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Neutron Stars

Neutron stars are born as a massive star runs out of nuclear fuel and undergoes a supernova explosion in which the core of the star collapses to form a compact object. By virtue of their compactness, with one and a half solar masses compressed inside ten kilometers, neutron stars are promising sources of detectable gravitational waves. Detection of gravitational waves originating in any dynamical process will provide unique information regarding many extremes of physics, and could shed light on the detailed supranuclear equation of state.

Within the European Network we are collaborating onseveral exciting research problems, with the ultimate goal of modelling and understanding neutron star physics from the gravitational-wave point of view:

General relativistic hydrodynamics

We are developing and testing fully three dimensional numerical codes for carrying out simulations of general relativistic hydrodynamics. These tools, that are developed within the framework of Cactus, will allow us to study neutron star processes in the nonlinear regime. The present GR hydrodynamics code, developed largely at Washington University in collaboration with various Network members, has recently been tested on evolutions of a single neutron star, providing interesting results regarding the oscillations of rapidly rotating relativistic stars.

These encouraging results will be combined and extended in view of the next, ambitious step in the analysis of compact object gravitational physics: the realistic and detailed computation of the merger phase of coalescing NS, which are one of the most favourite candidates for the emission of strong and periodic gravitational waves, soon to be detected.

Furthermore, this will build the initial scenario for the study of a high-matter density torus expected to form around a rapidly rotating black hole after the coalescence of a binary system of neutron stars. The aim of the project is to study the accretion of this matter onto the black hole and the emission of gravitational waves that will be produced during this process.

Gravitational-wave instabilities

Neutron stars are unstable! The instabilities set in through various modes of pulsation, which in the non-radial case will be associated with emission of gravitational waves. Within the Network we are particularly interested in the gravitational-wave driven instability discovered by Chandrasekhar, Friedman and Schutz (CFS) in the 1970s. It is currently believed that the so-called r-modes (which are analogous to the Rossby waves in the Earth's atmosphere and oceans) are particularly susceptible to this instability. If this is, indeed, the case this instability could have significant effect on the spin-evolution of a nascent neutron star. It would also provide a promising gravitational-wave source. The main thrust of the Network's research into the r-mode instability concerns issues related to the local radiation reaction on the fluid, the effects of differential rotation and various mechanisms that may serve to saturate an unstable mode.

Relativistic perturbation theory

In the last decade it has become clear that perturbative studies provide a crucial complement to fully nonlinear efforts. Not only do they allow us to test the nonlinear codes and decode complicated nonlinear results, they also provide a deep insight into the physics underlying many interesting phenomena (such as instabilities). The Network continues to make progress in the fully relativistic perturbative approach to neutron stars. Particular emphasis in the last year has been on the calculation of oscillation modes of rotating stars that may yield to the CFS-instability. We are also using the perturbation approach to investigate the possible excitation of pulsation modes during the inspiral of a relativistic binary. Finally, we are developing new framework for studying fully nonlinear "perturbations" of stars.

Neutron star asterology

The last few years have seen a revolution in "asteroseismology", with the SOHO satellite bringing unprecedented data for the Sun and many observations confirming our expectations that most stars oscillate. The observed data has helped put constraints on models of the solar interior, and one can hope that the observation of oscillations in distant neutron stars may lead to insight into the interior physics in a similar way. We are likely a decade or so away from detectors that are sensitive enough to make such observations regularly, but it is nevertheless important to develop a framework for solving the "inverse problem" for neutron star pulsation. In the Network we are trying to assess to which extent one can hope to extract information on the supranuclear equation of state from gravitational-wave data. Interesting results in this respect show that a strange star (entirely made up of strange, down and up quarks) would be effected by the r-mode instability in a way that is distinct from a neutron star. Thus, a detection of r-mode gravitational waves from a strange star could lead to an identification of a strange star, the existence of which would help constrain the parameters of QCD.

Gravitational collapse

It has long been believed that the gravitational collapse in which either a neutron star or a black hole is formed ought to be a promising source for gravitational waves. To date the predictions of the level of radiation are quite pessimistic, but this is likely to be (at least partly) due to the fact that there are as yet no fully three dimensional studies of supernova explosions and the ensuing collapse. Within the Network we are investigating this problem from the perturbative point of view. The idea is to investigate the evolution of non-axisymmetric perturbations of a collapsing background solution to Einstein's equations to see whether these perturbations lead to an observable gravitational-wave signature of the collapse event.

This work has been supported by the EU Programme 'Improving the Human Research Potential and the Socio-Economic Knowledge Base' (Research Training Network Contract HPRN-CT-2000-00137).