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Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations

Nature Astronomy volume 1, Article number: 0135 (2017) Cite this article

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Abstract

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.

First detected more than 40 years ago1, the Galactic positron annihilation signal poses two central, unresolved challenges: (1) the development of an understanding of the absolute positron production rate in the Galaxy; and (2) the finding of an explanation for the gross morphology of the positron annihilation distribution across the Galaxy. In particular, historical measurements2 have suggested a positron annihilation rate ∼1.4 times larger in the Galactic bulge than in the Galactic disk, despite the bulge hosting ≲50% the stellar mass3 and much less (≲10%) recent star formation than the disk (see Supplementary Information).

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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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Acknowledgements

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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Authors and Affiliations

  1. Research School of Astronomy and Astrophysics, Australian National University, Canberra 2611, Australia

    Roland M. Crocker, Ashley J. Ruiter, Ivo R. Seitenzahl, Fiona H. Panther, Anais Möller, David M. Nataf & Brad E. Tucker

  2. ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), Canberra 2611, Australia

    Ashley J. Ruiter, Ivo R. Seitenzahl, Fiona H. Panther, Anais Möller & Brad E. Tucker

  3. School of Physical, Environmental and Mathematical Sciences, UNSW Canberra, Australian Defence Force Academy, Canberra 2612, Australia

    Ivo R. Seitenzahl

  4. School of Mathematics and Physics, Queen’s University, University Road, Belfast BT7 1NN, UK

    Stuart Sim

  5. School of Mathematics and Physics, University of Queensland, Brisbane 4072, Australia

    Holger Baumgardt

  6. Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA

    David M. Nataf

  7. Center for Astrophysical Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, USA

    David M. Nataf

  8. Mathematical Sciences Institute, Australian National University, Canberra 2601, Australia

    Lilia Ferrario

  9. Department of Physics, University of Auckland, Auckland 1010, New Zealand

    J. J. Eldridge

  10. Department of Physics, University of Adelaide, Adelaide 5005, Australia

    Martin White

  11. Dublin Institute for Advanced Studies, 10 Burlington Road, Dublin 4, D04 C932, Ireland

    Felix Aharonian

  12. Max-Planck-Institut für Kernphsik, Saupfercheckweg 1, 69117 Heidelberg, Germany

    Felix Aharonian

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  1. Roland M. Crocker
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Contributions

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.

Corresponding author

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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