NASA satellite pins down timer in ’stellar bomb’

May 1st, 2008 - 2:20 pm ICT by admin  

Washington, May 1 (IANS) A NASA satellite helped astronomers pin down the timing mechanism in a ticking “stellar bomb” some 20,000 light years away. The twin-neutron star, designated as 4U 1636-53, produces between seven and 10 bursts daily, releasing more energy in 10 to 100 seconds than the sun radiates in an entire week.

The astronomers said they were equivalent to 100 hydrogen bombs detonating simultaneously on a city-sized surface.

“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 University of Amsterdam.

The system, comprising two objects orbiting each other every 3.8 hours, 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.

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, reports Sciencedaily.

The key to this discovery is Rossi X-Ray Timing Explorer (RXTE) satellite, 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 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, said Altamirano.

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