On the structure of accretion flows in compact accreting objects: a variability study across scales

Abstract

Accretion is the most efficient process in the Universe that we know of generating energy from matter. It is behind some of the most luminous phenomena ever observed and is present across all scale sizes, from supermassive black holes at the centres of galaxies through stellar mass black holes to degenerate dead cores of stars, such as white dwarfs and neutron stars. Accretion is fundamental for understanding how elements are redistributed across the Universe and what happens to stars after they run out of nuclear fuel.
This thesis focuses mostly on white dwarfs accreting material via Roche-Lobe overflow from a donor in a binary system and low-mass X-ray binaries, where the accretor is a neutron star. The goal of this thesis is to demonstrate the similarities in geometrical structure of accretion flow around these systems, especially in the context of broad-band aperiodic and non-coherent quasi-periodic variability.
Firstly, this thesis presents the first detection of an outer edge of the thin accretion disc in an ultra-compact accreting double white dwarf system SDSS J190+3940. I use this discovery to showcase the similarity to a disc dominated broad-band variability of X-ray binaries by adapting an X-ray binary model to model the accretion flows in SDSS J190+3940, showing consistent results with the detected disc edge region.
This thesis also reports on the discovery of a new type of quasi-periodic oscillation in accreting white dwarf systems, which are analogous to well known quasi-periodic oscillations in X-ray binaries. Here I propose to explain this behaviour as due to precession of an inner accretion flow lifted from the accretion disc plane by magnetic warping torque. This model originally proposed to explain similar phenomena in neutron star binaries also represents another link between accretion across scales. I further apply this model to a rare class of accreting neutron stars, so called pulsating ultra-luminous X-ray sources. Currently only three systems show these oscillations whilst being confirmed neutron stars. Therefore, measuring their intrinsic magnetic is still an open question. In this instance the model applied in this thesis also allows to constrain the lower limit of the magnetic field of the neutron stars to ~G.
Furthermore some transient phenomenology of accreting white dwarfs, referred to as micronovae are discussed. Micronova bursts serve as a link to thermonuclear bursts in neutron star X-ray binaries. Here two new systems displaying these bursts are shown. One of them, an intermediate polar DW Cancri, is also found to be undergoing transient behaviour of pole-flipping, which begins shortly prior to the burst.
In summary, this thesis pertains to varied observational phenomena in accreting systems. The overarching theme focuses on the links between different accreting systems, showcasing the scale-invariant nature of accretion physics as a tool in studying accretion flow geometry.