Neutron stars are mainly fluid, and can therefore support waves, rather like the waves of the Earth's ocean.  These fluid waves themselves emit gravitational waves, so we hope to learn about the fluid waves by studying gravitational waves.  Like other sorts of waves, gravitational waves carry energy.  We can therefore say that the energy of motion of the fluid waves is being converted into gravitational wave energy.  Under most circumstances, this loss of energy causes the fluid waves to become weaker.  However, if the neutron star rotates fast enough, this energy loss can actually make the fluid waves grow larger.  The larger fluid waves then emit gravitational wave energy even more rapidly, making the fluid wave bigger, and so on, leading to an instability.  To read about the details of this process, CLICK HERE.   Clearly, such instabilities are very interesting, as they may cause the star to emit a large quantity of gravitational wave energy.

My own research in this area has concentrated on accreting neutron stars.  `Accretion' simply means that hot plasma is falling onto the neutron star from another nearby star.  This plasma carries angular momentum, and will tend to make a slowly spinning neutron star spin faster.  I have recently investigated the possibility that  the rapid spin rate of some observed neutron stars represents a balance between the plasma tending to make the star spin faster, and gravitational wave energy loss, tending to make the star spin more slowly.   In order to describe such a scenario, many pieces of physics need to be supplied, such as how the falling plasma heats the star, how neutrinos (another sort of particle) then cool it, how viscous forces within the star tend to damp the perturbation, and how the gravitational wave field fights this damping to try and amplify the perturbation. My collaborators and I have assembled such a model, investigated its consequences, and compared our findings with observations of accreting stars. We were able to show that gravitational wave emission may play an important role in accreting neutron star systems.

Intriguingly, some theorists have suggested that the small dense objects at the centres of accreting systems aren't neutron stars at all, but are instead made up of a mixture of particles known as strange quarks. These rather exotic objects are known as strange stars. I investigated gravitational wave emission from this sort of star while a post-doc here at Southampton; my collaborators and I found a way of using gravitational wave observations to distinguish between the neutron star/strange star scenarios.