Using NASA’s Rossi X-ray Timing Explorer (RXTE) satellite, a team of four astronomers has discovered a timing mechanism that tells them exactly when a superdense star will let loose incredibly powerful explosions.
"We found a clock that ticks slower and slower, and when it slows down too much, boom! The bomb explodes," says team leader Diego Altamirano of the University of Amsterdam in the Netherlands.
The explosions occur on a neutron star, which is a city-sized remnant of a giant star that exploded in a supernova. But despite the neutron star’s small size, it contains more material than our sun. The neutron star is not alone in space. It has a companion star, and the two objects orbit each other every 3.8 hours. This double-star system is known as 4U 1636-53 for its sky coordinates in the Southern Hemisphere.
The system acts like a ticking time bomb. 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. The resulting explosion appears as a bright flash of X-rays that can be detected by satellites.
The neutron star in 4U 1636-53 produces about 7 to 10 bursts per day. These explosions are mind-boggling to contemplate. They release more energy in just 10 to 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 neutron star’s surface. It’s a good thing for us that this neutron star is 20,000 light-years from Earth, which is far enough away that the explosions pose no danger to humans or our planet. Fortunately for the neutron star, the explosion takes place only on its surface and in its atmosphere, so the neutron star survives the blast.
Scientists have observed thousands of similar X-ray bursts from about 80 different neutron stars. But until now, they had no way to predict when they would occur.
The key to this discovery is RXTE, which makes extremely precise timing measurements of objects that emit X-rays in a rapidly flickering pattern. 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. We have a clock that tells us when the bomb will explode!" says Altamirano.
"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," he says.
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