Scientists solve 30-year-old antimatter mystery
An international team of scientists has solved the 30-year mystery surrounding the origins of a giant cloud of antimatter at the centre of the Milky Way. The cloud, which is 10,000 light years across and generates the energy of 10,000 suns, was first discovered in the 1970s by gamma-ray detectors flown on balloons. The gamma rays are generated when individual particles of antimatter called positrons encounter electrons, their normal matter counterpart, and the two annihilate one another. Antimatter is extremely rare in the cosmos, and the discovery of a giant cloud of it at the heart of our own galaxy led to a rash of theories as to its origins. These included supernovae, neutron stars, colliding stellar winds and processes involving dark matter. In this latest piece of research, which was led by Georg Weidenspointner of the Max Planck Institute for Extraterrestrial Physics, the scientists analysed four years of observations from the European Space Agency's INTEGRAL (International Gamma-Ray Astrophysics Laboratory) satellite to gain a better picture of the cloud. Their findings are published in the latest edition of the journal Nature. To their surprise, the cloud turned out to have a rather lopsided shape, with twice as many positrons found on the western side of the galactic centre as on the eastern side. Integral also revealed that a population of binary stars has a similarly asymmetrical distribution, suggesting that these objects could be responsible for a large proportion of the antimatter. More specifically, the stars in question are low mass X-ray binary stars (LMXBs). In these systems, a relatively normal star is being eaten alive by a nearby stellar corpse such as a neutron star or a black hole. The strong gravitational field of the corpse rips gas away from the normal star, and as the gas spirals in towards the dead star, the intense heat causes positron-electron pairs to be spontaneously generated. 'Simple estimates suggest that about half and possibly all of the antimatter is coming from X-ray binaries,' said Dr Weidenspointner, the lead author of the paper. The rest could come from similar processes around the black hole at the centre of the galaxy and the various exploding stars found there. Although the study solves one mystery, it raises new questions. For example, stars are usually distributed more or less evenly around the galaxy, so why are the X-ray binaries unevenly distributed around the galactic centre? Furthermore, scientists do not yet understand how these systems are able to generate enough positrons to explain the cloud. Over the coming years, Dr Weidenspointner and his team will work to refine their results and answer these remaining questions. 'The link between LMXBs and the antimatter is not yet proven but it is a consistent story,' he said.