Gravitational waves from neutron stars

Lead Research Organisation: University of Southampton
Department Name: School of Mathematics

Abstract

Neutron stars are perhaps the most exotic objects that exist in our Universe. They are born when massive stars run out of nuclear fuel and collapse under their own weight. With a mass similar to that of our Sun compressed inside a radius of about 10 kilometres (the size of a medium sized city), neutron stars stretch our understanding of physics in several exciting directions. Their density is higher than what can be studied in our laboratories. This is thought to lead to the presence of exotic forms of matter like large-scale superfluids and a quark soup. In order to understand neutron stars, and be able to compare to X-ray and radio-wave observations, we need to improve our theoretical models considerably. That we do this is also important for gravitational-wave observations. Gravitational waves are ripples in the space-time fabric itself. They were predicted by Einstein ninety years ago, but we have not yet detected them directly. These gravitational waves carry information about the most violent processes in the Universe, like colliding black hole and neutron stars. Several, incredibly sensitive, detectors that are now in operation around the world should be able to detect these elusive waves in the next few years. British scientists are heavily involved in these projects in collaboration with international colleagues. In order to be able to dig the weak gravitational waves out of the unavoidable noise of a detector, one needs very accurate model of the astrophysical processes in which they were created. The aim of the proposed project is to improve our current models of neutron stars. This is of key importance since, essentially because of their extreme compactness, neutron stars are believed to generate gravitational waves in a number of different ways. One promising possibility is due to a wave on the star, similar to those in the Earth's oceans. A particular class of waves, known as Rossby waves, are expected to grow large in a rapidly rotating neutron star. In this project we will study the mechanisms that work against the growth of these waves, how the unstable waves affect the rotation of the star, and model the gravitational waves that are emitted in the process. We will also improve our models of how a neutron star interacts with its environment. Many neutron stars are in binary systems and accrete matter from their companions. This leads to the star spinning up. In order to derive reliable gravitational-wave models for both existing and future detectors, we need to understand how the stars spin rate varies. The new models that we will develop will be tested against data from observations with X-ray satellites. Another important piece of neutron star physics that must be considered is superfluidity. In the simplest case a superfluid flows without friction. Neutron stars are expected to contain a number of such components. These components are weakly coupled and can to a certain extent move independently from each other. This has many interesting effects. The most often discussed is known as a pulsar 'glitch', during which a neutron star is seen to spin up rapidly. Normally neutron stars spin down slowly, but in a glitch this trend is temporarily reversed. The glitches are believed to be associated with transfer of energy between various components in the star. We will develop the current theoretical framework in such a way that we can describe viscosity in a superfluid in a clear way. We will also investigate to what extent various waves may be unstable in a superfluid star. This requires a complicated generalisation of the mathematical models that we have used so far. The models that we build will be tested against data taken with radio observatories. The ultimate target of the project is to construct theoretical models which will enable us to use detected gravitational-wave signals to probe the detailed physics of neutron stars.

Publications

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