Cosmic ray nuclei, antiprotons and gamma rays in the galaxy: a new diffusion model

Abstract

We model the transport of cosmic ray nuclei in the Galaxy by means of a new numerical code. Differently from previous numerical models we account for a generic spatial distribution of the diffusion coefficient. We found that in the case of radially uniform diffusion, the main secondary/primary ratios (B/C, N/O and sub-Fe/Fe) and the modulated antiproton spectrum match consistently the available observations. Convection and re-acceleration do not seem to be required in the energy range we consider: 1 ≤ E ≤ 10 3 GeV/nucleon. We generalize these results accounting for radial dependence of the diffusion coefficient, which is assumed to trace that of the cosmic ray sources. While this does not affect the prediction of secondary/primary ratios, the simulated longitude profile of the diffuse γ-ray emission is significantly different from the uniform case and may agree with EGRET measurements without invoking ad hoc assumptions on the galactic gas density distribution. ‡ This unit of measure corresponds to the energy E competing on average to each nucleon within a nucleus with A nucleons and energy A × E.

Antiprotons from dark matter: Effects of a position-dependent diffusion coefficient

“…Such smooth variation was incorporated in our second model, which assumed an exponentially growing diffusion coefficient, which has been shown to produce consistent fits to conventional astrophysical cosmic ray observables in Ref. [4]. We found numerical solutions as well as analytic approximations valid at high antiproton energies for this model.…”

“…[4]. Using this code, it was demonstrated that a consistent fit to observed cosmic ray isotope ratios can be obtained in this model.…”

“…In reality, magnetic fields are known to decrease gradually with the distance from the disk, with exponential decay providing a reasonable fit to data. The diffusion coefficient likely follows a similar exponential profile [4]. Second, a typical dark matter halo is spherically symmetric and extends beyond the diffusion region and into the free propagation zone, in particular in the vertical direction: for example, for an isothermal dark matter profile in a model with L = 1 kpc, the diffusion zone contains only 10% of the dark matter mass of the full halo.…”

“…We find an analytic solution to this model, and show that in the limit when diffusion in the free-propagation zone is completely absent, the sources in this zone have no effect on the flux measured at Earth. We then consider a model with an exponentially varying diffusion coefficient [4] in Sec. 4, and obtain both a numerical solution and two analytic approximations which converge to it at high energies (above 50 GeV or so, depending on the desired accuracy).…”

“…We then consider a model with an exponentially varying diffusion coefficient [4] in Sec. 4, and obtain both a numerical solution and two analytic approximations which converge to it at high energies (above 50 GeV or so, depending on the desired accuracy). In Sec.…”