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

Local antimatter unveiled Antimatter is not an exotic rarity found only in the depths of the Universe: there are large quantities in our own Galaxy. We know this because we see the 511-keV γ-ray emission line, a signature of electron–positron annihilation, coming from the general direction of the Galactic Centre. The origin of the positrons has remained a mystery, but the distribution of the annihilation line radiation provides a clue. Astronomers now have the tools that can work out that distribution, and analysis of more than four years of spectroscopic data from the INTEGRAL satellite reveals an unexpected distribution of the 511-keV γ-ray emission from the inner Galactic disk, suggesting that the positrons originate in binary stars containing black holes or neutron stars. Gamma-ray line radiation at 511 keV is the signature of electron–positron annihilation, which comes from the general direction of the Galactic centre, but the origin of the positrons was a mystery. This paper reports a distinct asymmetry in the 511 keV line emission coming from the inner Galactic disk, which resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV, indicating that they may be the dominant origin of the positrons. 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 Centre 1 , but the origin of the positrons has remained a mystery. Stellar nucleosynthesis 2 , 3 , 4 , accreting compact objects 5 , 6 , 7 , 8 , and even the annihilation of exotic dark-matter particles 9 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 10 41 positrons escape per second from a typical hard LMXB. Positron production at this level f