a Galactic wind. We show that a small γ-ray gradient can be obtained, if the diffusion region does not extend far above the Galactic plane. Important ingredients of our model are: (i) anisotropic CR diffusion, (ii) strong radial and vertical gradients of the advection velocity (due to faster winds above higher CR source density regions).