Neutron Stars Ring to create GRBs

Short-lived gamma-ray bursts are thought to originate from a neutron star merging with another neutron star or with a black hole. Immense tidal forces will be produced during this merger, and—according to a new model proposed in Physical Review Letters—these forces may excite a resonant vibration in one of the neutron stars, causing its crust to shatter. The authors show that this sudden shattering could explain some pre-burst flares that have recently been observed.
Gamma-ray bursts (GRBs) come in two varieties: long and short. The long ones have been connected to certain supernovae, while the short ones appear to arise from binary mergers. While it’s been known for decades that some long GRBs are preceded by weaker gamma-ray flashes, it was only last year that astronomers detected a similar “precursor” emission arriving a few seconds before three short GRBs.
For short GRBs, one possible cause of these pre-burst flares is that the crust of one neutron star cracks open, releasing tremendous energy. Previous models considered tidal distortions, but this warping takes too long to form cracks, according to David Tsang of the California Institute of Technology, Pasadena, and his colleagues. They looked instead at vibrations that these tidal forces should stimulate as the merging heavyweights spiral in toward each other. Their calculations showed that the resonant excitation of a particular crust-core vibration could generate cracks capable of shattering the crust in a way consistent with the timing and energy release in the precursor data. They suggest that further observations could begin to probe the conditions inside of neutron stars.

A quick recap of neutron stars

Neutron stars are awesomely dense stellar objects. A star's structure is created through a balance between gravitational collapse and the radiation pressure created by burning nuclear fuel. At some point, the star runs out of fuel and collapses even further. This, if the star is large enough, is followed by a giant explosion. At the end of all the noise and fury, there is a very dense ball of neutrons, surrounded by detritus from the explosion, sitting in space, weakly radiating. These are neutron stars.
If the radiation is so weak, why doesn't the star collapse further? In this case, quantum mechanics takes a hand. Every particle is associated with a wave, and when particles are close enough that their waves overlap, then their combined waves dictate "where" each particles is and how much energy it has. This generates a force, called Fermi repulsion, that stabilizes the star against further collapse.
Nevertheless, these are very, very dense objects. Think of the mass of the sun or more, compacted into a sphere that has a diameter smaller than length of Manhattan. Above the neutrons on the outside of the star is a tasty layered crust, consisting of iron ions (all the electrons have been stripped away). Below that are heavy, neutron-rich elements that are also ionized, all arranged as the perfect crystal. Put together, this makes for one very strong material.

Do not collide neutron stars at home

When a neutron star enters a death spiral, the inevitable collision is a long time coming and, to make it all the more like a bad novel, all the action happens in the last few seconds. At this point, the neutron stars are orbiting each other faster than they are actually spinning on their own axis. Gravitational forces are distorting their shapes and heating the star interiors. In short, there is a lot of energy around, and nature is not afraid to use it.
Despite all that energy, at this stage of the death spiral, we still wouldn't see anything. In fact, it will take a sensitive gravitational wave observatory to see the build up to a neutron star collision. Even so, researchers had thought that the forces involved would be sufficient to rip the crust off a neutron star, resulting in a short burst of gamma rays. Unfortunately, when the calculations were done, it was found that the force only became sufficient at about the same time that the stars actually collided. In other words, it couldn't really explain these short gamma ray bursts.
In the latest bit of work, a group of researchers have looked at the various mechanical resonances between the crust and core of the neutron star. These resonances get excited by the tidal forces generated over the course of an orbit. The key point about resonances is that you have to excite them with a force that has a similar time period to that of the mechanical motion. Furthermore, the force has to be generated along the same axis as the mechanical motion. Finally, all this force has to be sufficient to get the crust moving.
Most of the mechanical modes do not meet all of these criteria. For instance, modes with high frequencies were excluded because they would only be excited at about the same time as the neutron stars collided. In the end, the researchers settled on a mode that corresponded to a mechanical vibration at the interface between the crust and the core. They calculated, that, depending on various factors, it had a frequency of 100-200Hz, and is optimally excited about two seconds before the neutron stars collide.
Further calculations showed that the build up of vibrational motion in the interface leads to cracks that would go right through the crust. However, the crust repairs itself as fast as it breaks. So what we have is a seismic wave traveling around the neutron star, breaking open the crust as it goes, getting larger and larger with time, and always finding new crust to break. At some point, the cracking process becomes too vigorous, and the entire crust shatters.
As it shatters, the neutron star rings like a bell, causing its surrounding magnetic field to ring in sympathy, generating a pulse of light. This pulse of light is what we observe just before the collision generates the main gamma ray burst.
To give you an idea of the time and energy scale involved in this: the neutron star enters resonance for the mechanical mode two seconds before it will collide. The crust explodes from the neutron star two milliseconds later. And, power emitted during the explosion is on the order of 10⁴¹W.