School of Physics - Theses
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ItemContinuous and stochastic gravitational wave emission from neutron star interior flows and oscillations( 2014)This thesis investigates continuous and stochastic gravitational wave signals from neutron star interior flows and oscillations. Neutron stars provide a unique laboratory to test physics at extremes of density, gravity, and magnetism. Gravitational waves directly probe the neutron star interior, carrying information about the properties of bulk nuclear matter. Glitches are rotational irregularities occasionally observed in pulsars. We calculate analytically the nonaxisymmetric Ekman spin-up flow following a glitch and its associated gravitational wave signal in the context of an idealised model. A large glitch with $\delta\Omega/\Omega = 10^{-4}$ in a pulsar rotating at $\sim 100$ Hz may be detectable by second- and third-generation interferometers. The signal depends on the inclination angle of the pulsar and the interior viscosity, compressibility, and stratification, which can be inferred gravitational wave data. Superfluid turbulence in neutron stars, driven by crust-core differential rotation, emits stochastic gravitational radiation. We calculate the stochastic background for a Universal neutron star population and two subpopulations: radio-loud pulsars and accreting millisecond pulsars. Non-detection of the stochastic background by LIGO implies an upper limit on the relaxation parameter $\tau_d = \Delta\Omega / \dot{\Omega}$, where $\dot{\Omega}$ is the spin-down rate, of $\tau_d \lesssim 10^5$ yr for radio-loud pulsars and $\tau_d \lesssim 10^7$ yr for accreting millisecond pulsars. Turbulent convection in main-sequence stars also emits gravitational radiation. We calculate the gravitational wave strain power spectral density for an individual star and a Universal stellar population. Due to its proximity, the signal from the Sun dominates the integrated background, but both fall well below the detection threshold of proposed space-based interferometers. Inside the gravitational wave near zone, the signal scales more steeply with distance ($\propto d^{-5}$) and is amplified relative to the far-zone signal ($\propto d^{-1}$). We calculate Rømer and Doppler timing residuals for a pulsar orbiting in the near zone of a high-mass main-sequence star and compare with observed of timing noise in three high-mass systems. The largest predicted root-mean-squared residuals, $\Delta T_{rms} = 2.8$ μs for PSR J0045-7319 at periastron, are a factor $\sim 10^3$ smaller than those observed. We propose a new gravitational wave detection statistic based on a modified form of higher criticism, a statistical method designed to indirectly detect a collection of sources too weak to be detected individually. Using higher criticism to reanalyse \mathcal{C}-statistic values for a simulated search of a low mass X-ray binary, we find higher criticism is sensitive to wave strain $\sim 6\%$ lower than the \mathcal{C}-statistic threshold. Higher criticism makes fewer assumptions about the source frequency and is more robust to error caused by accretion-driven phase wandering or an incorrect orbital period. Finally, we present preliminary results from two projects studying magnetar bursts. We propose a simple model of non-linear crust cracking and shear wave propagation to investigate the transient behaviour observed in the quasiperiodic oscillations detected in magnetar giant flares. The resulting frequency-time spectrograms contain features like frequency drifting, mode splitting and rotational phase dependence of oscillation frequencies. We also extend an existing smooth-particle-magnetohydrodynamics code to build a neutron star model. We validate the code against $f$-mode oscillation frequencies, observe rotational splitting, and present early progress towards implementing a rigid crust. Future applications include simulating crust-core magnetar oscillations as well as long-term spin down and post-glitch circulation.
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ItemNeutrino oscillations and the early universe( 2000)We construct a model which provides maximal mixing between a pseudo-Dirac Vµ/VT pair, based on a local U(1)Lµ-LT symmetry. Its strengths, weaknesses and phenomenological consequences are examined. A new intermediate range force is predicted, mediated by the light gauge boson of U(1)Lµ-LT. Through the mixing of µ, T and e, this force couples to electrons and thus may be searched for in precision “gravity” experiments.The generation of relic neutrino asymmetries in the early universe via the mechanism of partially coherent active-sterile neutrino oscillations is considered. We study how an approximate evolution equation for the growth of the asymmetry can be extracted from the exact Quantum Kinetic Equations which describe the evolution of the neutrino ensemble, and examine the nature of some of the approximations employed.