Gamma-Ray Burst Mystery Unraveled

From: http://www.wired.com/news/space/0,2697,69071,00.html?tw=rss.TOP

By Robert Zimmerman | Also by this reporter

02:00 AM Oct. 04, 2005 PT

Astronomers have long theorized that merging neutron stars produce massive explosions capable of wiping out nearby solar systems for thousands of light-years around. But no data was available to prove it one way or the other because the evidence -- incredibly short bursts of gamma rays -- was too ephemeral for systematic observation.

Now a flurry of research is coming to a head that offers the first detailed view of the origin of so-called short gamma-ray bursts, revealing a picture that is consistent with the merging neutron star theory. That means the universe could be far more hazardous than previously thought, given the number of known and probable neutron star pairs in relative proximity to Earth.

On Wednesday, NASA is expected to announce new findings related to short gamma-ray bursts, followed Thursday with the publication of four papers on the phenomenon in the science journal Nature. Though the papers have not yet been made public, they come amid a growing consensus among scientists about the origin of these most fleeting astronomical events.

Gamma-ray bursts have been one of astronomy's most challenging mysteries since their discovery in the early 1970s. Their ephemeral and sudden appearance -- about once per day for less than two minutes and often shorter than a second -- made it impossible to determine their distance, location or strength, and thus their cause.

What makes all gamma-ray bursts so intriguing, however, is what they represent. As astrophysicist Tsvi Piran stated at a Hubble Space Telescope symposium in 1999, "Every gamma-ray burst apparently signals the birth of a black hole."

Since the 1980s, the two most popular theories of the origin of gamma-ray bursts postulated stupendous explosions at gigantic distances, either from a supernova caused by the sudden collapse of a massive star's core, or the merger of two neutron stars or a neutron star and a black hole. The high-speed orbit of these dense binaries causes ripples of energy to radiate out from the system. The energy loss in turn causes the objects to slowly but inevitably spiral inward, eventually crashing together in a cataclysmic explosion.

Steve Thorsett of Princeton University has calculated the consequences if such a merger were to take place within 3,500 light-years of Earth, with its energy aimed at the solar system. The blast would bathe Earth in the equivalent of 300,000 megatons of TNT, 30 times the world's nuclear weaponry, with the gamma-ray and X-ray radiation stripping Earth of its ozone layer.

While scientists cannot yet predict with any precision which nearby stars will go supernova, the merger of neutron star binaries is as predictable as any solar eclipse. Three such binary systems have been discovered, and one, PSR B1534+12, presently sits about 3,500 light-years away and will coalesce in a billion years.

Scientists in the 1990s concluded that gamma-ray bursts lasting longer than 10 seconds appeared linked to unusual supernovas occurring many billions of light-years away.

For bursts shorter than 10 seconds, however, the mystery still remained. By the time telescopes could point to these brief events, their afterglows had disappeared, making it impossible to determine anything about them.

To solve this mystery, the Swift Gamma Ray Burst Mission was launched on Oct. 7, 2004. Comprising three different telescopes -- a gamma-ray detector, an X-ray telescope and an ultraviolet/optical telescope -- Swift was programmed to respond almost instantly whenever its gamma-ray detector sensed a gamma-ray burst in the sky.