Eclipses, bursts and outflows in highly inclined X-ray binaries

Thesis

Accretion, the capture of nearby material by the strong gravitational pull of an object, is ubiquitous throughout the universe and is often observed around newly forming stars, planets or compact objects like neutron stars (NSs) and black holes (BHs). X-ray binaries are transient systems in which a NS or BH heats material accreted from a companion star and thus glows brightly in X-rays. These bright X-rays are easy to observe with existing X-ray telescopes. Therefore, X-ray binaries serve as unique laboratories for fundamental physics, allowing us to test different gravitational regimes, probe the state of the ultra-dense matter in the cores of NSs and determine the maximum possible NS mass.

In this thesis, I pioneer the X-ray TRansit and EClipse model, x-trec to provide a novel and precise way of measuring NS masses, thus informing on the equation of state of ultra-dense matter and the classification of merging compact objects. My model exploits eclipsing X-ray binaries, in which the binary inclination, i, and mass ratio, q relate via the duration of totality, te, thus the degeneracy between i and q is disentangled through accurate mapping of the eclipse profile. I demonstrate the abilities of x-trec by applying it to archival observations of two eclipsing low-mass X-ray binaries, EXO 0748 – 676 and Swift J1858.6 – 0814, precisely determining the i and q for both systems. Furthermore, the novel consideration of the companion star’s outflow by x-trec allows it to diagnose the source of any absorbing material within the systems. For EXO 0748 – 676 and Swift J1858.6 – 0814, I use this modelling technique to diagnose an irradiation-driven ablated outflow in both systems, suggesting they are related to spider pulsars. I thoroughly explore this relation, infer that these two binaries are false widows, and predict that the ablated material in EXO 0748 – 676 leads to Type-I X-ray bursts being observable during the X-ray eclipses.

Authors: Knight, AH

• DOI: 10.5287/ora-wrygvmjgd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: X-ray binaries; neutron stars; false widows; pulsars; ablation
• Subjects: X-ray binaries; Astrophysics; X-ray sources, Galactic--Accretion; Accretion (Astrophysics)