Sep. 23, 1997 In a last waltz, a star in the evolutionary twilight of its long life orbits a black hole. As the star, a shrunken but very dense sun known as a neutron star, spirals toward the black hole, it is torn apart, sparking massive bursts of energy that, like cosmic beacons, are detected millions of light years away as gamma rays.
That, at least, is the picture painted by University of Wisconsin-Madison physicists Wlodzimierz Kluzniak and William Lee in a model that seeks to explain a conundrum of modern astrophysics. Their work was reported here last week (Sept. 19) at the Fourth Huntsville Gamma-Ray Burst Symposium.
Since their discovery 30 years ago, scientists have searched for, pondered and debated the origin of mysterious bursts of gamma rays. Appearing uniformly across the sky on an average of once a day, and detected only by satellites in orbit above the Earth's atmosphere, these mysterious bursts of high-energy radiation have been an astrophysical puzzle, bedeviling scientists with questions about their nature and origin.
"People have been struggling with this for 30 years," says Lee, a graduate student at UW-Madison, "and nobody's figured it out yet."
Each gamma ray burst is unique, says Lee. Some last just a fraction of a second. Others last for minutes. All come without warning and seem to come from all directions, briefly outshining entire galaxies and their billions of stars.
To be sure, the model presented today by Kluzniak, a professor of physics, and Lee is one of many competing ideas that seek to explain the mysteries associated with the brilliant bursts of radiation. There is, for example, no widespread agreement about what may cause the bursts. Some contend the sources are small black holes in nascent galaxies. Others suggest colliding neutron stars. Still other scientists think the bursts emanate from massive black holes that lie within quasars.
Another layer of debate centers on how distant the sources are from our own galaxy, the Milky Way. Most astrophysicists think that the bursts of gamma rays come from great distances beyond our galaxy, while some eminent astrophysicists argue that the source of the bursts could be very near the Milky Way.
One of the puzzles of gamma ray bursts, according to Lee, and one that his and Kluzniak's model seeks to answer, is the varying duration of the gamma ray bursts. In their model, the Wisconsin scientists suggest that a neutron star in orbit around a black hole is nearly pulled apart by the overpowering gravitational pull of the black hole.
The idea, says Lee, is the "star comes close and a little matter is dumped on the black hole." When that happens, according to their simulations, enough energy is released to produce a brilliant burst of gamma rays that can travel for millions of light years. The process, while violent, is not enough to destroy the neutron star right away. Instead, the star is repeatedly stripped of mass, each time releasing energy in the form of gamma rays.
"Our results show that if the neutron star is less massive than the black hole, it is not completely ripped apart," says Lee. "Instead, distinct, repeated episodes of mass transfer occur."
The neutron star could dump material on the black hole many times until the star has so little mass left that it becomes unstable and explodes, an event that could also give rise to a flash of radiation, the Wisconsin scientists suggest.
Computer simulations like Lee's and Kluzniak's will help set the stage for the solving of the gamma ray burst mystery. While satellites such as NASA's Compton Gamma Ray Observatory and the Hubble Space Telescope are investigating the mysterious bursts, no one has been able to identify a single source from which the bursts emanate.
But groups of observers in the United States and abroad are constructing fast-reaction telescopes, some of which will be able to be pointed at a suspected source less than 30 seconds after a burst is first detected.
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