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Satellite Explains Giant Cloud of Antimatter
The Integral satellite map of the galactic center Integral mapped the glow of 511 keV gamma rays from electron-positron annihilation. The map shows the whole sky, with the galactic center in the middle. The emission extends to the right. Credit: ESA/Integral/MPE/G. Weidenspointner
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After four years of observations, a European satellite named Integral may have solved one of the greatest mysteries about our own Milky Way Galaxy: the origin of a giant cloud of antimatter particles surrounding the galactic center.

Antimatter particles are similar to normal particles, but they have the opposite electric charge. For example, the types of particles responsible for electricity are known as electrons, which have a negative charge. The antimatter counterparts of electrons are known as positrons. Positrons are identical in every way to electrons except for the fact that they have a positive electric charge.

When a particle like an electron meets up with an antimatter particle like a positron, the particles destroy each other. The energy of this matter/antimatter annihilation is transformed into gamma rays. In Star Trek, it is this interaction that powers the Starship Enterprise.

In the 1970s, gamma-ray detectors flown on balloons discovered a huge cloud around the galactic center. This cloud radiates the kinds of gamma rays that are produced when electrons and positrons annihilate each other. The cloud itself is roughly 10,000 light-years across, and shines with the energy of about 10,000 suns.

Integral map of hard low mass Xray binaries This Integral image shows the distribution of hard low-mass X-ray binaries. This stellar population has a distribution that matches the extent of the 511 keV map. Credit: ESA/Integral/MPE/G. Weidenspointner
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The discovery of this cloud posed a big question for astronomers: Where does this antimatter come from? Normally, antimatter particles such as positrons are extremely rare in outer space, so some process must be creating the positrons near the center of our galaxy.

Thanks to the Integral satellite, astronomers may have an answer to their question. Integral, which was built and operated by the European Space Agency with help from NASA, discovered that the antimatter cloud is not perfectly centered on the galactic core. The cloud is longer on the western side than it is on the eastern side.

Integral also found that certain types of binary star systems are distributed in the same manner. These binaries consist of two stars: one star is a normal star, and the other is the dead remnant of an exploded star. These remnants can be so-called neutron stars or black holes. The black holes and neutron stars in these binaries are stealing gas from their companion stars, and when this gas falls onto the remnant, it heats up and emits X-rays. For that reason, these systems are known as X-ray binaries.

Artist concept of Integral This is an artist's impression of ESA's orbiting gamma-ray observatory, Integral. Credit: ESA
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"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, who is the lead author of the team’s scientific paper. This paper is being published in the January 10 issue of the journal Nature.

"The Integral discovery makes more exotic explanations for the antimatter unnecessary," says Gerry Skinner of NASA’s Goddard Space Flight Center in Greenbelt, Md., who is the second author of the Nature paper. But it leaves open the question of how these X-ray binaries are producing enough antimatter to explain the cloud. As Skinner says, "We expected something unexpected, but we did not anticipate this."
Bob Naeye
NASA's Goddard Space Flight Center