Capitalizing on a trick of light predicted by Albert Einstein, astronomers have captured their first-ever glimpse of a blast wave left by a gamma-ray burst from a distant edge of the cosmos.

Gamma-ray bursts, thought to originate near supernovas and black holes, flash powerfully through the sky at least once a day. They last from 10 milliseconds to more than 15 minutes and are followed by X-ray and visible light afterglows that endure several days. Astronomers can detect these intense radiation events with special instruments but the bursts emit no visible light even though they are the largest known explosions in the universe.

Now, a team of astronomers has taken advantage of a rare cosmic alignment that gave them an opportunity to focus, not on the burst itself, but on the small, expanding ring of light left in its wake. The object, called GRB 000301C, was discovered in March 2000.

"This discovery really confirms what we thought a gamma-ray burst shock should look like," said Peter Garnavich of the University of Notre Dame in a prepared statement. Garnavich, who worked also with Kris Stanek and Avi Leob of the Harvard-Smithsonian Center for Astrophysics, is the lead author of a report on the finding in the Astrophysical Journal Letters. "To be able to resolve an explosion so far away is really quite astounding."

Capturing evidence of a gamma-ray burst requires luck and precision equivalent to spotting a wedding ring 2 million miles away or seeing the letter "o" on a page of paper on Earth while observing from the moon.

The trick of physics that made the blast wave visible, called gravitational microlensing, occurs when light from a very distant source, such as a gamma-ray burst, is magnified by the gravity of an object that comes between the source and the observer -- an astronomer or telescope on Earth, in this case.

The blast-wave image came from observations made by researchers using the Smithsonian's Fred Lawrence Whipple Observatory and other instruments.

Loeb of the Harvard-Smithsonian Center for Astrophysics, along with his then-student Rosalba Perna, predicted in 1998 what a gravitationally magnified blast wave from a gamma-ray burst would look like.

The gravity of an ordinary star, perhaps half the mass of our sun and located halfway between a far edge of the universe and Earth, created the lensing phenomenon, Loeb said. The data from GRB 000301C matched their predictions for what a magnified blast-wave ring would look like if it were expanding faster than the speed of light. (The apparent expansion rate of the ring is faster than light due to a relativistic effect which occurs when an emitting source moves at speeds close to the speed of light almost along the line of sight).

"It's important because you can't really resolve gamma-ray bursts with ordinary telescopes because they have a size of a micro-arcsecond -- that's a million times smaller than the resolution of ordinary telescopes on Earth," Loeb said.

"But if you put a gravitational lens in front of it, then it can magnify the background source and you get a signature of the fact that it's related to the structure of the source."

Gamma-ray bursts, discovered by the Defense Department by mistake in the 1960s, are a hot topic in astronomy, offering a glimpse back in time. Scientists mourned the deorbiting earlier this year of NASA's Compton Gamma Ray Observatory, but a satellite built by the Massachusetts Institute of Technology could help out. It is set for launch next week on a mission to detect gamma-ray bursts.

The High-Energy Transient Explorer 2 (HETE 2) will be the first satellite dedicated entirely to studying gamma-ray bursts. HETE 1 was lost in 1996 after lift-off when the last stage of the rocket launching the spacecraft failed to separate from it.