SN1991bg-like supernovae are a compelling source of most Galactic antimatter

TL;DR

This paper proposes that SN1991bg-like supernovae are a significant source of the positrons responsible for the 511 keV gamma-ray emission observed in the Milky Way, explaining both the positron distribution and the calcium isotope abundance.

Contribution

It demonstrates that SN1991bg-like supernovae can account for the Galactic positron annihilation and calcium isotope observations, a novel explanation linking these phenomena.

Findings

SN1991bg-like supernovae produce enough $^{44}$Ti to explain positron levels.

The distribution of these supernovae matches the Galactic bulge emission.

The $^{44}$Ti decay explains the observed calcium isotope abundance.

Abstract

The Milky Way Galaxy glows with the soft gamma ray emission resulting from the annihilation of $\sim 5 \times 10^{43}$ electron-positron pairs every second. The origin of this vast quantity of antimatter and the peculiar morphology of the 511keV gamma ray line resulting from this annihilation have been the subject of debate for almost half a century. Most obvious positron sources are associated with star forming regions and cannot explain the rate of positron annihilation in the Galactic bulge, which last saw star formation some $10\,\mathrm{Gyr}$ ago, or else violate stringent constraints on the positron injection energy. Radioactive decay of elements formed in core collapse supernovae (CCSNe) and normal Type Ia supernovae (SNe Ia) could supply positrons matching the injection energy constraints but the distribution of such potential sources does not replicate the required morphology. We show that a single class of peculiar thermonuclear supernova - SN1991bg-like supernovae (SNe 91bg) - can supply the number and distribution of positrons we see annihilating in the Galaxy through the decay of $^{44}$Ti synthesised in these events. Such $^{44}$Ti production simultaneously addresses the observed abundance of $^{44}$Ca, the $^{44}$Ti decay product, in solar system material.