Enormous Plume of Antimatter Alters View of Milky Way
By MALCOLM W. BROWNE
WILLIAMSBURG, Va. -- Astrophysicists announced on Monday that they had discovered what appears to be a monster fountain of antimatter erupting outward from the core of the Milky Way.
They said the discovery would 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 here, was made using the Compton Gamma Ray Observatory, a satellite launched by the National Aeronautics 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 X-rays.
The antimatter was discovered as a result of a series of observations made by the satellite since last November.
Antimatter -- a form of matter in which the electrical charge or other property 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, the two kinds of matter 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 concentration of 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 by 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 announced on Monday, said the cause and the nature of the antimatter fountain were puzzling. It might be a more or less continuous shaft of antimatter streaking northward from the galactic center, or it might be a cloud, separated from the main part of the galaxy.
Dr. Charles Dermer of the Naval Research Laboratory surmised that the fountain might be a mixture of gas, boiling away from violently dying stars near the center of the galaxy, and a stream of positrons.
When positrons (also called positive electrons) collide with ordinary, negatively charged electrons, they mutually 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 center. But dust and gas obstruct any light from the galactic center, and so astronomers 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 center, but scientists realized that it would 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 center 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, by the six-year-old Compton satellite, could account for the missing gamma rays.
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 a Milky Way spiral arm, the fountain, some 25,000 light-years distant, is about 4,000 light-years across and rises some 3,500 light-years above the disk.
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 here in the course of a symposium of more than 200 astrophysicists specializing in gamma ray research.
Although none of the scientists here 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.
Positrons can be produced by heated gas as it spirals into black holes, but there are other ways that the positrons in the fountain may have originated. The explosions of supernovas and white dwarf stars produce positrons, which can be generated as well in the slow decay of a radioactive isotope of aluminum created in such explosions.
While the galactic fountain emits continuous gamma rays, much more elusive is a class of objects known as gamma ray bursters: points in the sky that suddenly emit intense gamma rays for a few hours or days and then disappear. Because bursters are so brief, satellite controllers trying to track them have trouble aiming at the right point in the sky quickly enough.
But several European teams reported on Monday that they had found a lingering afterglow of radiation occurring after the gamma bursts fade out. These scientists, using an Italian-Dutch spacecraft called Beppo-SAX, reported that they had recorded a slowly fading X-ray image of a burster, which opens the way to studying these enigmas over somewhat longer time periods. Many astrophysicists believe that bursters are very distant events in which the collision of neutron stars releases almost inconceivable energy.
Many participants in this week's symposium said the universe -- even Earth's general neighborhood of the Milky Way -- was much more violent than was believed until a few years ago. Dr. Isabelle Grenier, an astrophysicist at the Saclay Center of Studies, in France, said nearby stellar explosions could be common enough that their effects might be detected in the composition of the polar icecaps.
In another development, Dr. John Mattox, an astrophysicist at Boston University, announced the possible discovery of a large planet orbiting a gamma-ray-emitting neutron star called Geminga. Until recently, Geminga was known only as a mysterious source of very bright gamma rays. But it is now known to be a tiny neutron star, the surviving cinder of a supernova explosion 300,000 years ago.
Geminga, composed of matter so dense that a piece of it the size of a sugar cube would weigh as much as the whole human race, rotates four times a second. Mattox measured the arrival times of gamma-ray pulses using data from American and European spacecraft alike, and was able to calculate a slight anomaly that could be explained by the existence of a planet twice the mass of Earth orbiting Geminga some 300 million miles from the star.
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Copyright 1997 The New York Times