Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations

Our Galaxy hosts the annihilation of a few 10⁴³ low-energy positrons every second. Radioactive isotopes capable of supplying such positrons are synthesized in stars, stellar remnants and supernovae. For decades, however, there has been no positive identification of a main stellar positron source, leading to suggestions that many positrons originate from exotic sources like the Galaxy’s central supermassive black hole or dark matter annihilation. Here we show that a single type of transient source, deriving from stellar populations of age 3–6 Gyr and yielding ∼0.03 M⊙ of the positron emitter ⁴⁴Ti, can simultaneously explain the strength and morphology of the Galactic positron annihilation signal and the Solar System abundance of the ⁴⁴Ti decay product ⁴⁴Ca. This transient is likely the merger of two low-mass white dwarfs, observed in external galaxies as the sub-luminous, thermonuclear supernova known as SN 1991bg-like.Roland M. Crocker, Ashley J. Ruiter, Ivo R. Seitenzahl, Fiona H. Panther, Stuart Sim, Holger Baumgardt, Anais Möller, David M. Nataf, Lilia Ferrario, J.J. Eldridge, Martin White, Brad E. Tucker, and Felix Aharonia