Saturday, January 18, 2014

Mysterious Cloud Of Antimatter Found In Milky Way

The shape of the mysterious cloud of antimatter in the central regions of the Milky Way has been revealed by ESA’s orbiting gamma-ray observatory Integral. The unexpectedly lopsided shape is a new clue to the origin of the antimatter.

The observations have significantly decreased the chances that the antimatter is coming from the annihilation or decay of astronomical dark matter.

Georg Weidenspointner at the Max Planck Institute for Extraterrestrial Physics and an international team of astronomers made the discovery using four-years-worth of data from Integral. The cloud shows up because of the gamma rays it emits when individual particles of antimatter, in this case positrons, encounter electrons, their normal matter counterpart, and annihilate one another.

One signature of positron-electron annihilation is gamma rays carrying 511 thousand electron-volts (keV) of energy. There has been a vigorous debate about the origin of these positrons ever since the discovery of the 511 keV emission from the centre of the galaxy by gamma-ray detectors flown on balloons during the 1970s.

Some astronomers have suggested that exploding stars could produce the positrons. This is because radioactive nuclear elements are formed in the giant outbursts of energy, and some of these decay by releasing positrons. However, it is unclear whether these positrons can escape from the stellar debris in sufficient quantity to explain the size of the observed cloud.

The galactic centre

Other astronomers wondered whether more exotic processes were at work. From earlier results using much less data, the positron cloud seemed to be spherical and centred on the centre of the galaxy. Such a shape and position corresponds to the expected distribution of dark matter in the centre of our galaxy, so it was suggested that dark matter was annihilating or decaying into pairs of electrons and positrons, which then annihilated to produce the gamma rays.

The trouble with this idea, however, was that the dark matter particles needed to be much less massive than most theories were predicting.

The new results give astronomers a valuable new clue and point away from dark matter as the origin of the antimatter. Beyond the galactic centre, the cloud is not entirely spherical. Instead it is lopsided with twice as much on one side of the galactic centre as the other. Such a distribution is highly unusual because gas in the inner region of the galaxy is relatively evenly distributed.

Equally importantly, Integral found evidence that a population of binary stars is also significantly off-centre, corresponding in extent to the cloud of antimatter. That powerfully suggests these objects, known as hard (because they emit at high energies) low mass X-ray binaries, are responsible for a major amount of antimatter.

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