鈥淪tarquakes鈥� that rip open the crust of stellar corpses can be predicted with unprecedented accuracy in one such neutron star, new research reveals. The finding bolsters the theory that the quakes are set off when the star鈥檚 solid crust begins to rotate much more slowly than its fluid core.
Neutron stars form when massive stars explode at the end of their lives and leave behind spinning corpses that are so dense their electrons and protons combine into neutrons. The stars are just 30 kilometres across and are thought to have a solid crust that covers a 鈥渟uperfluid鈥� core, which flows without frictional resistance.
These two components can rotate independently 鈥� which is what researchers believe leads to the starquakes, also called 鈥済litches鈥�. The discrepancy arises because the neutron star鈥檚 magnetic field is frozen into the crust. This field emits 鈥渕agnetic dipole鈥� radiation that slows the crust down over time, explains Tod Strohmayer of NASA鈥檚 Goddard Space Flight Center in Maryland, US.
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鈥淭he superfluid component is not slowing down as much as the crust,鈥� he told New 杏吧原创. 鈥淲e think that can鈥檛 go on indefinitely 鈥� eventually, the extra angular momentum in the spinning superfluid gets transferred to the crust and that causes the glitch.鈥�
During the glitch, the hard crust cracks open and the superfluid flows into the cracks, making the crust rotate faster, he says.
Quake-ridden star
Now researchers led by John Middleditch of Los Alamos National Laboratory in New Mexico, US, have analysed these increases in spin to predict when the next glitch should occur in one particularly quake-ridden neutron star.
The star is classified as a pulsar because it emits radiation from its poles that sweeps across the Earth like a lighthouse beam. Called PSR J0537-6910, it lies about 170,000 light years from Earth and appears to be just 4000 years old. Its crust normally spins about 62 times per second 鈥� making it the fastest-spinning young pulsar known.
But when it glitches, it speeds up by one rotation every seven hours. The team discovered that the more it speeds up, the longer it will be until the next starquake.
鈥淏y monitoring the pulsar spin rate and changes in the spin, we can pin down a starquake event to within a couple of days,鈥� says Middleditch. 鈥淚f only predicting earthquakes were this straightforward.鈥�
鈥淚t鈥檚 a pretty exciting result,鈥� says Strohmayer, who is not part of the research team. 鈥淚t鈥檚 the first time anyone鈥檚 demonstrated they can predict this kind of glitching behaviour.鈥�
Unstable youth
The research team suspects the reason this star is so active is because it is so young. 鈥淭here are many very old pulsars that do not seem to glitch at all,鈥� says team member William Zhang of Goddard. 鈥淓very pulsar might do this when it鈥檚 at this age.鈥�
Pulsars may stop quaking in their old age because 鈥渢hey may have synchronised themselves鈥�, Zhang told New 杏吧原创. 鈥淭he entire star may begin rotating at the same rate.鈥�
The researchers predict the next big quake will occur around midnight GMT on 4 August. 鈥淲e鈥檒l see how this holds up, but it pretty much always has,鈥� says Middleditch.
Neutron stars of another type, called magnetars, are rocked by even more powerful starquakes in which particles and radiation flood out of the crust when it rips open (view a NASA animation ).
Middleditch presented the research on Monday at a meeting of the American Astronomical Society in Calgary, Canada.
