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Radiation Flashes Traced to Neutron Star Collisions

October 06, 2005|Karen Kaplan | Times Staff Writer

Scientists have solved one of the most elusive mysteries of the universe, tracing the cause of the brilliant flashes of cosmic radiation known as short gamma-ray bursts to the collision of neutron stars.

When two of the super-dense, burned-out stars slam into each other, they emit gamma rays that release more energy in a fraction of a second than the sun has produced in its entire history, according to a series of papers published today in the journal Nature.

"Our observations do not prove the ... model, but we surely have found a lady with a smoking gun next to a dead body," said Caltech astronomer Shri Kulkarni, who co-wrote one of the papers.

The first big clue came in May, when NASA's Swift spacecraft captured evidence of a fleeting burst of gamma rays.

Although it lasted only 70 milliseconds, it was followed by a stream of X-rays that for the first time allowed scientists to trace their source, said Don Lamb, a professor of astronomy and astrophysics at the University of Chicago and author of one of the papers.

The researchers calculated that the gamma rays came from the edge of a galaxy 1 billion light-years from Earth. Though the X-rays were too faint to determine the exact location of the burst, the evidence supported the theory that a neutron star collision was responsible.

For better proof, they would have to wait for a brighter gamma-ray burst.

It came two months later. That burst was also over in 70 milliseconds, but it was much brighter and produced a bigger stream of X-rays. Telescopes on Earth and in orbit were able to pinpoint its location to the outskirts of a spiral galaxy, also about 1 billion light-years away.

The location was important because it ruled out a prime candidate -- supernovas, which are found toward the center of galaxies.

The only other possibility was neutron stars, the collapsed remnants of supernovas, propelled by their explosions out to the fringes of galaxies.

Some of these neutron stars wind up in pairs, circling each other for tens of millions or even billions of years as gravity pulls them together.

The eventual collision produces gas jets that stream out in two directions and are recognized on Earth as gamma rays, the highest-energy form of electromagnetic radiation.

Though satellites and telescopes did not observe the collision directly, scientists said the evidence built a solid case for the collision theory.

In addition, thefindings confirmed that a collision between a neutron star and a black hole could also generate a short gamma-ray burst.

"There's no other theory that fits this data," said George Ricker, a senior research scientist at the MIT Kavli Institute for Astrophysics and Space Research who co-wrote one of the papers.

Scientists have been contemplating gamma-ray bursts since the mid-1960s, when they were first detected by U.S. satellites.

"The people who made the initial discoveries had the idea that it might reflect surreptitious nuclear testing by the Soviet Union," Ricker said. "They quickly got past this idea because they were seeing so many of them and nobody could possibly be popping off that many nuclear weapons in space without us knowing about it."

Scientists remained befuddled until the late 1990s, when they discovered that gamma-ray bursts were followed by an X-ray afterglow.

Tracing the X-rays allowed them to determine that one type of burst, now known as a long gamma-ray burst, was produced by the collapse of giant stars. Their heavy cores spin so rapidly that they throw off streams of radiation and matter at nearly the speed of light, generating bursts that last for two to 100 seconds.

Short gamma-ray bursts were over so quickly that scientists didn't get a chance to gather much data. The Swift satellite was designed to react quickly to the first sign of a burst and pivot to capture data before it disappeared.

Discovering the source of short bursts now gives scientists hope that they will soon nail down proof of the existence of gravitational waves, which were proposed by Albert Einstein but have never been directly measured.

In addition to throwing off gamma rays, neutron star collisions also produce enormous amounts of gravitational waves, which squeeze the fabric of space-time in some directions and stretch it out in others.

Scientists are hoping to measure that warping by bouncing lasers off mirrors spaced 2.5 miles apart in underground detectors. If gravitational waves cause the mirrors to move even slightly, the lasers will detect it.

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