Abstract
The annihilation of positrons in the Milky Way Galaxy has been observed for ∼50 years; however, the production sites of these positrons remains hard to identify. The observed morphology of positron annihilation gamma-rays provides information on the annihilation sites of these Galactic positrons. It is understood that the positrons responsible for the annihilation signal originate at MeV energies. The majority of sources of MeV positrons occupy the star-forming thin disk of the Milky Way. If positrons propagate far from their sources, we must develop accurate models of positron propagation through all interstellar medium (ISM) phases in order to reveal the currently uncertain origin of these Galactic positrons. On the other hand, if positrons annihilate close to their sources, an alternative source of MeV positrons with a distribution that matches the annihilation morphology must be identified. In this work, I discuss the various models that have been developed to understand the origin of the 511 keV line from the direction of the Galactic bulge, and the propagation of positrons in the ISM.
Highlights
The annihilation of positrons at rest results in the characteristic emission of a gamma-ray line at ∼0.5 MeV
In [2], 11 years of INTEGRAL/SPI exposures are combined, and the resulting positron annihilation rate is consistent with earlier analyses [3,4], which reveal the annihilation of positrons in the bulge region of the Galaxy
As the results based on SPI/INTEGRAL data are presented as varying best-fit models, one way of quantifying the morphology is to use the ratio of positron annihilation fluxes in the bulge and disk
Summary
The annihilation of positrons (anti-electrons) at rest results in the characteristic emission of a gamma-ray line at ∼0.5 MeV. Galaxies 2018, 6, 39 suggest the initial kinetic energy of the annihilating positrons does not exceed a few MeV This rules out the origin of the positrons being cosmic ray (CR) secondaries produced in the decay of pions, as well as other sources of relativistic positrons, such as pulsars, millisecond pulsars, and magnetars [11]. The presence of the 511 keV line and the ortho-positronium continuum at low gamma-ray energies strongly suggests that the majority of positrons annihilate via interactions with neutral hydrogen at thermal energies. The origin of these annihilating positrons has puzzled astronomers for half a century. I summarise the various transport mechanisms that have been explored in the literature to date to explain the annihilation of Galactic bulge positrons
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