An Anisotropic Propagation Model for Galactic Cosmic Rays

TL;DR

This paper introduces an anisotropic cosmic ray propagation model that accounts for high galactic wind speeds and explains gamma-ray and positron observations by allowing different diffusion coefficients in the halo and disk.

Contribution

It proposes a novel anisotropic diffusion model with variable halo diffusion coefficients and radial wind dependence, addressing limitations of isotropic models.

Findings

High galactic wind speeds are compatible with cosmic ray data.

Radial dependence of wind velocity explains gamma-ray flux gradients.

Positron escape from the disk accounts for INTEGRAL observations.

Abstract

Isotropic diffusion models for Galactic cosmic ray transport put tight constraints on the maximum convection velocity in the halo. For a half halo height of 4 kpc the maximum convection speed is limited to 40 km/s in the halo, since otherwise the constraints from local secondary to primary ratios and radioactive isotopes cannot be met. The ROSAT Galactic wind observations of wind speeds up to 760 km/s therefore constitute a problem for diffusion models. It is shown that such wind speeds are possible, if the diffusion coefficient in the halo is different from the diffusion coefficient in the disk. The radial dependence of the wind velocity was taken to be proportional to the source strength, as expected from winds which are sustained by cosmic ray pressure. In this case the cosmic ray density and with it the diffuse $\gamma$-ray production from nuclear interactions are suppressed near the sources. This solves in a natural way the problem of the soft gradient in the radial dependence of the $\gamma$-ray flux. Furthermore, the large bulge over disk ratio in positron annihilation as observed by INTEGRAL, can be explained by positron escape from the disk in such a model.