Antimatter geyser discovered at galaxy's centre

BY MALCOLM BROWNE
New York Times Service
Williamsburg, Va.

ASTROPHYSICISTS say they have discovered what appears to be a monster fountain of antimatter erupting from the core of the Milky Way.

They said the discovery could compel them to alter their image of the disk-shaped galaxy. In the revised image, it is as if a burst of steam were spurting upward from the yolk of a fried egg.

The discovery, reported at a meeting on Monday, was made using the Compton Gamma Ray Observatory, a satellite launched by the National Aer onautics and Space Administration six years ago. The four instruments aboard the observatory detect, measure and record gamma rays: invisible rays that have higher energies than all other forms of radiation, including Xrays.

The antimatter was discovered as a result of a series of observations made by the satellite since November.

Antimatter-a form of matter in which the electrical charge of each constituent particle is the reverse of that in the usual matter of our universe-cannot be directly detected in space. But when antimatter comes into contact with ordinary matter, they instantly annihilate each other, producing gamma rays, which can be detected by instruments outside Earth's shielding atmosphere.

The newly discovered plume of antimatter rises some 3,500 light-years above the disk of Earth's galaxy, which is about 100,000 light-years across. But even if this cloud of antimatter were to reach Earth, the scientists reassured their audience, it would cause no harm, because the antimatter particles in the cloud are extremely diffuse.

Moreover, only positrons are believed to be present, not antiprotons or entire antimatter atoms. Although forms of antimatter other than positrons have been created in laboratories on Earth, they have never been unequivocally identified elsewhere.

(A prevailing theory is that the Big Bang of creation produced approximately equal amounts of matter and antimatter, which promptly annihilated each other, but that a small excess of ordinary matter was enough to create the universe as we know it, with very little surviving antimatter.)

Astrophysicists representing the Naval Research Laboratory, Northwestern University and the University of California at Berkeley, who collaborated in the discovery, said the cause and nature of the antimatter fountain were puzzling. It may be a more or less continuous shaft of antimatter streaking northward from the galactic center, or it may be a cloud separated from the main part of the galaxy.

Charles Dermer of the Naval Research Laboratory surmised that the fountain is a mixture of gas, boiling away from violently dying stars near the centre of the galaxy, and a stream of positrons.

When positrons (also called positive electrons) collide with ordinary, negatively charged electrons, they destroy each other and spawn gamma rays that have a very specific energy: 511,000 electron volts. The Compton satellite is able to identify the specific energies of the gamma rays it sees, and the "fountain" was seen by tuning the instrument to the characteristic electron-positron annihilation energy.

It has long been known that Earth's galaxy looks something like a fried egg with pinwheel spirals. Earth lies in one of these spirals, and as we look up at the Milky Way on clear nights, we look inward toward the galactic centre. But dust and gas obstruct any light from the galactic centre, so astronomers must depend on other types of radiation to deduce the galaxy's innermost structure.

Earlier gamma-ray observations have suggested that there is an enormous black hole at the center of the galaxy and that, as matter is drawn into the hole, it is so intensely heated that antimatter positrons are formed.

This would account for some of the bright gamma rays flowing from the galactic centre, but scientists realized that it could not explain the much larger flow of gamma radiation coming from the Milky Way as a whole. Even after a relatively small black hole dubbed the Great Annihilator, was discovered fairly near the galaxy's centre several years ago, there still seemed to be too few gamma rays to account for all the gamma rays seen by wide-angle detectors in space.

The latest discovery could account for the missing gamma rays.

Dr. Dermer, of the Naval Research Laboratory, described the big feature as "a fountain of annihilating death from exploding stars." Viewed from Earth's position within the Milky Way, the fountain, about 25,000 light-years distant, is about 4,000 light-years across and rises 3,500 light-years above the galactic disc.

Asked why the fountain should rise from only one side of the galaxy and not the other, Dermer compared the galaxy to a pressure cooker, in which rising pressure "from a boiling cauldron of exploding stars" blew off the lid.

The results of the latest Compton satellite measurements were kept secret until Monday, when the research team disclosed that their findings had been submitted for publication in The Astrophysical Journal. Few scientists not involved in the study have had time to evaluate the discovery, reported in the course of a symposium of more than 200 astrophysicists specializing in gamma-ray research.

Although none of the scientists questioned the existence of the "annihilation fountain" suggested by the satellite's measurements, much debate is expected concerning the origin of the fountain's positrons.

Drew Williamson

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