Antiproton Flux in Cosmic Ray Propagation Models with Anisotropic Diffusion

TL;DR

This study implements an anisotropic cosmic ray diffusion model in GALPROP, analyzing its impact on antiproton flux predictions and fitting observational data to assess the model's validity.

Contribution

It introduces a height-dependent diffusion coefficient parameter in cosmic ray models, demonstrating improved fits to galactic wind observations and antiproton flux data.

Findings

Models with diffusion coefficient D_{xx} proportional to |z| fit high-velocity winds.

Predicted antiproton flux lower than PAMELA measurements at 2-20 GeV.

Combined fit suggests only partial explanation of observed antiproton flux.

Abstract

Recently a cosmic ray propagation model has been introduced, where anisotropic diffusion is used as a mechanism to allow for $\mathcal{O}(100)$ km/s galactic winds. This model predicts a reduced antiproton background flux, suggesting an excess is being observed. We implement this model in GALPROP v50.1 and perform a $\chi^2$ analysis for B/C, $^{10}$Be/$^{9}$Be, and the recent PAMELA $\bar{p}/p$ datasets. By introducing a power-index parameter $\alpha$ that dictates the dependence of the diffusion coefficient $D_{xx}$ on height $|z|$ away from the galactic plane, we confirm that isotropic diffusion models with $\alpha=0$ cannot accommodate high velocity convective winds suggested by ROSAT, while models with $\alpha=1$ ($D_{xx}\propto |z|$) can give a very good fit. A fit to B/C and $^{10}$Be/$^{9}$Be data predicts a lower $\bar{p}/p$ flux ratio than the PAMELA measurement at energies between approximately 2 GeV to 20 GeV. A combined fit including in addition the $\bar{p}/p$ data is marginal, suggesting only a partial contribution to the measured antiproton flux.