WASHINGTON -- Scientists have found fresh clues but no convincing explanations to one of the most baffling mysteries in astronomy, the source of bursts of gamma rays that flash briefly in the sky and then disappear.
Brenda Dingus of NASA said yesterday that on Super Bowl Sunday, Jan. 31, the Compton Gamma Ray Observatory satellite spotted a 50-second burst 10 times more powerful than any studied in the 25 years since they were discovered.
The unexpected strength of the "Super Bowl burst" and other recent satellite data, she said, suggest that these rays are created by matter moving at tremendous velocity, almost 99 percent of the speed of light. And the rays, she said, are probably streaming out in a narrow beam from their sources, the nature and location of which are unknown.
That means that astronomers may be seeing only the beams that happen to be traveling within Earth's line of sight.
For every burst seen by Compton, she said, there may be 10 or 100 others not visible in our corner of the universe.
Despite recording more than 600 bursts with Compton since April 1991, astronomers seem as divided as ever on the fundamental questions: Just where and how are these bursts created?
If, as some astronomers suspect, the sources of these bursts are very distant, then the mechanism that makes them must be more powerful than any yet discovered.
Stanford Woosley, an astronomer with the University of California at Santa Cruz, said that up to two years ago, almost all astronomers thought that gamma rays were born in a halo of neutron stars -- collapsed stars, made up entirely of neutrons, with tremendous gravitational energy -- swirling close to the disc of the Milky Way.
The bursts were caused, they thought, by the energy released when a piece of rock or other matter would fall on the surface of a such a star.
But the Compton satellite findings seem to rule out this explanation.
Dr. Woosley said that it would be very difficult, perhaps impossible, for an omelet-shaped galaxy like the Milky Way to wrap itself with a perfectly spherical halo of stars.
If the cloud were close by, the gravity from the galaxy would most likely tug the halo into an irregular shape. That would make the halo look uneven -- thicker in some places, thinner in others -- from Earth.
Instead, NASA scientists said, the bursts -- and presumably the objects that cause them -- are smoothly scattered across the sky.
If there's a halo out there, it's a perfect sphere.
Chryssa Kouveliotou, an astronomer with the University of Athens, said that if there is a halo, it is no closer than 200,000 light years. That would place a vast cloud of neutron stars in an area of interstellar space where nothing has been detected before.
Dr. Woosley said the evidence seems to be mounting that the bursting objects are very far away.
If so, he said, they generate tremendous energy -- as much in 10 seconds as what 1,000 stars the size of the sun give off in 10 billion years.