Modelling the radio synchrotron outbursts from the nova-like variable star AE Aquarii
Venter, Louis Albert
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This thesis proposes a model for the origin of the observed radio outbursts of the binary star AB Aquarii. The system consists of a white dwarf (WD) and a red dwarf (RD) orbiting each other with a period of Porb ~ 10 h. The compact white dwarf and its relatively strong (B ~ 106 G) magnetic field rotates about its spin axes with period PWD ~ 33 s. Plasma clouds fall from the RD to the WD and a part of this mass transfer reaches the surface of the WD where X-ray and optical emission results. An important aspect for the model is that most of the transfer flow is expelled from the system by the rapidly rotating magnetosphere of the WD. An assumtion of the model is that a part of the transfer is magnetized with a field strength of up to Bblob = 3000 G. This magnetic field in the transfer originates on the red dwarf star. The blobs originate at the point where the gravitational and centrifugal forces of the two star combination is in equilibrium and where plasma is pushed from the RD to the WD by pressure gradients between the RD's surface and the vacuum-like space around the WD. In this process the RD's magnetic field is pinched into the clouds and electrons are accelerated to mildly relativistic energies (1-15 MeV). These electrons in the the magnetic field then radiate via the synchrotron emission process in the radio to infra-red frequency range. The radiation loses intensity as the blobs expand due to the weakening field and the electrons losing energy. However, in the model it is suggested that the electrons are reaccelerated in the propeller ejection process. The electrons are energized by the compressing action of the magnetophere on the blobs in terms of acceleration mechanisms like shock drift acceleration and magnetic pumping. It is also assumed that the magnetic field is tangled in the blobs in a highly turbulent medium. The tangled field ensures knots of high magnetic energy density where acceleration can take place. The field also weakens slower with expansion. The combination of the re-acceleration and the strengthened field means that a blob can stay a radio source for a longer time. This prolonged life time of a radio blob is important to explain the observed time variation of the radio flux from AE Aquarii. The Van der Laan model describes the time evolution of a synchrotron cloud due to its expansion. This idea is applied to the plasma blobs of AB Aqr that are ejected from the system and expand as they drift away. The flux is calculated for a single blob in the radio to IR frequency range. The flux from blobs at different stages of expansion are integrated in all frequency bands above the plasma frequency of each individual blob. The result is a spectrum that can be compared to the average observed spectrum.