The influence of Galactic stellar structures on 511 keV positron annihilation morphology

Abstract

Galactic positrons are mostly observed indirectly from the products of their annihilation. The 511 keV photons produced by their annihilation remain the most convincing probe into their origin as they move relatively unperturbed through the interstellar medium. Presently, observations of these gamma-rays are conducted exclusively by space-bourne instruments to avoid the atmospheric background but these measurements can be difficult, costly, and, due to the energies involved, have inherently poor resolution. The current iteration of one such experiment is ESA's INTEGRAL satellite which measures 2 x 10^43 e+ s-1, primarily originating from the central region of the Galaxy with a low-flux background permeating the disc; there have been suggestions that the e+ flux favours negative longitudes in either the peak or the disc flux. These observations have revealed no obvious source that can account for the radiation, and the source of Galactic positrons is still intensely debated. This study takes a holistic approach to the problem, focusing on the nature and limitations of gamma-ray observations, the large scale distribution of stars involved in positron nucleosynthesis in the Galaxy and positron propagation in the interstellar medium. We use detailed density models of stars and their Galactic distribution, including the bulge/bar, thin and thick discs, to produce a density-dependent measurement of the expected flux from positron nucleosynthesis in these stars. After introducing positron propagation, we generate maps of relative Galactic annihilation flux for our models and correlate these to observations of Galactic 511 keV morphology, investigating the connection between stellar distribution and positron annihilation morphology. While considering negligible e+ diffusion in the bulge/bar, we find that the natural peak in luminosity flux to be at l_0 = - 0.35 deg, for a bulge/bar tilt angle of 11.1 deg, at a FWHM of 6 deg. This produces a negative to positive flux ratio of 1.1, providing a mechanism for a central 511 keV flux asymmetry. Although this asymmetry is unconfirmed, our results suggest that a smooth distribution of e+ annihilation sources in an extended population can be attributed to the natural distribution of stars in the Galaxy without requiring any asymmetric sources. Furthermore, we perform a best-fit between our models of relative e+ flux and Galactic disc and 511 keV data, finding a best-fit for stars in the Galaxy older than 8 Gyr, i.e., classes of older stars with a diffusion length of 1.5 kpc. Given this diffusion, the initial e+ kinetic energy we estimate to be between 100 - 200 keV. For positron nucleosynthesis, this energy corresponds most closely to the radioactive decay of 22Na, which occurs predominantly in novæ, and 26Al, from massive stars. Additionally, if we consider that positrons partially lose momentum when escaping their supernovæ sources, there may also be contributions from 56Ni and 44Ti