Date of Award
Doctor of Philosophy (PhD)
Leising, Mark D
Hartmann , Dieter H
King , Jeremy R
Drymiotis , Fivos
The Galactic 511 keV positron annihilation emission has a strong bulge component to the emission and a weak disk component. The bulge emission is about 1.4 -- 6 times as the disk emission. The bulge emission is defined to be a diffuse emission centered at the Galactic center and extends to about a FWHM of 8 degrees in Galactic longitude and latitude. An asymmetry has also been observed in the disk emission, where the negative longitudes are brighter than the positive longitudes by a factor of 1.8.
This research examines the morphology of the Galactic 511 keV positron annihilation emission by modeling each stage of the positron from production, propagation, to finally annihilation. The production of positrons is modeled using the most probable positron production sources, radioactive isotopes, Al-26, Ti-44, and Ni-56. After the positron is produced the model accounts for the energy loss and annihilation mechanisms a positron from a radioactive isotope may undergo as it propagates through a warm neutral and ionized Galactic medium. The propagation of positrons is modeled in three different scenarios: positrons are trapped in the turbulent Galactic magnetic field and annihilate where they are produce, positrons escape the turbulent field and travel along the Galactic poloidal field, and positrons travel along the poloidal field but slow and annihilate in an extra density of gas placed at the Galactic center.
Results show the model of a turbulent field and propagation along a poloidal field can explain the flux from positron annihilation emission and the observed asymmetry using positrons from radioactive isotopes. However, the bulge emission cannot be explained by positron propagation from the disk to the bulge. In the transport model along the Galactic poloidal field there is no emission from the bulge region because there is no gas for positrons to slow and annihilate with. The model that includes an extra density of gas in the Galactic center shows that even with gas to slow and annihilate with the positrons are produced a an energy such that they may slow in the extra gas, but do not necessarily annihilate there.
Therefore, positron propagation is not sufficient to explain the Galactic 511 keV positron annihilation emission morphology. The conclusion is that there must be a central positron production source or population of sources the produces positrons at low energy, such that they will be confined to and annihilate in the bulge region.
Johns, Bethany, "Modeling the Galactic 511 keV Positron Annihilation Emission, Production, and Propagation" (2010). All Dissertations. 652.