CATACLYSMIC VARIABLES

Using the Multipurpose Fotometer mounted on the JKT, researchers observed several eclipses of the eclipsing cataclysmic variable UX Uma in four wavebands and reconstructed the accretion disk structure using a maximum-entropy technique. The reconstruction shows that the bulk of the optical radiation comes from a more or less symmetric structure centered on the white dwarf, but that there is also a smaller spot, where the gas stream from the cool star hits the accretion disc.
AM Her systems are an important class of cataclysmic variables in which the secondary star loses material to a strongly magnetic white dwarf primary. The magnetic field on the primary is enormously strong, some tens of megagauss, so that the stream of material does not form a disk, but is captured by the field, and accretes on to the white dwarf in one or more shock regions at the magnetic poles. The energy of the system is released partly as X-rays in the shocks, and partly as optical cyclotron radiation from free electrons in the stream spiralling in the magnetic field. This radiation is emitted at low-order harmonics of the fundamental cyclotron frequency, broadened into humps by Doppler motion of the electrons in the emission region. Observing these humps in the spectra of AM Her systems is very difficult, but some reseachers using the WHT Faint Object Spectrograph were able to detect humps in a total of five systems. From the observations it is possible to determine the magnetic field strengths and temperatures of the emitting regions. The fields lie between 25 and 44 megagauss and the estimated temperatures lie between 5.0 and 23 keV. The data are now subject to detailed modelling.