UNEXPECTED CLOUD OF ANTIMATTER ABOVE THE GALACTIC CENTER

EVANSTON, Ill. - New maps of gamma rays from NASA's Compton Gamma Ray Observatory show evidence of a previously unknown and unexpected cloud of positrons, a form of antimatter, extending 3,000 light years above the center of our Galaxy.

The researchers expected the maps to show a large cloud of antimatter near the galactic center and along the plane of the Galaxy, caused by the explosions of young massive stars. The maps show that gamma ray activity, but, surprisingly, they also show a second cloud of antimatter well off the galactic plane.

"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 on Monday, April 28 at the 4th Compton Symposium being held in Williamsburg, Va. The results have been submitted for publication in The Astrophysical Journal.

A second paper presented at the conference, entitled "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 maps of the positrons were produced by NASA's Compton Gamma Ray Observatory, which was launched into orbit in April of 1991. One of the instruments on board, called the Oriented Scintillation Spectrometer Experiment (OSSE), is sensitive to gamma rays produced by the annihilation of positrons, the antimatter counterpart of the ordinary electron. These gamma rays have an energy of 511,000 electron volts, or about 250,000 times the energy of normal visible light. They are produced when positrons (antimatter) and electrons (matter) collide and annihilate, converting all of their mass into energy according to Einstein's famous equation E=MC2.

The center of our Galaxy is located about 25,000 light years away in the direction of the constellation Sagittarius. Because of the intervening interstellar dust and gas, the center of our Galaxy cannot be observed in normal visible light. The dust and gas, however, are transparent to gamma rays. Since the Earth's atmosphere absorbs gamma rays, the gamma ray telescopes must be carried by high altitude balloons or on spacecraft such as GRO.

"By combining all our observations from the direction of the center of our Galaxy, we have been able to generate maps of where positron annihilation is occurring," Purcell said. "Variations in the number of 511,000 electron volt gamma rays would suggest the presence of a single source of positrons, such as a massive black hole," he added. "Unfortunately, we have not yet seen any such variations, but what we do see is perhaps more exciting - an unexpected cloud of positrons."

"It's possible that this mapping effort could turn up evidence for other unexpected clouds of positrons," Kurfess said. "We will also keep monitoring the center of the Galaxy in the hope of seeing evidence for a black hole 'turning on' and producing positrons," he added.

Positrons, and antimatter in general, are thought to be relatively rare in the Universe. There are several ways in which positrons can be created, however. One way is through the decay of naturally occurring radioactive elements. Such radioactive materials can be created in astrophysical sources such as supernovae, novae, and Wolf-Rayet stars, which are massive stars having violent surface activity. Because these objects are relatively common in the Galaxy, the radioactive materials, and so the resulting positrons, will be distributed throughout the Galaxy. It is very likely that the same types of stars responsible for creating these radioactive materials were also responsible for creating all of the matter making up the Earth.

Another way positrons might be created is when matter falls into the extremely high gravitational field of a black hole. As matter falls into the black hole, its temperature will increase until it is hot enough to create pairs of positrons and electrons which then may stream away from the black hole at high velocities. The number of positrons created near a black hole may change abruptly as blobs of matter fall into the black hole from a companion star, while the number of positrons created by radioactive decay would be steady over long periods of time.

A third possibility is that within the last million years this region was the site of a merger of two neutron stars which created a massive galactic fireball. Such events are widely believed to cause the enigmatic gamma-ray burst phenomenon which has baffled astronomers for over twenty years.

Because the Universe appears to contain more matter than antimatter, however, once the positrons are created it is only a matter of time before they are destroyed. The positron is the anti-particle of the electron, so when a positron and electron collide they annihilate, converting the entire mass of the positron and electron into energy in the form of gamma rays. In many cases the resulting gamma rays have an energy of 511,000 electron volts, so the detection of these gamma rays indicate the presence of annihilating positrons.

Gamma rays having an energy of 511,000 electron volts were first observed from the direction of the center of our Galaxy in the early 1970's. Then, in the early 1980's, observations seemed to show a sharp decrease in the number of 511,000 electron volt gamma rays emanating from the direction of the center of our Galaxy. Such a sharp decline in the observed number of 511,000 electron volt gamma rays indicates the annihilating positrons are being produced by a small, discrete source. One possibility that generated intense interest in the scientific community is that they originate in the vicinity of a black hole, dubbed the "Great Annihilator," located at or near the center of the Galaxy.

The launch of the GRO spacecraft began a new era in our understanding of the source of positrons in our Galaxy. The OSSE instrument, ten times more sensitive than earlier gamma ray experiments, is providing scientists with he first opportunity to undertake comprehensive observations of the distribution and variability of the sources producing the positrons in the Galaxy. To date, OSSE has spent over a year mapping the distribution of the 511,000 electron volt gamma rays coming from the center of our Galaxy and searching for variations in the number of gamma rays observed.

The OSSE experiment team is headed by Kurfess and includes Purcell and a team of researchers at Northwestern University headed by Melville P. Ulmer, professor of physics and astronomy. Other scientists involved in the 511 keV mapping study included David Dixon from the University of California, Riverside, Institute for Geophysics and Planetary Physics, Lingxiang Cheng and Marvin Leventhal from the University of Maryland Astronomy Department, Robert Kinzer and Jeffrey Skibo from the Naval Research Laboratory, David Smith from the University of California, Berkeley, Space Sciences Laboratory, Jack Tueller from the NASA Goddard Space Flight Center, and Michael Saunders from the Stanford University Systems Optimization Laboratory.



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