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Autopsy Of An Explosion

March 26, 1999

Scientists analyze what occurs during a gamma-ray burst

The crime: catastrophic explosions deep within the Universe.
The clues: emissions ranging all around the spectrum, from gamma-rays to radio waves.
The motive: Unknown.

Determining what happens during a gamma-ray burst reads like a detective story. Scientists are hot on the trail of unraveling the mystery, using telescopes that are far more complex than Sherlock Holmes' simple magnifying glass.

Every aspect of a burst, from the electromagnetic emissions to the intensity of the explosion, tells its own tale. By figuring out how all these different "points-of-view" fit into the main event, scientists hope to determine what really happens. A gamma-ray burst detected on Jan. 23, 1999 - GRB 990123 - has provided scientists with even more clues to help interpret the postmortem examinations.

"This burst makes us ask two questions," says Michael Briggs, a University of Alabama at Huntsville research scientist working at NASA's Marshall Space Flight Center. "First, what caused the initial emission of visible light, and second, was that light beamed?"

These questions are addressed in a paper co-authored by Briggs and Dr. Titus J. Galama, a University of Amsterdam astrophysicist, along with colleagues from other institutions around the world. The paper, "Spectral Energy Distributions and Light Curves of GRB 990123 and its Afterglow," is due to be published in the April 1 issue of the journal Nature. By answering these questions, scientists can come closer to figuring out what happened during the explosion.

Revealing Light

With GRB 990123, scientists for the first time saw visible light emitted during a gamma-ray burst explosion. This light was so bright, observers on Earth could've seen it with a pair of binoculars. Scientists see the emission of this visible light as a clue that helps determine the structure of the explosion.

The explosion of a gamma-ray burst is like a bizarre, intersteller traffic jam. Because material is flowing out from the explosion at different velocities, collisions occur.

"Imagine the opening gate at the Indy 500," says Dr. Chryssa Kouveliotou of the Universities Space Research Association at NASA/Marshall, who also contributed to the Nature paper. "Because everyone is not moving at the same exact speed, you get pile-ups." Such collisions in a gamma-ray burst create shock waves that generate various energy wavelengths.

Three kinds of shock waves are associated with gamma-ray bursts: external, internal, and reverse. As the source of a gamma-ray burst explodes, material blows outward, creating an external shock wave ring traveling away from the source of the explosion. The impact of this fast-moving material pushing against the interstellar medium creates reverse shock waves. Meanwhile, matter still racing outward from the explosion at different speeds generates internal shock waves. These internal shock waves push the reverse shock waves outward. The reverse shock waves still appear to be traveling inward, however, because they are slower and colder than the internal shock waves.

"We think these reverse shock waves may be the source of the initial visible light emission," says Briggs.

The amount of energy released from a gamma-ray burst boggles the imagination. Exploding with the power of ten million billion suns, only collisions between objects like super-dense neutron stars and black holes have enough energy potential to create such a cataclysmic event. But no one is sure what causes a gamma-ray burst; the motive remains a mystery.

As powerful as all gamma-ray bursts are, GRB 990123 was at the top one percent of its class. GRB 990123 was so powerful that scientists began to wonder if the burst was beamed rather than dispersed evenly, or isotropically. A beamed explosion is directed like a flashlight, while an isotropic explosion is dispersed outward like the emission from a lightbulb.

"Beaming is actually quite common," says Dr. Charles Meegan, an astrophysicist at NASA/Marshall. "Radio sources, for instance, tend to beam in specific directions."

Data from the Hubble Space Telescope showed a rapid decay of the GRB 9990123 light curve. This data provides evidence for the beaming theory, because beamed light appears to dim much more rapidly than isotropic light.

A beamed explosion would have all of its power concentrated to a specific area. But an isotropic burst explodes outwards to all points in space, so we would only see the part of the energy directed toward us. Isotropic explosions, therefore, are more powerful than they look. The calculation of a gamma-ray burst's total energy depends on the dynamics of the explosion - an isotropic explosion would be calculated to have much more power than a beamed burst.

"If gamma-ray bursts are beamed," says Meegan, "then the energies we're seeing are less than we first thought, but that also means there are more of them out there that we don't see."

If the explosions are beamed in just one direction, only those lying in the path of the beams would see them. That means that there could be gamma-ray bursts exploding all the time, but because the beams are focused in other directions we don't see them.

But we would be able to see their afterglows, because afterglows are always isotropic. If we found afterglows without seeing the initial bursts, that would prove gamma-ray burst explosions are beamed.

"Early in the history of gamma-ray bursts, many people thought supernovas might be GRB afterglows," says Meegan. But from the thousands of gamma-ray bursts detected thus far, not a single one has resulted in a supernova afterglow. While there has been a search for supernovas coincident with gamma-ray bursts, there has not been a search for afterglows without gamma-ray bursts.

"Afterglows are hard to spot," says Meegan. "They're so faint that they're just at the limit of our detection devices. Supernovas are easier to find because they're generally much brighter than GRB afterglows."

Gamma-Ray Burst, alias "GRB"

Although gamma-ray bursts emit most of their energy in the gamma-ray part of the spectrum, they emit other wavelengths as well. Gamma-ray bursts were thus named because when they were discovered by accident in the early 1970s, they seemed to only emit radiation in the gamma portion of the spectrum. Scientists thought it was odd that the bursts appeared to only give off one form of energy. Most other energy sources give off several forms of energy simultaneously. A flame, for instance, gives off infrared (heat) and visible light.

As it turns out, gamma-ray bursts are more than just explosions emitting gamma rays. Now we know that it was our ability to see, rather than the source, which was limited. Telescopes are only designed to see one part of the electromagnetic spectrum. When a gamma-ray telescope detects a burst, there is usually not enough time to direct other telescopes to look at the explosion. The bursts are extremely short-lived. For GRB 990123, the Robotic and Optical Transient Search Experiment (ROTSE) coordinated with the Compton Gamma Ray Observatory within 20 seconds of the start of the explosion - just quick enough to catch a burst in action.

Although the explosion only lasts for a few seconds, the afterglow can linger for weeks or even months. The afterglow follows a path down the electromagnetic spectrum, first mostly emitting gamma rays, then peaking at x-rays, and so on, all the way down to radio waves. At this point, however, peak emissions drop off quickly as radio waves are absorbed back into the source. Eventually, the afterglow fades completely from our view.

Because the afterglow is much longer-lived than the initial explosion, various types of telescopes have been used to study the afterglow. Most of our knowledge about gamma-ray bursts comes from these studies of the afterglow.

The meaning of other GRB 990123 emissions is currently under debate. The gamma-ray burst emitted a large amount of X-rays, far in excess of predicted models. And there is also some debate about the radio emissions of the burst. Scientists are currently trying to untangle these clues to better understand the mechanics of the explosion.

Gamma-ray bursts are mysterious objects. A literal flash in the night, they are as fleeting and elusive as a passing thought. But as our ability to track these explosions improves, scientists can use their data to piece together an image of gamma-ray burst anatomy. Such knowledge will perhaps lead to a deeper understanding of what creates these enigmatic eruptions, and suggest why they occur at all.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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