Clues to the Huge Antimatter Cloud at Center of the Milky Way

6a00d8341bf7f753ef013480807511970c.jpg Several years of observations from the European Space Agency’s Integral (International Gamma-Ray Astrophysics Laboratory) satellite have solved one of the most vexing mysteries in our Milky Way: the origin of a giant cloud of antimatter surrounding the galactic center.

The shape of the mysterious cloud of antimatter in the central regions of the Milky Way has been revealed by Integral. The unexpectedly lopsided shape is a new clue to the origin of the antimatter. The observations have debunked the chances that the antimatter is coming from the annihilation or decay of astronomical dark matter.

The cloud extends farther on the western side of the galactic center than it does on the eastern side. This imbalance matches the distribution of a population of binary star systems that contain black holes or neutron stars, strongly suggesting that these binaries are churning out at least half of the antimatter, and perhaps all of it.

The cloud itself is roughly 10,000 light-years across, and generates the energy of about 10,000 Suns. The cloud shines brightly in gamma rays due to a reaction governed by Einstein’s famous equation E=mc^2. Negatively charged subatomic particles known as electrons collide with their antimatter counterparts, positively charged positrons. When electrons and positrons meet, they can annihilate one another and convert all of their mass into gamma rays with energies of 511,000 electron-volts (511 keV).

The antimatter cloud was discovered in the 1970s by gamma-ray detectors flown on balloons. Scientists have proposed a wide range of explanations for the origin of the antimatter, which is exceedingly rare in the cosmos. For years, many theories centered around radioactive elements produced in supernovae, prodigious stellar explosions. Others suggested that the positrons come from neutron stars, novae, or colliding stellar winds.

In recent years, some theorists championed the idea that particles of dark matter were annihilating one another, or with atomic matter, producing electrons and positrons that annihilate into 511-keV gamma rays. But other scientists remained skeptical, noting that the dark matter particles had to be significantly lighter than most theories predicted.

"The Integral results seem to rule out dark matter as the major source of the gamma rays," said Gerry Skinner, who currently works at NASA’s Goddard Space Flight Center in Greenbelt, Md., Skinner is a co-investigator of Integral’s SPI (SPectrometer for Integral) instrument, which made this discovery.

Integral found certain types of binary systems near the galactic center are also skewed to the west. These systems are known as hard low-mass X-ray binaries, since they light up in high-energy (hard) X-rays as gas from a low-mass star spirals into a companion black hole or neutron star. Because the two "pictures" of antimatter and hard low-mass X-ray binaries line up strongly suggests the binaries are producing significant amounts of positrons.

"Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries," says Georg Weidenspointer of the Max Planck Institute for Extraterrestrial Physics in Germany.
While Integral’s discovery clears up one mystery, it raises a new one. Scientists don’t understand how low-mass X-ray binaries could produce enough positrons to explain the cloud, and they also don’t know how they escape from these systems. "We expected something unexpected, but we did not expect this," says Skinner. The antimatter is probably produced in a region near the neutron stars and black holes, where powerful magnetic fields launch jets of particles that rip through space at near-light speed.
NASA’s Gamma-ray Large Area Space Telescope (GLAST), scheduled to launch in 2008, may help clarify how objects such as black holes launch particle jets. Conceivably, it could even detect higher-energy gamma rays from heavier types of dark matter particles annihilating one another.

The new results give astronomers a valuable new clue and point away from dark matter as the origin of the antimatter. Beyond the Milk Way's center, the cloud is not entirely spherical. Instead it is lopsided with twice as much on one side of the galactic center as the other. Such a distribution is 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-center, 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.

The researchers calculate that a relatively ordinary star getting torn apart by a black hole or neutron star orbiting around it — a so-called "low mass X-ray binary" — could spew on the order of one hundred thousand billion billion billion billion positrons (a 1 followed by 41 zeros) per second. These could account for a great deal of the antimatter that scientists have inferred, reducing or potentially eliminating the need for exotic explanations such as ones involving dark matter.

“Simple estimates suggest that about half and possibly all of the antimatter is coming from the X-ray binaries,” says Weidenspointner. The other half could be coming from a similar process around the galaxy’s central black hole and the various exploding stars there. The discovery has real astrophysical importance because it decreases the need for dark matter at the center of our galaxy.

The mystery of where antimattter goes may now be solved by a latecomer to the field of antimatter research. Dragan Hajdukovic, a physicist at CERN near Geneva, who says that the current generation observatories designed to see neutrinos could answer this question, pointing out that particle-antiparticle pairs are constantly leaping in and out of existence in any field sufficiently strong to allow this phenomenon.

Normally, Hajdukovic told MITs Technology Review, gravity is too weak to support this process. However, he says that all changes inside a black hole where field strengths reach extraordinary values. Here, Hajdukovic calculates that the field ought to be strong enough to generate a regular stream of neutrino-antineutrino pairs.

If gravity attracts both matter and antimatter, then we'll be none the wiser since matter can never escape from a black hole and we'd never see either of these particles.

But if gravity repels antimatter, the antineutrinos would be hurled out of the black hole with great energy. "While neutrinos must stay confined inside the horizon, the antineutrinos should be violently ejected," says Hajdukovic,

That would make black holes powerful antineutrino sources. Hajdukovic calculates that the supermassive black holes at the center of the Milky Way and Andromeda galaxies should be bright enough to be seen by the generation of the neutrino telescopes currently being built.

The biggest and most sensitive of these is IceCube, a detector currently being assembled in the ice beneath the South Pole. It should be complete next year. Hajdukovic hedges his bet, however. He says that the discovery of antineutrinos streaming from black holes would not automatically settle the question of which way antimatter falls. The reason is that another previously unknown force may have generated the neutrino antineutrino pairs inside the balck hole. 

In 2008 the puzzle of where the mysterious antimatter at the heart of our galaxy comes from was solved, according to a top Italian space expert, Giovanni Fabrizio Bignami, head of the Italian Space Agency. He said the cloud of antimatter at the centre of the Milky Way, which scientists had known about for 30 years, appears to derive from 'binary' star systems distributed in the same area.

Binary star systems are ones in which a normal star is gradually being sucked towards a black hole or a neutron star. Neutron stars are stars that have collapsed under their own gravity and become incredibly dense.

''We have taken a big step forwards in understanding the antimatter at the centre of our galaxy,'' he said, noting that the findings were the result of four years of data supplied by the European Space Agency's satellite Integral. ''We used to think that the source of the anti-matter was a single point, like a black hole,'' Bignami said.

The Integral satellite showed that positrons – one of the key components of antimatter – are spread out over a wide area around the centre and that there are more on one side than the other.

''This seemed very strange and it gave us a big clue, putting us on the trail of the possible source of antimatter''.

The group then noticed that the distribution of binary systems in the galaxy matched the distribution of positrons almost perfectly. The researchers immediately made the connection and concluded that at least half of the antimatter comes from these binaries..

The principle they deduced may well be valid elsewhere in the universe but for now there are no instruments powerful enough, until IceCube, to let scientists observe other galaxies in enough detail to tell.

Antimatter is made up of three sorts of subatomic particles: positrons, antiprotons and antineutrons. Their equivalents in normal matter are the negatively charged electrons, positively charged protons and neutral neutrons. When two like particles of matter and antimatter meet they 'annihilate', disappearing in an explosion.

The existence of antimatter was deduced by British physicist Paul Dirac in the 1920s. Later scientists managed to create it in laboratories. Then astronomers found a mysterious cloud of it at the centre of the Milky Way. Because Dirac's theory is that matter and antimatter are produced in equal amounts from energy, a question that intrigues many is whether other places exist in the universe which are almost entirely antimatter.

Sounds like we'll have to wait for the results from IceCube in Antarctica.

Casey Kazan via newscientist.com and technologyreview.com

Additional Sources:
http://www.esa.int/esaCP/SEMKTX2MDAF_index_0.html
http://www.nasa.gov/centers/goddard/news/topstory/2007/antimatter_binary.html

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