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An asymmetric distribution of positrons in the Galactic disk revealed by γ-rays


Gamma-ray line radiation at 511 keV is the signature of electron–positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre1, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis2,3,4, accreting compact objects5,6,7,8, and even the annihilation of exotic dark-matter particles9 have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (10–50° from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV (‘hard’ LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron–positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 1041 positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

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Figure 1: A sky map in the 511-keV electron–positron annihilation line, and the sky distribution of hard LMXBs.
Figure 2: The longitude profile of a model for the sky distribution of 511-keV electron–positron annihilation line radiation for Galactic latitudes | b | < 10°.
Figure 3: Energy spectra of the γ-ray emission from the inner Galactic disk.


  1. Leventhal, M., MacCallum, C. J. & Stang, P. D. Detection of 511 keV positron annihilation radiation from the galactic center direction. Astrophys. J. 225, L11–L14 (1978)

    Article  ADS  Google Scholar 

  2. Clayton, D. D. Galaxy-Positronium origin of 476 keV galactic feature. Nature Phys. Sci. 244, 137–138 (1973)

    Article  CAS  Google Scholar 

  3. Clayton, D. D. & Hoyle, F. Gamma-ray lines from novae. Astrophys. J. 187, L101–L103 (1974)

    Article  ADS  CAS  Google Scholar 

  4. Prantzos, N. & Cassé, M. On the production of 26Al by Wolf-Rayet stars—Galactic yield and gamma-ray line emissivity. Astrophys. J. 307, 324–331 (1986)

    Article  ADS  CAS  Google Scholar 

  5. Ramaty, R. & Lingenfelter, R. E. Gamma-ray line astronomy. Nature 278, 127–132 (1979)

    Article  ADS  CAS  Google Scholar 

  6. Guessoum, N., Jean, P. & Prantzos, N. Microquasars as sources of positron annihilation radiation. Astron. Astrophys. 457, 753–762 (2006)

    Article  ADS  CAS  Google Scholar 

  7. Cheng, K. S., Chernyshov, D. O. & Dogiel, V. A. Annihilation emission from the Galactic black hole. Astrophys. J. 645, 1138–1151 (2006)

    Article  ADS  CAS  Google Scholar 

  8. Totani, T. A RIAF interpretation for the past higher activity of the Galactic Center black hole and the 511 kev annihilation emission. Publ. Astron. Soc. Jpn 58, 965–977 (2006)

    Article  ADS  CAS  Google Scholar 

  9. Boehm, C., Hooper, D., Silk, J., Cassé, M. & Paul, J. MeV dark matter: Has it been detected? Phys. Rev. Lett. 92, 101301 (2004)

    Article  ADS  Google Scholar 

  10. Knödlseder, J. et al. The all-sky distribution of 511 keV electron-positron annihilation emission. Astron. Astrophys. 441, 513–532 (2005)

    Article  ADS  Google Scholar 

  11. Teegarden, B. J. & Watanabe, K. A comprehensive search for gamma-ray lines in the first year of data from the INTEGRAL spectrometer. Astrophys. J. 646, 965–981 (2006)

    Article  ADS  CAS  Google Scholar 

  12. De Cesare, G. et al. INTEGRAL/IBIS search for e-e+ annihilation radiation from the galactic center region. Adv. Space Res. 38, 1457–1460 (2006)

    Article  ADS  CAS  Google Scholar 

  13. Weidenspointner, G. et al. The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL. Astron. Astrophys. 450, 1013–1021 (2006)

    Article  ADS  CAS  Google Scholar 

  14. Milne, P. A., Kurfess, P. A., Kinzer, R. L., Leising, M. D. & Dixon, D. D. in The Fifth Compton Symposium. (eds McConnell, M. L. & Ryan, J. M.) 21–30 (AIP Conf. Proc. Vol. 510, American Institute of Physics, Melville, 2000)

    Google Scholar 

  15. Kinzer, R. L. et al. Positron annihilation radiation from the inner galaxy. Astrophys. J. 559, 282–295 (2001)

    Article  ADS  CAS  Google Scholar 

  16. Weidenspointner, G. et al. The sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI. In Proceedings of 6th INTEGRAL Workshop (ESA SP-622, ESA Publications Division, Noordwijk, in the press); preprint at 〈〉 (2007)

    Google Scholar 

  17. Gehrels, N. et al. GRIS observations of positron annihilation radiation from the Galactic center. Astrophys. J. 375, L13–L16 (1991)

    Article  ADS  CAS  Google Scholar 

  18. Benjamin, R. A. et al. First GLIMPSE results on the stellar structure of the Galaxy. Astrophys. J. 630, L149–L152 (2005)

    Article  ADS  CAS  Google Scholar 

  19. Knödlseder, J. et al. A multiwavelength comparison of COMPTEL 1.8 MeV 26Al data. Astron. Astrophys. 344, 68–82 (1999)

    ADS  Google Scholar 

  20. Plüschke, S. et al. in Proceedings of the Fourth INTEGRAL Workshop (ed. Battrick, B.) 55–58 (ESA SP-459, European Space Agency, ESA Publications Division, Noordwijk, 2001)

    Google Scholar 

  21. Diehl, R. et al. 26Al in the inner Galaxy. Large-scale spectral characteristics derived with SPI/INTEGRAL. Astron. Astrophys. 449, 1025–1031 (2006)

    Article  ADS  CAS  Google Scholar 

  22. Jean, P. et al. Spectral analysis of the Galactic e+e- annihilation emission. Astron. Astrophys. 445, 579–589 (2006)

    Article  ADS  CAS  Google Scholar 

  23. Gillard, W., Jean, P., Marcowith, A. & Ferrière, K. Transport of positrons in the interstellar medium. in Proceedings of 6th INTEGRAL Workshop (ESA SP-622, ESA Publications Division, Noordwijk, in the press); preprint at 〈〉 (2007)

    Google Scholar 

  24. Bird, A. J. et al. The 3rd IBIS/ISGRI soft gamma-ray survey catalog. Astrophys. J. Suppl. Ser. 170, 175–186 (2007)

    Article  ADS  Google Scholar 

  25. Bazzano, A. et al. INTEGRAL IBIS census of the sky beyond 100keV. Astrophys. J. 649, L9–L12 (2006)

    Article  ADS  Google Scholar 

  26. Grimm, H.-J., Gilfanov, M. & Sunyaev, R. The Milky Way in X-rays for an outside observer. Astron. Astrophys. 391, 923–944 (2002)

    Article  ADS  Google Scholar 

  27. Schanne, S., Cassé, M., Sizun, P., Cordier, B. & Paul, J. Type Ia supernova rate in the Galactic Center region. In Proceedings of 6th INTEGRAL Workshop (ESA SP-622, ESA Publications Division, Noordwijk, in the press); preprint at 〈〉 (2007)

    Google Scholar 

  28. Knödlseder, J. et al. Imaging the gamma-ray sky with SPI aboard INTEGRAL. in Proceedings of 6th INTEGRAL Workshop (ESA SP-622, ESA Publications Division, Noordwijk, in the press); preprint at 〈〉 (2007)

    Google Scholar 

  29. Robin, A. C., Reylé, C., Derrière, S. & Picaud, S. A synthetic view on structure and evolution of the Milky Way. Astron. Astrophys. 409, 523–540 (2003)

    Article  ADS  Google Scholar 

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This work is based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA. A.W.S. is supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DLR).

Author Contributions G.W. led the work and performed the main analysis. G.S. was involved in editing and some of the statistical analysis and P.J. with the background modelling and spectral analysis. J.K. was responsible for the modelling and fitting software used. P.v.B., R.D., B.C., S.S. and C.W. critically discussed the analysis methods and the scientific interpretation. G.B. made general scientific contributions and shared his historical memory. A.W.S. provided critical evaluation of the manuscript and pointed out relevant literature. All the authors discussed the results and commented on the manuscript.

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Correspondence to Georg Weidenspointner.

Supplementary information

Supplementary Information

This file contains Supplementary Notes concerning the SPI instrument, the observations used, data analysis techniques, definitions of sky models, the statistical determination of the inner disk flux ratio and its significance, the statistical comparison with the distribution of observed X-ray binaries, and comparisons to previous measurements. Also included there are the associated Supplementary Figures 1-4 with Legends; Supplementary Table 1, providing fit results for different models, and additional references. (PDF 1087 kb)

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Weidenspointner, G., Skinner, G., Jean, P. et al. An asymmetric distribution of positrons in the Galactic disk revealed by γ-rays. Nature 451, 159–162 (2008).

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