Modelling the radio synchrotron outbursts from the nova-like variable star AE Aquarii

[摘要] This thesis proposes a model for the origin of the observed radio outbursts of the binarystar 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 relativelystrong (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 thesurface of the WD where X-ray and optical emission results. An important aspect for themodel is that most of the transfer flow is expelled from the system by the rapidly rotatingmagnetosphere of the WD. An assumtion of the model is that a part of the transfer ismagnetized with a field strength of up to Bblob = 3000 G. This magnetic field in thetransfer originates on the red dwarf star. The blobs originate at the point where thegravitational and centrifugal forces of the two star combination is in equilibrium andwhere plasma is pushed from the RD to the WD by pressure gradients between the RD'ssurface and the vacuum-like space around the WD. In this process the RD's magneticfield is pinched into the clouds and electrons are accelerated to mildly relativisticenergies (1-15 MeV). These electrons in the the magnetic field then radiate via thesynchrotron emission process in the radio to infra-red frequency range.The radiation loses intensity as the blobs expand due to the weakening field and theelectrons losing energy. However, in the model it is suggested that the electrons are reacceleratedin the propeller ejection process. The electrons are energized by thecompressing action of the magnetophere on the blobs in terms of accelerationmechanisms like shock drift acceleration and magnetic pumping.It is also assumed that the magnetic field is tangled in the blobs in a highly turbulentmedium. The tangled field ensures knots of high magnetic energy density whereacceleration can take place. The field also weakens slower with expansion.The combination of the re-acceleration and the strengthened field means that a blob canstay a radio source for a longer time. This prolonged life time of a radio blob isimportant 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 itsexpansion. This idea is applied to the plasma blobs of AB Aqr that are ejected from thesystem and expand as they drift away.The flux is calculated for a single blob in the radio to IR frequency range. The flux fromblobs at different stages of expansion are integrated in all frequency bands above theplasma frequency of each individual blob. The result is a spectrum that can be comparedto the average observed spectrum.

[发布机构] University of the Free State