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.

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Figure 1: Binned CO white dwarf–He white dwarf mergers within our BPS model.
Figure 2: SFRs of the disk and bulge adopted in this work and resulting SN Ia and SN 91bg formation rates vSN.
Figure 3: Constraints on the characteristic delay time tp of the main source of Galactic positrons.
Figure 4: Modelled white dwarf masses and inferred synthesized masses of 56Ni and 44Ti.


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R.M.C. was the recipient of an Australian Research Council Future Fellowship (FT110100108). Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics through project number CE110001020. D.M.N. is supported by the Allan C. and Dorothy H. Davis Fellowship. The authors thank J. Avila, J. Beacom, N. Bell, G. Bicknell, D. Clayton, K. Freeman, O. Gerhard, J. Hurley, T. Ireland, A. Karakas, M. Kerr, J. Machacek, F. Melia, D. Murtagh, R. O’Leary, R. Pakmor, T. Siegert, P. Tisserand, R. Volkas, A. Wallner, R. Wyse and F. Yuan for very useful discussions. They particularly thank B. Schmidt for pointing out the potential importance of SN1991bg-like SNe to the positron problem.

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All the authors discussed the results and commented on the manuscript. R.M.C. wrote the paper. A.J.R. performed the BPS modelling and provided theoretical input. I.R.S. provided theoretical input, helped with calculating the yields of the helium detonations and contributed to the writing of the paper. F.H.P., A.M. and B.E.T. provided advice about the rates, prevalence and distribution of 91bg in supernova searches. H.B., L.F. and J.J.E. provided advice on the BPS modelling. A.M. and M.W. provided statistical analysis. D.M.N. provided advice about the star formation history of the Galactic bulge and other theoretical input. S.S. provided input on the phenomenology of SN explosions. F.A. provided input on the phenomenology of positron transport and annihilation radiation. All the authors commented on the draft text.

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Correspondence to Roland M. Crocker.

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Crocker, R., Ruiter, A., Seitenzahl, I. et al. Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations. Nat Astron 1, 0135 (2017). https://doi.org/10.1038/s41550-017-0135

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