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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.


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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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