PHILADELPHIA, JAN. 14 -- Astronomers today reported the discovery of one of the most spectacular overachievers in the cosmos -- a quasar that suddenly doubled its output and in three minutes emitted the equivalent of almost 1 million years of the sun's radiation.

The burst, which stunned its observers, may have represented violent activity on the brink of a black hole.

The Japanese Ginga ("Galaxy") satellite first recorded the flare, which emanated from a galaxy about 2 billion light years from Earth, in November 1989. But the enormous energy level was so far at odds with prevailing theories of astrophysics that the joint Japanese-U.S. team spent a year verifying the measurements.

"Finally," said astronomer Ronald Remillard of the Massachusetts Institute of Technology, a Ginga project scientist, "we decided that there was no other plausible explanation."

Quasars, or "quasi-stellar objects," are believed to lie at the center of certain galaxies and draw their power from black holes -- objects in which matter is so densely compressed and the pull of gravity so strong that even light cannot escape.

As vast whorls of gas, dust or even passing stars are drawn into the maelstrom, their atoms become highly energized, turning into clouds of electrically charged subatomic particles and releasing radiation.

The hotter these clouds, the higher the energy emitted, reaching the super-energetic X-ray range before slipping over the black hole's "light-escape boundary" into oblivion. When additional matter slides into the clouds, the quasar can flare.

That same radiation, however, begins to exert an opposing outward pressure on matter that is falling toward the black hole's vortex. This sets a physical limit on the amount of radiation that any quasar should be able to emit.

"However," Remillard said, "our flare exceeds this limit by a factor of 20." Moreover, the quasar doubled its output in three minutes -- less time than it would take light to travel from one side of the black hole's light boundary to the other.

The Ginga scientists theorize that the burst was caused by a phenomenon called "relativistic beaming."

In certain circumstances, the whirling clouds may form into a focused jet of gas. According to Einstein's theory of relativity, if that jet is directed toward Earth at a velocity approaching the speed of light, then it would appear to be much stronger and brighter than it would to an observer in its home galaxy.

That is, Remillard said, "it would turn from a light bulb to a flashlight."

If this hypothesis is correct, there may be many more quasars (and black holes) than presently believed. If their "flashlights" are pointed in other directions, Earth observers would never see them.

The Ginga results were reported at the annual meeting of the American Astronomical Society here.