Scientists using data from an instrument on NASA’s Compton Gamma Ray Observatory (CGRO) have discovered two unexpected clouds of antimatter in the Milky Way Galaxy which scientists call “antimatter annihilation radiation.”

Scientists from Northwestern University, Evanston, IL, the Naval Research Laboratory (NRL), Washington, DC, and other institutions used CGRO’s Oriented Scintillation Spectrometer Experiment (OSSE) to make the discovery, which points to the existence of a hot fountain of gas filled with antimatter electrons rising from a region that surrounds the center of the Milky Way galaxy. The nature of the furious activity producing the hot antimatter-filled fountain is unclear, but could be related to massive star formation taking place near the large black hole at the center of the galaxy. Other possibilities include winds from giant stars or black hole antimatter factories.

The researchers used maps of gamma ray sources from CGRO which they expected to show a large cloud of antimatter near the galactic center and along the plane of the galaxy. The maps, surprisingly, also show a second cloud of antimatter well off the galactic plane. The second cloud may be caused by the explosions of young massive stars.

"The origin of this new and unexpected source of antimatter is a mystery," said William R. Purcell, research scientist and assistant professor of physics and astronomy at Northwestern University.

"The antimatter cloud could have been formed by multiple star bursts occurring in the central region of the galaxy, jets of material from a black hole near the galactic center, the merger of two neutron stars, or it could have been produced by an entirely different source," said James D. Kurfess, head of the Gamma and Cosmic Ray Astrophysics Branch at the Naval Research Laboratory.

The researchers presented their findings today at the fourth Compton Symposium in Williamsburg, VA. The results have been submitted for publication in the Astrophysical Journal. A second paper presented at the conference, titled "The Annihilation Fountain in the Galactic Center Region," examines theoretical models for one possible source of the antimatter -- star bursts in the central region.

The second paper is authored by Dr. Charles Dermer and Dr. Jeffrey Skibo of NRL. They note that the gamma-ray observations permit us to see clearly, for the first time, a new part of our galaxy made of a hot column of gas filled with antimatter electrons (also called positrons by scientists), and they argue that the antimatter electrons come from newly created elements produced by exploding stars formed near the center of our galaxy.

"It is like finding a new room in the house we have lived in since childhood," comments Dr. Dermer. "And the room is not empty -- it has some engine or boiler making hot gas filled with annihilating antimatter. No one is certain whether the antimatter comes from exploding stars, black holes or something entirely different, and that is what makes this discovery so exciting."

Evidence points to the existence of a black hole with the mass of a million Suns at the very center of our galaxy. Unlike in other galaxies which harbor huge black holes, very little light comes from this source. Huge dense clouds of gas also surround the galactic center. Prolific star formation, powerful stellar winds from massive stars, and supernovae are all found here. Another theory, based on observation of radio emissions showing some black holes produce X-rays and jets, is that such outflowing jets could be made of antimatter.

The Compton Gamma Ray Observatory, launched from the Space Shuttle in 1991, views the universe in a search for gamma rays and their source. Gamma rays are extremely energetic light photons produced by high-energy particles, by the decay of excited nuclei, and when matter and antimatter annihilate each other. Antimatter cannot be found in large quantities on Earth because it would instantly vaporize anything it came into contact with. All evidence points to the universe being composed almost entirely of normal matter, though opinions differ on this.

Using the OSSE experiment, the OSSE team found antimatter positrons to be annihilating with normal matter electrons at an astonishing rate. Scientists are speculating on the origin of this antimatter, with a "black-hole lobby" favoring antimatter production in the jets of black holes.

Other scientists favor freshly synthesized radioactive material in stellar explosions being ejected up above our galaxy in an annihilating fountain of gas. Drs. Dermer and Skibo favor the latter scenario, because exploding stars will eject large quantities of hot gas made up of normal matter. This hot gas provides a target with which the antimatter electrons can annihilate.

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