Positron Annihilation in the Milky Way: Searching for the source of Galactic antimatter

Abstract

For more than 50 years, the rate and distribution of positron (anti-electron) annihilation in the Milky Way has puzzled astronomers. Positrons with low (around an MeV) initial energies are likely to be predominantly produced by beta-plus unstable radioactive isotopes. However, the observed rate of positron annihilation in the interstellar medium of the Milky Way exceeds the predicted rate of positron production by 'conventional sources' such as core-collapse and Type Ia supernovae. Moreover, the observed distribution of positron annihilation in the Milky Way appears to trace the older stellar populations of the Galaxy (the Galactic bulge and a thick, truncated disk), while massive stars, core-collapse supernovae, and Type Ia supernovae - all important sites for synthesis of radioactive material and potential positron sources - largely occur in the thin disk of the Galaxy. In this thesis, I will constrain scenarios for positron injection and transport in the interstellar medium and energetic outflows of the Milky Way. I will also show that positron annihilation on alkali metal atoms constrains the annihilation timescale of the positrons we observe annihilating today to be around 1 Myr. Finally, I will show that the delay time distribution of SN1991bg-like supernovae, a highly plausible source of up to 90% of Galactic positrons, is consistent with that required to explain the total Galactic positron annihilation line.