A study of quasi-periodic oscillations in magnetic cataclysmic variable stars

Breytenbach, Hannes

A study of quasi-periodic oscillations in magnetic cataclysmic variable stars

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Astronomy

Faculty

Faculty of Science

Abstract

Accreting compact binary systems are unique astrophysical laboratories. The presence of a mass transferring donor star, spilling material into the magnetosphere of a rotating compact companion, creates a fascinating variety of phenomena. In Magnetic Cataclysmic Variables (mCV) the strong magnetic field of a White Dwarf (WD) channels infalling material from a red dwarf donor star, disrupting the formation of an accretion disc, often even being powerful enough to enforce spin-orbit synchronisation. The flow of plasma from the donor star, which is highly supersonic by the time it nears the WD, undergoes a strong hydrodynamic shock before impacting the stellar surface. The shock thermalises the bulk kinetic energy of the flow, producing a hot (mega Kelvin) settling flow that emits primarily in the X-ray through bremsstrahlung, but also in the optical and infrared through cyclotron emission and X-ray reprocessing. Almost all the luminous radiation in these types of systems emanates from magnetically confined plasma in the post-shock accretion column just above the surface of the WD. The variation in observed brightness of this region, driven by the irregularities of the accretion flow, can be rapid and dramatic. Most intriguingly, fast quasiperiodic brightness variations have been observed in a few of these systems. According to our current best understanding, Quasi-Periodic Oscillations (QPO) are generated by a thermal instability in the post-shock accretion region that drives the position of the shock front to oscillate, thereby modulating the luminosity of the Post-Shock Region (PSR). While this model provides a plausible mechanism for generating QPOs, it has a number of shortcomings, most conspicuous the absence of X-ray QPOs. QPOs are therefore a phenomenon still not fully understood, but intriguing in its potential to reveal details about the dynamics of plasma under incredibly extreme conditions. In this thesis, we present the first results from a recent observational campaign at South African Astronomical Observatory (SAAO) to search for- and characterise QPOs in Polar type Cataclysmic Variables. We develop a statistically justified technique for the extraction of light curves from Electron Multiplying CCD (EMCCD) data based on the development of a generative model for our data. We then utilise spectral estimation techniques such as the Fast Fourier Transform (FFT), Lomb Scargle Periodogram (LSP) and Time-Frequency Representationss (TFR), to search the light curves for QPOs in the presence of red noise. We detail the discovery a weak QPO in the recently identified eclipsing Polar, CTCV J1928–5001, the first new QPO detection in mCVs in over two decades. We also present the discovery, observations and modelling of a new eclipsing Polar, MASTER OT J061451.70–272535.5, which presents a double eclipse profile due to obscuration from an accretion stream.

Keywords

astronomy

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