EVEN a material 10 billion times as strong as steel has a breaking point. It seems neutron stars may shatter under extreme forces, explaining puzzling X-ray flares.
Neutron stars are dense remnants of stars gone supernova, packing the mass of the sun into a sphere the size of a city. Their cores may be fluid, but their outer surfaces are solid and extremely tough - making graphene, the strongest material on Earth, look like tissue paper by comparison.
These shells may shatter, though, in the final few seconds before a pair of neutron stars merges to form a black hole - a union thought to generate explosions known as short gamma-ray bursts.
David Tsang of the California Institute of Technology in Pasadena and colleagues have calculated how the mutual gravitational pull of such stars will distort their shape, creating moving tidal bulges. As the stars spiral towards each other, orbiting ever faster, they squeeze and stretch each other ever faster too.
A few seconds before the stars merge, the frequency of this squeezing and stretching matches the frequency at which one of the stars vibrates most easily. This creates a resonance that boosts the vibrations dramatically, causing the star's crust to crack in many places - just as a wine glass may shatter when a certain note is sung, the team says (Physical Review Letters, DOI: 10.1103/physrevlett.108.011102).
The star's gravity is too powerful to let the pieces fly away, but the sudden movement can disturb its magnetic field, accelerating electrons and leading to a powerful X-ray flare. That could explain observations by NASA's Swift satellite in which a blast of X-rays preceded some short gamma-ray bursts by a few seconds.
Combining observations of X-ray flares with those of gravitational waves emitted by the stars as they spiral together could fix the exact frequency at which the shattering occurs, which would reveal more about the stars' mysterious interiors, says Tsang.
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