The most detailed computer simulations of X-ray bursts ever produced show fireballs piercing the atmosphere of a neutron star, shock waves racing across its surface at supersonic speeds and crashing sheets of lava splashing to heights of 9 miles.

The simulations were motivated by the flickering X-ray bursts, called oscillations, discovered by the Rossi X-ray Timing Explorer satellite in 1996. The research points to specific X-ray emission characteristics that astronomers should be able to find in future observations, said the study's co-authors, led by Michael Zingale, a doctoral student in astronomy & astrophysics at the University of Chicago.

A supercomputer simulation of the surface of a neutron star 60 microseconds after the detonation of an X-ray burst. Horizontally, the plot represents a 1.2 mile portion along the surface of the neutron star. Vertically, the plot extends .93 of a mile above the star's surface. The colors indicate different values in density of matter. The greatest densities occur in the red area, where matter is 100 million times more dense than water. The greeen line near the bottom marks the boundary between the unburned nuclear fuel ahead of the detonation front and the ash behind it. A shock wave races ahead of the detonation front, reaching a height of .62 of a mile. Also visible is the first surface wave breaking ahead of the detonation front. A dark blue line marks a constant density concetration of matter that is 100 million times greater than the human body, indicating how much the surface of the star has been disturbed by the explosion. Photo: University of Chicago

"The material that you're going to see with the Rossi satellite will balloon to heights far higher than I think anyone imagined," Zingale said. "This will only happen if something as violent as we've simulated is actually occurring."

Zingale described the work Monday at the 196th national meeting of the American Astronomical Society in Rochester, N.Y., His co-authors include Frank Timmes, Bryce Fryxell and Don Lamb Jr. of the University of Chicago's Center for Astrophysical Thermonuclear Flashes, one of five centers supported by the U.S. Department of Energy's Accelerated Strategic Computing Initiative.

X-ray bursts are associated with binary stars, in which a normal star and a neutron star orbit each other. Neutron stars, the burned out cores of dead stars, measure no more than 10 miles in diameter, about the size of Chicago. But they are densely packed, giving them a gravitational force so powerful that a marshmellow striking the surface of the stars would release the energy of a hydrogen bomb.

This gravitational force causes the neutron star to draw gas from its companion. When the gas piles up on the neutron star surface and reaches a critically high temperature, it ignites in a spectacular thermonuclear flash.

The oscillations discovered by the Rossi satellite came as a surprise to many astronomers. The oscillations indicate that nuclear fuel burns unevenly over the surface of a neutron star during an X-ray burst.

"Further observations and at least one analysis suggest that the fuel was pooled in two places, perhaps the magnetic polar regions of the neutron star," said Lamb, Professor in Astronomy & Astrophysics. The observations also show that the flashes spread from pole to pole in less than 30 milliseconds. This is much faster than a normal flame front -- such as one might see in a gas fireplace -- can travel, Lamb said.

As the simulations show, only a detonation wave, a shock wave traveling slightly faster than the speed of sound, could spread that fast, Lamb said. The shock wave heats the material as it passes and ignites everything in its path.

The computer simulations also show this shock wave setting off a series of surface waves that behave like water on the surface of the ocean, except that they travel at one-tenth the speed of light. "They resemble glowing hot lava waves rolling like breakers across the surface of the neutron star," Lamb said.

The simulated surface of a neutron star 150 microseconds after the detonation of an X-ray burst. Here, the detonation front is about to reach the right side of the illustration, while several surface waves form behind the detonation front. Photo: University of Chicago

When these waves break on the unburned portion of the star, they crash heavily under a gravitational force billions of times stronger than on Earth. The crashing waves send sheets of burning fuel splashing nine miles above the star's surface. Evidence of such an event would be visible to X-ray astronomers, Lamb said.

"There would be a huge spike of X-rays visible right at the beginning and visible from all directions," Lamb explained. "That would be a signature that in fact a detonation has gone off. You wouldn't expect that kind of violent behavior in an ordinary flame front."

The Rossi satellite can make extremely precise measurements of X-ray emissions from neutron stars but is unable to make direct images of the surface, Zingale said, hence the need for simulations.

"People come up with theories describing what they believe is actually taking place on the surface, and we test these theories using supercomputers to see how they would actually play out, putting as much of the actual physics and computational resources as we can into our calculations," he said.

Previous theoretical calculations of neutron-star explosions, which were cutting-edge 15 years ago, covered a region 400 meters (440 yards) long and 180 meters (198 yards) high at a resolution of 2 or 3 meters.

But Zingale and his colleagues now have access to U.S. Department of Energy supercomputers. The supercomputer at Los Alamos National Laboratory in Albuquerque, N.M., that they used for their simulations is thousands of times faster than anything available 15 years ago. The Los Alamos supercomputer enabled them to produce simulations covering a region 2 kilometers (1.2 miles) long and 1.5 kilometers (nine-tenths of a mile) high at a resolution of 1 meter. The Chicago scientists also have more sophisticated computer code to help them simulate the physics of a neutron-star explosion.

"The flash code that Mike and his colleagues at Chicago have built to do the X-ray burst calculation will be the basic tool for proceeding to look at other problems in comparable levels of detail," said study co-author James Truran Jr., Professor in Astronomy & Astrophysics at Chicago. Other simulations on the Flash Center's agenda that also involve binary systems include classical novae, or stars that undergo less violent thermonuclear flashes, and type Ia supernovae, or exploding stars.

"I'm sure we'll find just as many surprises there as we have here," Truran said.