Accretion characteristics in intermediate polars

Parker, Tracey Louise (2006). Accretion characteristics in intermediate polars. PhD thesis The Open University.



This thesis concerns the class of interacting binaries known as intermediate polars (IPs). These are semi-detached magnetic cataclysmic variable systems in which a red dwarf secondary transfers material via Roche lobe overflow onto a white dwarf (WD). The magnetic field of the white dwarf (~ 106 to 107Gauss) plays an important part in determining the type of accretion flow from the secondary. In chapter 1 I discuss binary systems in general, moving on to a more in depth look at Intermediate polars (IPs), their geometry and characteristics, ending with a brief look at all known IPs to date.
In the first part of the thesis I present an analysis of the X-ray lightcurves in 16 IPs in order to examine the possible cause of the orbital modulation. I show that X-ray orbital modulation is widespread amongst IPs, but not ubiquitous. The orbital modulation is most likely due to photoelectric absorption in material at the edge of the accretion disk. Assuming a random distribution of inclination angles, the fact that such a modulation is seen in seven systems out of sixteen studied (plus two eclipsing systems) implies that modulations are visible at inclination angles in excess of 60°. It is also apparent that these modulations can appear and disappear on timescales of ~years or months in an individual system, which may be evidence for precessing, tilted accretion disks.
In the second half of the thesis I use a particle hydro dynamical code known as HyDisc, to investigate the accretion flows in IPs, as a function of parameter space for two dipole models. One where we assume that the density and size scale of the blobs being accreted are constant which we refer to as the n6 model, and the other where the size scale and density of the accreted blobs are not constant refered to as the n3 model. I show that the accretion flow can take the form of an accretion disk, accretion stream, propeller accretion and ring accretion for the n3 model and stream and disk accretion in the n6 model, depending on the magnetic field strength, orbital period and spin period of the system.
violate some of the assumptions of Doppler tomography, such as motion parallel to the orbital plane due to the accretion curtains and that accretion flow is constant throughout the orbital period, making the analysis more complex and the interpretations of observational tomograms flawed as they are based on false assumptions. We have therefore generated simulated tomograms from the simulated accretion flows so we can compare them with real tomograms from observed data and begin to interpret them better. In this way we can discover the nature of the accretion flows in real systems. We show that some of the tomogram features that are produced are in good agreement with those of published observations, but there are also a number of new features which arise corresponding to each of the accretion mechanisms of disks, streams, propellers, weak propellers and ring systems.

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