Gamma-Ray Constraint on Galactic Positron Production by MeV Dark Matter

TL;DR

The Galactic positrons, as observed by their annihilation gamma-ray line at 0.511 MeV, are difficult to account for with astrophysical sources, and it has been proposed that they are produced instead by dark matter annihilation or decay in the inner Galactic halo.

Abstract

The Galactic positrons, as observed by their annihilation gamma-ray line at 0.511 MeV, are difficult to account for with astrophysical sources. It has been proposed that they are produced instead by dark matter annihilation or decay in the inner Galactic halo. To avoid other constraints, these processes are required to occur "invisibly," such that the eventual positron annihilation is the only detectable signal. However, electromagnetic radiative corrections to these processes inevitably produce real gamma rays (``internal bremsstrahlung"); this emission violates COMPTEL and EGRET constraints unless the dark matter mass is less than about 20 MeV.

Figures (2)

• Figure 1: Diagram (a) shows the dark matter annihilation channel $\chi \chi \rightarrow e^+ e^-$ assumed by Boehm et al.Boehm04a; the positron production and propagation occur invisibly, with only the eventual positron annihilation being detectable. The open circle represents unspecified new physics. Diagram (b) shows the observation of this paper, that positron production must be accompanied by detectable internal bremsstrahlung (a similar diagram with radiation from the electron is not shown).
• Figure 2: The solid lines show the internal bremsstrahlung spectra labeled by the assumed dark matter masses in MeV, and normalized to produce the observed Galactic 0.511 MeV flux (with the light dotted lines corresponding to its uncertainty); we assumed that the FWHM of the 0.511 MeV emission region is 9 degrees. The approximate COMPTEL (a) and EGRET (b) data and their uncertainty band are shown by the box, and the long-dashed line below it indicates the rough constraint on the maximum allowed contribution of internal bremsstrahlung gamma-rays.