An international team of scientists using data from a small NASA spacecraft has put into place a large piece of the astronomical puzzle of short-duration gamma-ray bursts.

Scientists say they have seen tantalizing, first-time evidence of a black hole eating a neutron star-first stretching the neutron star into a crescent, swallowing it, and then gulping up crumbs of the broken star in the minutes and hours that followed, as illustrated in these images. Credit: Dana Berry/NASA

With observations from the High Energy Transient Explorer 2 (HETE-2) satellite, the team has obtained the best evidence yet that short-duration gamma-ray bursts are caused by the collision of two compact stars.

The team, which includes Kevin Hurley, senior space fellow at the University of California, Berkeley's Space Sciences Laboratory, will announce its results in the Oct. 6 issue of the journal Nature.

"This is the first secure measurement that links the short bursts to the compact object collision theory," said Garrett Jernigan, a research physicist at the laboratory.

One of the nagging mysteries of cosmic gamma-ray bursts, the most luminous explosions in the universe, is whether all are produced the same way.

"These bursts come in a bewildering array of shapes and varieties, and when you're shopping for a theory to explain them, one size may not fit all," Hurley said.

The breakthrough burst came on July 9 from the constellation of Grus (the crane) in the southern sky, and lasted only one-tenth of a second. The burst was captured by HETE-2, a spacecraft launched in 2000 by an international collaboration involving the United States, Japan, France and other partners.

"That was the clue we were waiting for," said Jernigan. "Bursts seem to come mainly in two varieties - the long ones, which last about 20 seconds, and the short ones, which last a few tenths of a second. It's the short ones that have still been puzzling us."

Evidence has mounted over the years that the longer bursts are caused by the collapse of massive stars in distant galaxies. These stars, which contain about 30 times the matter that our sun does, run out of nuclear fuel and collapse into black holes, emitting prodigious amounts of energy in gamma-rays, which are a very energetic version of X-rays.

For the short bursts, though, there was no evidence that this was the case. On the contrary, there were many theories that these bursts are produced by the collision of two small, compact objects - either two neutron stars, or one neutron star and a small black hole. In either case, a larger black hole is formed as the result of a collision.

The HETE-2 spacecraft pinpointed the source of the short burst on July 9 and triggered a series of observations by NASA's Chandra X-Ray Observatory and Hubble Space Telescope, as well as by a host of ground-based observatories. What they found, for the first time, was a faint, short-lived optical afterglow, which represents the embers of the explosion which produced the burst. This, in turn, allowed astronomers to identify the galaxy in which the burst had occurred, and to measure its distance.

To their surprise, it was relatively nearby, by astronomical standards.

"This burst took place only one billion light years from Earth, instead of the usual 10 billion light-years for the long bursts," said Nat Butler, a Townes Fellow at UC Berkeley and part of the HETE team. "When you do the math, it turns out that this was about a thousand times less energetic than a typical long burst."

Another clue came from the position of the burst within the galaxy. It was on the outskirts, where double neutron star systems are expected to be. The details of the Hubble and Chandra observations, which clinch the case, are being published in a companion Nature paper by Derek Fox of Pennsylvania State University and his colleagues.

Neutron stars are the remnants of normal stars after they have burned out. They measure about 10 miles in diameter, but contain the same amount of mass as our sun. They can exist in binary systems, where two neutron stars orbit each other, or where one neutron star orbits a black hole. Eventually, though, these systems must decay and the compact stars must collide, producing gravitational waves predicted by Albert Einstein's general theory of relativity. These waves have never been detected directly, and that is the goal of the Laser Interferometer Gravitational Wave Observatory (LIGO), which has just begun operation.

"It's possible now that the first gravitational wave source that LIGO observes will also be a gamma-ray burst source," said Hurley. "Now that would be a spectacular discovery."