Magnetic CVs consist of a dwarf secondary star and a white dwarf primary. The white dwarf accretes material from the secondary through Roche lobe overflow. This occurs in the form of an accretion stream, which in the case of the strongly magnetic AM Her systems, impacts directly onto the surface of the white dwarf. There are a number of emission processes which occur, the most important being X-ray and cyclotron emission. The later type of emission is due to electrons spiralling down the magnetic field lines of the white dwarf.

The fact that cyclotron emission from AM Her systems is strongly polarised (up to 10's of percent) makes polarimetric observations of magnetic CVs an important diagnostic tool. From such observations we can obtain important system parameters, such as the binary system inclination and the offset between the spin and magnetic axes of the white dwarf.

It is worth re-capping some basic properties of polarised light. Polarised light is made up of two components - one linearly polarised, the other circularly polarised. The linear component makes a maximum contribution when the magnetic field is viewed roughly perpendicular to the line of sight, while the circular component peaks when the magnetic field is parallel to the line of sight.

At MSSL we have obtained a number of polarimetry data sets over the years. As an example of data from an AM Her system, we show polarimetry data of RE1844-74 taken using the Anglo Australian Telescope

The left hand panel of the above figure shows the polarisation data through a broad green filter, while the right hand panel shows the red band data. From top to bottom, it shows the intensity data, circular polarisation, linear polarisation and its position angle. Initially workers modelled polarisation curves assuming that the accretion regions on the surface of the white dwarf were small and circular in shape. As higher quality data became available it was found that to model the curves succesfully, the accretion regions were required to be long thin arcs. The solid lines show the best fit to the data. These fits enabled us to obtain the system geometry and also the magnetic field strength at both accretion regions. (See Ramsay et al 1996 for the full paper on RE1844-74).

Up until now it has been a rather laborious task to fit models to polarisation data. The model parameters have had to be modified by `hand'. Recently, however, several members of the CV group (Potter, Hakala & Cropper) have used genetic alogrithims to obtain fits to the polarisation data. These methods are described in our page on one particular application of Genetic Alogrithms.

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Written by Gavin Ramsay:
Last up-date: 23 July 1997