GALPROP cosmic-ray propagation code: recent results and updates

Abstract

Information on cosmic-ray (CR) composition comes from direct CR measurements while their distribution in the Galaxy is evaluated from observations of their associated diffuse emission in the range from radio to gamma rays. Even though the main interaction processes are identified, more and more precise observations provide an opportunity to study more subtle effects and pose a challenge to the propagation models. GALPROP is a sophisticated CR propagation code that is being developed for about 20 years. It provides a unified framework for interpretations of data from many different types of experiments. It is used for a description of direct CR measurements and associated interstellar emissions (radio to gamma rays), thereby providing important information about CR injection and propagation in the interstellar medium. By accounting for all relevant observables at a time, the GALPROP code brings together theoretical predictions, interpretation of the most recent observations, and helps to reveal the signatures of new phenomena. In this paper we review latest applications of GALPROP and address ongoing and near future improvements. We are discussing effects of different propagation models, and of the transition from cylindrically symmetrical models to a proper 3D description of the components of the interstellar medium and the source distribution.

Keywords: Cosmic rays; Galaxy; astroparticle physics; diffusion; gamma rays; radio.

Figure 1:

Voyager 1 counting rate dominated by protons ≥70 MeV from 1977 through 2015 [6]. The inset shows the time period since 2012. The vertical dotted lines show the period selected for the analysis in the paper.

Figure 2:

Posterior distributions, mean and best-fit, with 1 and 2-sigma credible intervals for the p, $\overline{p}$, and He scan (blue), and for the light element scan (magenta). Adapted from [7].

Figure 3:

Spatial differences of the Inverse Compton emission at 100 MeV (upper) and pion-decay emission at 1 GeV (lower) between a diffusion-reacceleration (DR) and a diffusion-only (PD) models. Skymaps show (DR-PD)/DR.

Figure 4:

Spatial differences of the Inverse Compton intensity at 100 MeV between a reacceleration model with no convection (DR) and a reacceleration model with convection (DRC). Skymaps shows (DR-DRC)/DR.

Figure 5:

Figure taken from [8]. Ratio of the gamma-ray skymaps at 10 GeV calculated with GALPROP for two gas and CR source models: a model incorporating the spiral arm structure in the gas and CR source distributions vs. a standard model with cylindrical symmetry. The enhanced emission at the spiral arm tangents are visible in the map. Details can be found in [8].