Gravitational waves from accreting neutron stars and Cassiopeia A

Abstract

This thesis is concerned with the mysteries of neutron stars and the quest for gravitational waves. Rapidly-rotating neutron stars are anticipated sources of periodic gravitational waves, and are expected to be detectable within the next decade using kilometre-scale laser interferometry. We first perform ideal-magnetohydrodynamic axisymmetric simulations of a magnetically confined mountain on an accreting neutron star. Two scenarios are considered, in which the mountain sits atop a hard surface or sinks into a soft, fluid base. We quantify the ellipticity of the star, due to a mountain grown on a hard surface, and the reduction in ellipticity due to sinking. The consequences for gravitational waves from low-mass x-ray binaries are discussed. We next present two approaches to reducing the computational cost of searches for periodic gravitational waves. First, we generalise the PowerFlux semi-coherent search method to estimate the amplitudes and polarisation of the periodic gravitational wave signal. The relative efficiencies of the generalised and standard methods are compared using simulated signals. Second, we present an algorithm which minimises the number of templates required for a fully coherent search, by using lattice sphere covering to optimally place templates in the search parameter space. An implementation of the algorithm is tested using Monte Carlo simulations. Finally, we present a coherent search for periodic gravitational waves targeting the central compact object in the supernova remnant Cassiopeia A, using data from the fifth science run of the Laser Interferometer Gravitational-Wave Observatory. The search parameter space is determined by the sensitive frequencies of the detectors, by the age of the compact object, and a range of braking indices. No gravitational wave signal is detected. We set an upper limit on the strength of gravitational waves from the compact object in Cassiopeia A, which surpasses the theoretical limit based on energy conservation. Cassiopeia A is thus one of only a few astronomical objects, to date, where gravitational wave observations are beginning to constrain astrophysics.