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Using observations from NASA’s Rossi X-ray Timing Explorer (RXTE), an international team of astronomers has discovered a timing mechanism that allows them to predict exactly when a neutron star will unleash its wrath on the Universe. The powerful explosions occur in the collapsed remnants of massive stars - neutron stars - that have already exploded in a supernova. Hydrogen and helium gas from a companion star spirals onto the neutron star, slowly accumulating on its surface until it heats up to a critical temperature, sparking uncontrollable fusion into heavier elements and igniting a thermonuclear flame that rapidly engulfs the entire star. A bright flash of X-rays emanates from the explosion, releasing more energy in a 10 to 100 second burst than our Sun radiates in an entire week. The bursts can even occur from the same neutron star several times in one day.
Four still frames from a computer animation that illustrate a thermonuclear explosion as it ignites, spreads and engulfs an entire neutron star. Images: NASA. RXTE has made extremely precise timing measurements of these rapidly flickering X-ray emitting objects, based on the repetitive nature of the fusion processes resulting in a regular X-ray signal. According to theory, the frequency of these oscillations should be about 0.009 cycles per second, which is very close to the frequency measured at the 4U 1636-53 system, 20,000 light years away, by Diego Altamirano and colleagues in the new study. But the key result is that the team also found the frequency to decrease over time from about 12 millihertz to 8 millihertz (1 cycle per 125 seconds), and every time it dipped into this lower limit it unleashed a powerful X-ray burst. “We found a clock that ticks slower and slower, and when it slows down too much, boom! The bomb explodes," says Altamirano of the University of Amsterdam, "We are able to predict when these explosions are happening. We have a clock that tells us when the bomb will explode!" "We do not yet know if this sequence of events means the oscillations cause the explosion, or if they are just telling us the time has come for an outburst,” adds Michiel van der Klis, who also works at the University of Amsterdam, “Further observations from RXTE will be essential to figure this out.” The same group is now studying more than 50 other neutron stars to see if it can identify similar behavior. The frequencies of the X-ray emissions are also related to the mass and size of the neutron star, providing a new tool to study the physical properties of neutron stars.
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