NASA Predicts Huge Cosmic Explosions
The image from a computer animation shows a neutron star sucking in gas from a companion star’s atmosphere. The gas spirals onto the neutron star, slowly building up on its surface until it heats up to a critical temperature for an explosion.
Credit: NASA.

Astronomers are now able to predict when a certain type of star will let loose a powerful eruption.

The explosions occur on a neutron star, a city-sized remnant of a giant star that exploded in a supernova long ago and collapsed into a hyperdense ember. It now siphons material from a companion star while the two objects orbit each other every 3.8 hours.

The neutron star has incredibly strong gravity, so it sucks in some of the gas from the companion star?s atmosphere. The gas spirals onto the neutron star, slowly building up on its surface until it heats up to a critical temperature. Suddenly, the gas at one small spot on the neutron star?s surface ignites a powerful explosion, and the flame quickly spreads around the entire star.

"We found a clock that ticks slower and slower, and when it slows down too much, boom! The bomb explodes," said team leader Diego Altamirano of the University of Amsterdam in the Netherlands.

The explosion appears as a bright flash of X-rays.

The new study was done using NASA?s Rossi X-ray Timing Explorer (RXTE) satellite.

The neutron star produces about 7 to 10 bursts per day. These explosions release more energy in 100 seconds than our sun radiates in an entire week. The energy is equivalent to 100 hydrogen bombs exploding simultaneously over each postage-stamp-size patch of the dying star?s surface.

Scientists have observed thousands of similar bursts from about 80 different neutron stars, according to a NASA statement today. But until now, they had no way to predict when they would occur.

As gas gradually builds up on the neutron star?s surface, the atoms that make up the gas slam together to form heavier atoms in a process known as fusion. Sometimes, the fusion occurs in a stable and almost perfectly repetitive fashion, producing a nearly regular X-ray signal known as a quasi-periodic oscillation (or QPO for short). Think of the QPO as a clock that ticks with near-perfect precision.

Scientists expect that the QPO clock should tick about once every two minutes (120 seconds). This is what Altamirano?s team found when the astronomers observed the system with RXTE. But the team also found that the QPO clock starts ticking slower and slower as gas builds up on its surface. Whenever it slows down to one cycle every 125 seconds, the neutron star lets loose a powerful explosion.

"We can predict when these explosions are happening," Altamirano said.

This double-star system is called 4U 1636-53 and is about 20,000 light-years away. Of course that means the "predictions" involve explosions that actually occurred 20,000 years ago; the light is just now arriving.

"It's an exciting discovery," adds Tod Strohmayer of NASA?s Goddard Space Flight Center in Greenbelt, Md. Strohmayer is an expert in neutron stars who was not involved in this study. He notes that the ticking of the QPO clock depends on the size and weight of the neutron star.

"It gives us a new tool to study these fascinating objects," Altamirano said.