Astronomers Probe Space Mystery

Orbiting Gamma-Ray Observatory tries to pinpoint sources of unexplained bursts of energy

GERALD FISHMAN is a happy astronomer. He has gotten his teeth into what he calls "probably one of the most intriguing unsolved mysteries in astronomy."

It's the riddle of the now-you-see-'em, now-you-don't gamma-ray sources that pop up unpredictably all over the sky. They shine brilliantly with gamma-ray "light" for a few seconds or less only to fade and never be seen again. Searches of the locations in the sky where these "bursters" have appeared have so far found nothing unusual.

Astronomers don't know what they are or where they are. They could be close to Earth or billions of light years away. But Dr. Fishman says he thinks that, whatever they turn out to be, "it's going to be something new."

That's the kind of challenge astronomers love to find when they open up a new research field - in this case, the observation of the universe with gamma rays. It also is the major challenge that has emerged from the millions of bits of data taken by the National Aeronautics and Space Administration (NASA) orbiting gamma-ray observatory. Fishman, who works at NASA's Marshall Space Flight Center in Huntsville, Ala., is one of the principal investigators using that facility.

The space shuttle Atlantis deployed the $557-million, 17.5-ton observatory on its 273-by-280 mile-high (440-by-450 kilometer) orbit April 7, 1991. A year ago next Wednesday, NASA gave the satellite its official name - the Arthur Holly Compton Gamma Ray Observatory - in honor of the late Nobel Prize laureate who discovered a fundamental process involved when gamma rays interact with matter. Earlier this month, Fishman joined dozens of other scientists to review the observatory's findings during the World

Space Congress here.

Gamma rays - the "light" by which the observatory sees cosmic phenomena - are an extremely high energy form of electromagnetic radiation. Gamma- ray photons range in energy from about 10,000 times to about 10 billion times the energy of a photon (particle) of visible light.

Gamma rays arise in a variety of ways. Particles accelerating in strong magnetic fields may emit them. Electrons may collide with photons and boost them to gamma-ray energies - the process Compton discovered. Gamma rays may come from fast electrons slowing down in the presence of other matter - so-called braking radiation - or from the collision of atomic nuclei. Matter and antimatter - such as electrons and positrons - may collide and annihilate each other in gamma-ray bursts.

Gamma rays also come from decay of radioactive elements. One of the Compton Observatory's long-term objectives is to search the sky for radioactive material produced in the giant star explosions called supernovae.

These processes all involve high energies. Scientists expect that gamma rays will be a signature of the most violent and spectacular phenomena in the universe, including supernovae. They should flow from the vicinity of black holes. These are objects so compact that not even light can escape their strong gravity.

Matter circling or falling into black holes, however, may send out gamma rays. The radiation can come from violent processes in the cores of galaxies and quasars, objects much smaller than a galaxy that radiate more power than a whole galaxy of stars.

The Compton observatory has seen the gamma-ray signatures of some of these phenomena. But some of its most significant results have been in what it has not seen. For example, British cosmologist Stephen Hawking at Cambridge University has suggested that tiny black holes left over from the universe's birth may be reaching the end of their days. They may evaporate in a burst of gamma rays.

Carl Fichtel of NASA's Goddard Space Flight Center in Greenbelt, Md., a Compton principal investigator, told the symposium: "I've been looking like crazy for those gammas from evaporating black holes, and I haven't seen anything yet."

THE biggest news that's no news, however, deals with the bursters. The fact that Compton scientists have nothing novel to report about this phenomenon deepens its mystery.

American Vela satellites watching for violations of the nuclear-weapons partial test ban discovered gamma-ray bursts in the late 1960s. Subsequent observations by instruments on balloons and satellites spotted more of them. These few observations showed no preferred direction for the bursts.

Scientists tended to think that the bursts must be coming from objects within our own Milky Way galaxy. They anticipated that, as more bursts were observed, they would show a concentration to the plane of the galaxy and toward the galactic center. Compton, which keeps continuous watch over large areas of sky, was expected to reveal this asymmetry. But the more its instruments have looked, the more symmetrical the burster distribution has become. Yet the unknown sources do not seem to be distributed unifo rmly in space.

Fishman explains: "We're surrounded by gamma-ray bursts. But they're confined to the distance `D'. We don't know what `D' is." He added that there could be objects "we never knew existed before" out toward the edge of the universe.

Alternatively, there could be "a previously unknown component" of our galaxy surrounding the galaxy or even surrounding just our solar system.

Then there is the Great Annihilator. That's the name astronomers gave to the unknown source or sources close to our galactic center that emit gamma rays at energies characteristic of the mutual annihilation of electrons and positrons. Earlier observations suggested there may be a point source that varies its strength and may even turn on and off.

Compton principal investigator William Purcell of Northwestern University in Evanston, Ill., says his group has seen the characteristic annihilation radiation. But it has not seen any sign of a variable point source, which might be a black hole or other exotic object in our own galactic center. "I would really appreciate it if the galactic center would cooperate and turn on or something," he says.

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