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Gamma-Ray Mystery Solved

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A 30-year-old puzzle about the origin of short bursts of high-radiation energy in the cosmos has been solved. In the current issue of Nature, four different teams of astronomers provide a variety of evidence that, for the first time, establishes the cosmological distance of the so-called short gamma-ray bursts and points to the source as either the collision of two small but dense stars, known as neutron stars, or the collision of a neutron star with a black hole. The finding finally confirms a theory called the merger model and opens the door not only to more detailed studies of these unusual events but to the potential for detecting gravitational waves, the elusive oscillations in spacetime created by gravity.

Short gamma-ray bursts are cousin to long gamma-ray bursts, which are displays of enormous amounts of energy that last for more than two seconds and briefly outshine other energy sources before fading. In 1997 astronomers determined that the source of a long gamma-ray burst was a supernova explosion, the collapse of a massive star. They also discovered that a monthlong afterglow followed the event that could be observed with instruments used to detect optical frequencies, x-rays or radio waves.

Unfortunately, attempts to find the afterglow and therefore the source of short gamma-ray bursts proved fruitless. Some researchers theorized that these afterglows eluded detection because they occurred in a less dense region of a galaxy, where ejected material wouldn't have the opportunity to interact with lots of particles and produce a bright enough burst. Others suggested that the bright energy was there, but astronomers just didn't have their instruments pointed in the right direction.

In November 2004 NASA launched the Swift satellite, designed specifically to sense x-ray emissions and then turn rapidly in their direction to image their source. By May of this year, a team lead by Neil Gehrels from NASA's Goddard Space Flight Center, detected the first short burst with the satellite. They determined it was coming from an elliptical galaxy, which typically consists of older stars.

Shortly thereafter in July, another team lead by Jesus Noel Villasenor of the Massachusetts Institute of Technology observed a second short burst using a satellite named HETE 2. Two other teams, one lead by Derek Fox of Caltech and the other lead by Jens Hjorth of the University of Copenhagen, detected x-ray and optical afterglows from the burst. They calculated that the luminosity was about 1,000 times lower than that coming from a long gamma-ray burst.

The data indicated that although the energy output was much lower than that seen after massive stars collapsed, it was too high to be explained by other theories suggesting that the energy derives from quakes on neutron stars. "All the evidence we presented comes to provide a solid case for the merger model and it's pretty deadly to the massive star model," says Fox.

The merger model posits that two stars, which have begun life as massive entities orbiting each other, burn through their fuel in about 10 million years and collapse into highly dense neutron stars about the size of New York City. Over another 100 million to a few billion years the two objects continue to lose energy, and as they do, their orbits shrink. Eventually they collide and produce the short gamma-ray burst. (The theory also works if one star becomes a black hole that eventually devours its partner.)

From the data gathered, astronomers now estimate that for every short gamma burst that occurs, another 30 go undetected. That knowledge could advance studies being conducted using gravitational wave detectors, such as Caltech¿s Laser Interferometer Gravitational Wave Observatory. These instruments are being used to measure gravitational waves from sources such as black holes, which evade understanding in part because they do not emit radiation.

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