Unexpected Cloud of Antimatter off the Galactic Plane

New maps of gamma rays from NASA's Compton Gamma Ray Observatory (CGRO) show evidence of a previously unknown and unexpected cloud of positrons, a form of antimatter, lying some 3,000 light years above the center of our galaxy.

Researchers from the Naval Research Laboratory (NRL) and Northwestern University 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 this 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 Dr.William R. Purcell, research scientist and assistant professor of physics and astronomy at Northwestern University.

Dr. James D. Kurfess, head of the Gamma and Cosmic Ray Astrophysics branch at NRL and OSSE principal investigator, noted that, "The antimatter cloud could have been formed by an ejection from a black hole at the galactic center, or from star bursts in the central region, or it could have been produced by an entirely different source."

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. (See NRL Press Release 30-97R.)

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, the NRL-developed Oriented Scintillation Spectrometer Experiment (OSSE), is sensitive to gamma rays produced by the annihilation of positrons, the antimatter counterpart of the ordinary electron.

The 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 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 placed in 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 yet not seen any such variations, but what we do see is perhaps more exciting - an unexpected region of annihilation radiation extending above the galactic center."

"It's possible that this mapping effort could turn up 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. 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's 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 because of the small size of the black hole, while the number of positrons created by radioactive decay would be steady over long periods of time.

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. Because the positron is the anti-particle of the electron, 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 indicates 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 1970s. Then, in the early 1980s, 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. A sharp decline in the number of 511,000-electron volt gamma rays produced would indicate that the annihilating positrons were 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 our 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 the 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 more than 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 Dr. James D. Kurfess of NRL, and includes a team of researchers at Northwestern University headed by Dr. Melville P. Ulmer, professor of physics and astronomy. The scientists involved in the 511 keV mapping study also included: Dr. David Dixon from the University of California, Riverside, Institute for Geophysics and Planetary Physics; Drs. Lingxiang Cheng and Marvin Leventhal from the University of Maryland Astronomy Department; Drs. Robert Kinzer and Dr. Jeffery Skibo of NRL; Dr. David Smith from the University of California, Berkeley, Space Sciences Laboratory; Dr. Jack Tueller from the NASA Goddard Space Flight Center; and Dr. Michael Saunders from the Stanford University Systems Optimization Laboratory.

Copies of the press release and related images may be obtained via the World Wide Web at:
http://www.astro.nwu.edu/astro/purcell/511kev_press_release.

Figure Caption: Map of the anti-matter annihilation radiation at 511,000-electron-volts from the galactic center region observed by NASA's Compton Gamma Ray Observatory. The unexpected cloud of emission is the region just above the strong part of the galactic center.

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