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The Milky Way hosts the annihilation of a few *10^43 positrons every second. Radionuclides capable of supplying such positrons are synthesised in stars, stellar remnants, and supernovae. For decades, however, there has been no positive identification of a main stellar positron source. This has led to suggestions that many positrons originate from exotic sources like the Galaxy’s central super-massive black hole or dark matter annihilation, but such sources would not explain the recently-detected positron signal from the extended Galactic disk. We show that a single type of transient source, deriving from stellar populations of age 3-6 Gyr and yielding ∼ 0.03 MSun of the positron emitter 44Ti, can simultaneously explain the absolute positron luminosity of the Galaxy and the morphology of the annihilation signal. Our binary population synthesis modelling demonstrates that this transient is likely the merger of two low-mass white dwarfs, likely observed in external galaxies as a particular sub-luminous, thermonuclear supernova, known as SN1991bg-like. |
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