PITTSBURGH -- Almost every day, a huge explosion somewhere in outer space sends bursts of high-energy gamma rays sweeping by Earth.
Scientists are fascinated by them, and they have lots of theories about what the bursts are and where they come from. But they have almost no answers.
"We really as yet have no clue to what they are," said Dr. Geoffrey Pendleton of the Marshall Space Flight Center in Huntsville, Ala.
Although they are invisible to humans, the bursts are among the most impressive celestial phenomena detectable from Earth.
If the human eye were sensitive to gamma rays and if they penetrated atmosphere, the bursts would appear in the sky like flashbulbs, popping off almost daily, briefly shining as brightly as the planet Venus.
Some are as fleeting as 20 thousandths of a second. Others last as long as 13 minutes.
They were first detected by classified Vela military satellites launched in the 1960s to detect Chinese or Russian nuclear tests in outer space or on the far side of the moon.
The Vela satellites never spotted a nuclear test, but they did record about a dozen gamma ray bursts each year from deep in outer space.
Astronomers have tracked the bursts since 1991 with the Burst and Transient Source Experiment (BATSE) on the orbiting Compton Gamma Ray Observatory. With more sensitive instruments, BATSE is detecting about 300 bursts each year.
Scientists say the bursts appear unpredictably, at random, scattered all over the sky. But efforts so far to match them to visible stars and galaxies have failed.
The bursts also appear to destroy their sources. "We've never seen them repeat from the same location," said Dr. Gerald J. Fishman, of the Marshall Space Flight Center, an astrophysicist with the BATSE project.
Interviewed yesterday at a meeting of the American Astronomical Society, Dr. Fishman said there are at least 120 different theories about what is producing the bursts.
"It's some sort of explosive phenomenon, but what is causing the explosions, we do not know," he said.
A network of 25 ground-based observatories is on constant alert for the bursts. When the BATSE detectors spot one, all 25 are signaled within five seconds. Their telescopes swing around to look for the source. So far, they have found nothing.
Scientists don't even know yet how far away the sources of the bursts are, Dr. Fishman said. Calculations thus far suggest only that they are scattered evenly throughout, but are contained within, a sphere of space extending out in all directions from observers on Earth.
"They are either in a large halo around our galaxy," he said, "or . . . they are at much larger distances, at the edge of the universe."
But what could be producing such bursts of high-energy radiation?
Gamma rays are created when high-energy electrons smash into another material. Exploding stars, solar flares and jets spewed from collapsing stars can produce enough turbulence to generate gamma rays.
Such phenomena should also produce X-rays. But X-rays are almost totally absent from gamma ray bursts, suggesting they are absorbed by something before reaching Earth.
One of the more promising theories is that the bursts are generated when a neutron star merges with a black hole. Neutron stars are the extremely compact, highly energetic spinning cores of collapsed stars. Black holes are the collapsed remains of giant stars, so massive that not even light can escape their immense gravitational pull.
Such a collision would produce an immense explosion and gamma ray emissions, Dr. Fishman said.