A possible approach to three-dimensional cosmic-ray propagation in the Galaxy IV. Electrons and electron-induced gamma-rays

TL;DR

This paper models three-dimensional cosmic-ray electron and gamma-ray propagation in the Galaxy, comparing results with recent observational data, and concludes that many features are consistent with standard models, except for some high-latitude gamma-ray excess.

Contribution

It introduces a detailed diffusion-halo model including stochastic reacceleration and compares its predictions with recent cosmic-ray and gamma-ray data, clarifying the origins of observed spectral features.

Findings

Electron spectrum follows E^{-3} up to 1 TeV without prominent features.

EGRET GeV-excess is attributed to instrumental issues, not astrophysical or dark matter origins.

High-latitude gamma-ray enhancement suggests possible local or model uncertainties.

Abstract

Based on the diffusion-halo model for cosmic-ray (CR) propagation, including stochastic reacceleration due to collisions with hydromagnetic turbulence, we study the behavior of the electron component and the diffuse $\gamma$-rays (D$\gamma$'s) induced by them. The galactic parameters appearing in these studies are essentially the same as those appearing in the hadronic CR components, while we additionally need information on the interstellar radiation field, taking into account dependences on both the photon energy, $E_{\scriptsize {ph}}$, and the position, $\vct{r}$. We compare our numerical results with the data on hadrons, electrons and D$\gamma$'s, including the most recent results from FERMI, which gives two remarkable results; 1) the electron spectrum falls with energy as $E_e^{-3}$ up to 1\,TeV, and does not exhibit prominent spectral features around 500\,GeV, in contrast to the dramatic excess appearing in both ATIC and PPB-BETS spectra, and 2) the EGRET GeV-excess in the D$\gamma$ spectrum is due neither to an astronomical origin (much harder CR spectrum in the galactic center) nor a cosmological one (dark matter annihilation or decay), but due to an instrumental problem. In the present paper, however, we focus our interest rather conservatively upon the internal relation between these three components, using {\it common} galactic parameters. We find that they are in reasonable harmony with each other within both the theoretical and experimental uncertainties, apart from the electron-anomaly problem, while some enhancement of D$\gamma$'s appears in the high galactic latitude with $|b| > 60^\circ$ in the GeV region.