Science Collected Works

Multi-messenger observations of neutron stars provide a rich database from which fundamental properties, such as their ellipticities, the strengths and structures of their magnetic fields, and their spin histories since birth can be inferred. This thesis consists of three projects aimed at measuring these properties using radio, X-ray, and gravitational wave emission. Firstly, we analyse X-ray timing data from the accreting millisecond pulsar XTE J1814�338, and attempt to reproduce the data using a model of a freely precessing pulsar. Precessing pulsars have nonzero ellipticities and hence are sources of continuous gravitational waves. Precession in an accretion-powered pulsar is expected to produce characteristic variations in the pulse properties. Assuming surface intensity maps with one and two hotspots, we compute theoretically the periodic modulation of the mean flux, pulse-phase residuals and fractional amplitudes of the first and second harmonic of the pulse profiles. We then search for these signatures in 37 days of X-ray timing data from XTE J1814�338. We analyse a 12.2-d modulation observed previously and show that it is consistent with a freely precessing neutron star only if the inclination angle is < 0.1�, an a priori unlikely orientation. We conclude that if the observed flux variations are due to precession, the surface intensity map must be over-simplified. Our model allows us to place an upper limit on e of 3.0 x 10-9 independently of the surface intensity map, and we estimate the tilt angle 0 to he roughly between 5� and 10�. On the other hand, if the observed flux variations are not due to precession, the detected X-ray modulation serves as a firm upper limit on any underlying precession signal. We then place an upper limit on the product eros# of < 9.9 x 10-10. The first scenario translates into a maximum gravitational wave strain of ~ 10-27 from XTE J1814�338 (assuming a distance of 8 kpc), and a corresponding signal-to-noise ratio of < 10-3 (for a 120 day integration time) for the advanced Laser Interferometer Gravitational Wave Observatory (LIGO) ground-based detector. Secondly, we design and implement a semi-coherent cross-correlation algorithm which will be used to search for a young neutron star in the supernova remnant SNR 1987A using data from LIGO. We introduce an astrophysical model for the gravitational wave phase which describes a neutron star�s spin down in terms of its magnetic field strength B and ellipticity e, instead of its frequency derivatives. This model allows accurate tracking of the gravitational wave phase from a neutron star spinning down rapidly, an issue which has hindered previous searches for such young objects. We calculate the semi-coherent phase metric and estimate the range of search parameters achievable given our computational resources. We also present results of software verification tests. We verify that when searching over pure noise, the cross-correlation detection statistic is distributed as a zero mean, unit variance Gaussian. In the presence of a signal, the mean and variance of the detection statistic depend on the gravitational wave strain. We also compare results obtained by searching over exact source inclination and polarization angles to those obtained by averaging over these angles. For standard LIGO sensitivity, in the frequency band between approximately 100 Hz and 300 Hz, we will be able to place limits of B > 1013G and e < 10-4. Monte Carlo sensitivity estimates show that the smallest detectable gravitational wave strain at 150 Hz for a search using 109 templates is ~ 6 x 10-25. Finally, we use radio pulsar polarimetry to investigate the magnetic geometry and orientation of pulsars. Polarimetrie studies of pulsar radio emission traditionally concentrate on how the Stokes vector (/, Q, U, V) varies with pulse longitude, with special emphasis on the position angle (PA) swing of the linearly polarized component. The interpretation of the PA swing in terms of the rotating vector model is limited by the assumption of an axisymmetric magnetic field and the degeneracy of the output with respect to the orientation and magnetic geometry of the pulsar; different combinations of the latter two properties can produce similar PA swings. We introduce Stokes phase portraits as a supplementary diagnostic tool with which the orientation and magnetic geometry can be inferred more accurately. The Stokes phase portraits feature unique patterns in the I-Q, I-U, and Q-U planes, whose shapes depend sensitively on the magnetic geometry, inclination angle, beam and polarization patterns, and emission altitude. We construct look-up tables of Stokes phase portraits and PA swings for pure and current-modified dipole fields, filled core and hollow cone beams, and two empirical linear polarization models, including the effects of relativistic aberration. We apply Stokes tomography to observations of 26 radio pulsars, and show that for 60% of the sample, the observed emission is incompatible with a pure magnetic dipole at low emission altitudes. We model in detail two pulsars, PSR J0827+2637 and PSR J0304+1932, using a pure and current-modified dipole respectively. We also apply Stokes tomography to two millisecond radio pulsars, PSR J1939+2134 and PSR J0437�4715. The Stokes phase portraits for PSR J1939+2134 at 0.61 GHz are consistent with a current-modified dipole. However the fit is less accurate for PSR J1939+2134 at 1.414 GHz, and for PSR J0437�4715 at 1.44 GHz, indicating that these objects have a more complicated magnetic field geometry, such as a surface quadrupole, a force-free or vacuum-like field, or a polar magnetic mountain.

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