A study of compact objects: stellar evolution models and X-ray binaries

Thesis

Compact and double compact binary systems are excellent natural laboratories for the study of a variety of physical phenomena. In this Thesis I study black hole binary systems and neutron star low-mass X-ray binaries in order to investigate binary stellar evolution pathways and accretion mechanisms.

Estimating the binary black hole merger rate and comparing it with predictions from gravitational-wave observatories is an important method to constrain binary stellar evolution channels. I investigate the chemically homogeneous evolution of close binary systems consisting of rapidly rotating, massive stars at low metallicities, by combining, for the first time, realistic binary models with detailed cosmological calculations of the chemical and star-formation history of the Universe. By constraining the population properties and determining the cosmological and detection rates of binary black hole mergers more precisely than before, I find that the chemically homogeneous evolution pathway can be an important source of aLIGO events.

Probing accretion processes in neutron star low-mass X-ray binaries offers unique tests of general relativity and helps to constrain the elusive equation of state of neutron stars. In the next part of the Thesis, I explore one of the most promising models to explain the accreting quasi-periodic oscillations observable in the power density spectra of neutron stars and black holes in binary systems: the relativistic precession model. I analyse the RXTE data of the neutron star X-ray binary 4U1608-52, presenting the first case study where more than one usable set of the necessary three quasi-periodic oscillations (a so-called triplet) are found for one single source, and for which multiple tests of the relativistic precession model can be carried out. I find that the mass and spin values resulting from such triplets cluster around physically realistic values, which can be considered reliable estimates of the neutron star fundamental parameters. If confirmed, my results indicate that 4U1608-52 would be one of the heaviest neutron stars known to date.

Finally, I analyse the RXTE data of the 11 Hz accreting pulsar IGR J17480-2446. Previous studies failed to find in this source the very low-frequency quasi-periodic oscillation associated with frame-dragging in the relativistic precession model, thus casting doubt on the validity of the model itself. In this study I find previously undetected very low-frequency quasi-periodic oscillations and show that these are consistent with being produced via frame-dragging, thus re-establishing the validity of the relativistic precession model.

Authors: du Buisson, L

• DOI: 10.5287/ora-mqd5arpbw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Accretion (Astrophysics); Gravitational waves--Detection; Black holes (Astronomy); X-ray binaries; Stars--Evolution