Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 103-109).

Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references.

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

Millisecond pulsars (MSP) are old neutron stars that have been spun up to high spin frequencies(as fast as 716 Hz) through the accretion of matter from a companion star. The extreme stellar densities in the core of globular clusters creates numerous accreting neutron star systems through exchange interactions; this leads to the formation of MSPs in larger numbers than in the galactic disk. Over the course of this project, we have collected over 17 TB of data on the 3 globular clusters M28, NGC6440 and NGC6441 plus 2 observations on NGC6522 and NGC6624 as part of the recently begun S-band survey using the Green Bank telescope. I have analyzed and conducted acceleration searches on 70% of the data and discovered 7 of the 23 new millisecond pulsars reported in this work. One year of timing observations of the pulsars in M28 and NGC6440 has led to the phase connected solution for 12 of the 15 new pulsars in those two clusters, 7 of which are in binaries. We have measured the rate of advance of periastron for two highly eccentric binaries and assuming this is purely due to general relativity, this leads to total system masses of (1.616 - 0.014)M and (2.2 - 0.8)M for M28C and NGC6440B respectively. The small mass function combined with this information imply that the most likely neutron star mass of NGC6440B is either very large or else there could be significant contribution to the advance of periastron from a nonzero quadrupole moment due to tidal interaction with the companion. Measurements of the period derivatives for many of the pulsars show that they are dominated by the dynamical effect of the gravitational field of the clusters. Finally, we have discovered the potential presence of a Mars-mass planet orbiting the pulsar NGC6440C with a period of 21 days. A dedicated timing campaign will be necessary to confirm the presence of such an object.

Millisecond pulsars (MSP) are old neutron stars that have been spun up to high spin frequencies(as fast as 716 Hz) through the accretion of matter from a companion star. The extreme stellar densities in the core of globular clusters creates numerous accreting neutron star systems through exchange interactions; this leads to the formation of MSPs in larger numbers than in the galactic disk. Over the course of this project, we have collected over 17 TB of data on the 3 globular clusters M28, NGC6440 and NGC6441 plus 2 observations on NGC6522 and NGC6624 as part of the recently begun S-band survey using the Green Bank telescope. I have analyzed and conducted acceleration searches on 70% of the data and discovered 7 of the 23 new millisecond pulsars reported in this work. One year of timing observations of the pulsars in M28 and NGC6440 has led to the phase connected solution for 12 of the 15 new pulsars in those two clusters, 7 of which are in binaries. We have measured the rate of advance of periastron for two highly eccentric binaries and assuming this is purely due to general relativity, this leads to total system masses of (1.616 - 0.014)M and (2.2 - 0.8)M for M28C and NGC6440B respectively. The small mass function combined with this information imply that the most likely neutron star mass of NGC6440B is either very large or else there could be significant contribution to the advance of periastron from a nonzero quadrupole moment due to tidal interaction with the companion. Measurements of the period derivatives for many of the pulsars show that they are dominated by the dynamical effect of the gravitational field of the clusters. Finally, we have discovered the potential presence of a Mars-mass planet orbiting the pulsar NGC6440C with a period of 21 days. A dedicated timing campaign will be necessary to confirm the presence of such an object.

Thesis (M. Phil.)--University of Hong Kong, 1993.

M.Sc. / Just as the observations of oscillations of ordinary stars can be used to determine their composition and structure, the oscillations of neutron stars could potentially be used to determine the nature of the dense nuclear matter from which they are made. The superfluidity of the interiors of neutron stars is normally probed by observations of pulsar glitches. It turns out that the superfluidity affects the oscillations in a neutron star core. In particular, it results in a class of oscillation modes specifically associated with the superfluid core. Although these modes have not been detected from observations, it is hoped by some that gravitational wave data may be used to probe the superfluidity of neutron star cores. In this dissertation, a simple equilibrium model is used in order to calculate the superfluid modes in the context of newtonian gravity. The equilibrium model that is used is the same combination of the Serot equation of state and the Harrison-Wheeler equation of state that was used formerly by Lee and by Lindblom & Mendell. Numerical calculations of the superfluid modes are done for 20 different neutron star models ranging in mass between 0.5 and 2 solar masses. The frequencies of the oscillations for the 0.5 and 1.4 solar masses agree fairly well with Lee's results, which strongly validates the computer code written for numerical calculation in this work. In all the models, the eigenfrequencies of the super uid or s-modes are found among those of the f and p-modes. For the equation of state that is used, it is shown that the dimensionless frequencies of the p-modes increase with an increase in mass of the neutron star while those of the s-modes decrease with an increase in neutron star mass. The plan of the dissertation is as follows. Chapter 1 gives a short introduction to stellar oscillations and mentions the oscillations of neutron stars. Chapters 2 and 3 provide the general theoretical background of stellar structure and stellar oscillations respectively. Chapter 4 is a review of the equations of state of neutron star matter derived previously in the literature. Chapter 5 provides the method of calculation as well as the results. Chapter 6 provides a discussion of the results. Chapter 7 briefly gives a review of a mathematical framework for fluids that could be used in order to calculate the oscillations in a general relativistic context and then briefly describes the effects of rotation and magnetic fields. Appendix B liststhe source code for the programs used to do the calculations and also explains some of the extra numerical procedures used for the computation.

Millisecond pulsars (MSP) are old neutron stars that have been spun up to high spin frequencies(as fast as 716 Hz) through the accretion of matter from a companion star. The extreme stellar densities in the core of globular clusters creates numerous accreting neutron star systems through exchange interactions; this leads to the formation of MSPs in larger numbers than in the galactic disk. Over the course of this project, we have collected over 17 TB of data on the 3 globular clusters M28, NGC6440 and NGC6441 plus 2 observations on NGC6522 and NGC6624 as part of the recently begun S-band survey using the Green Bank telescope. I have analyzed and conducted acceleration searches on 70% of the data and discovered 7 of the 23 new millisecond pulsars reported in this work. One year of timing observations of the pulsars in M28 and NGC6440 has led to the phase connected solution for 12 of the 15 new pulsars in those two clusters, 7 of which are in binaries. We have measured the rate of advance of periastron for two highly eccentric binaries and assuming this is purely due to general relativity, this leads to total system masses of (1.616 - 0.014)M and (2.2 - 0.8)M for M28C and NGC6440B respectively. The small mass function combined with this information imply that the most likely neutron star mass of NGC6440B is either very large or else there could be significant contribution to the advance of periastron from a nonzero quadrupole moment due to tidal interaction with the companion. Measurements of the period derivatives for many of the pulsars show that they are dominated by the dynamical effect of the gravitational field of the clusters. Finally, we have discovered the potential presence of a Mars-mass planet orbiting the pulsar NGC6440C with a period of 21 days. A dedicated timing campaign will be necessary to confirm the presence of such an object. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

We consider the question of the magnetic field configuration of central compact objects (CCOs), specifically if their observed spectra allow uniform surface temperatures and carbon atmospheres. Although it is theoretically plausible that young hot neutron stars will deplete their hydrogen and helium atmospheres through diffusive nuclear burning, we find that there is no strong observational evidence to suggest that any particular CCO has a uniform temperature carbon atmosphere. In fact, they all may have small hot spots, similar to what we have measured on the surface of RX J0822−4300, and what has been observed in the cases of two other CCOs, 1E 1207.4−5209 and PSR J1852+0040. We find it is likely that most CCOs have small magnetic inclination angles. We also study the magnetic field configurations of two particular young neutron stars through general relativistic modeling of the X-ray light curves produced by their thermal surface emission. In particular, we have analyzed over a decade of XMM-Newton observations of the central compact object RX J0822−4300 and also the transient magnetar XTEJ1810−197. We show that the CCO RX J0822−4300 has two heated regions with very dif-ferent sizes and temperatures, and we measure a significant deviation angle from a purelyantipodal geometry. This measurement can inform theoretical models of the strength and geometry of the crustal magnetic fields that conduct heat to toward these hot spots. We measure the size, temperature, angular emission pattern and viewing geometry toward the heated surface regions of the magnetar XTE J1810−197 in the years following its 2003 outburst. We demonstrate that, after the size and the temperature of the heated region shrank from what was measured in the initial outburst, the magnetar eventually entered a steady state with the hot spot luminosity powered by magnetic field decay. We find that the magnitude of the flux from the whole surface of XTE J1810−197, combined with several distance estimates, indicates that the mass of XTE J1810−197 must be significantly larger than the canonical 1.4 solar mass neutron star.