Positron Annihilation in the Nuclear Outflows of the Milky Way

Observations of soft gamma rays emanating from the Milky Way from SPI/\textit{INTEGRAL} reveal the annihilation of $\sim2\times10^{43}$ positrons every second in the Galactic bulge. The origin of these positrons, which annihilate to produce a prominent emission line centered at 511 keV, has remained mysterious since their discovery almost 50 years ago. A plausible origin for the positrons is in association with the intense star formation ongoing in the Galactic center. Moreover, there is strong evidence for a nuclear outflow in the Milky Way. We consider here whether the Galaxy's nuclear outflow can advect positrons in such a way that their annihilation distribution and spectrum replicates the observations, in particular that positrons annihilate in a warm, partially ionized plasma. We find that a nuclear outflow cannot advect positrons into the Galactic bulge and replicate the observed morphology and spectrum of positron annihilation. This rules out a scenario where a steady-state nuclear outflow advects positrons produced near the Galactic center into the Galactic Bulge.