Indirect imaging of AM Herculis-stars

Abstract

AM Herculis stars (or 'polars') are close binaries consisting of a white dwarf with a strong magnetic field and a main sequence star (the 'secondary') which is losing mass to the white dwarf. The magnetic field channels the matter to the magnetic poles, where the released potential energy is radiated away - mainly in X-rays and the UV. The spatial structure of these objects cannot be observed directly, because their apparent size is too small. Indirect imaging methods are capable of restoring information which is only contained in observational data as projections onto a sub-space. These tomographic methods are known from medicine, but can also be applied to astronomical data, when a number of measurements can be regarded as views of the same object under different aspect angles. In this work tomographic methods are applied to time-resolved spectroscopic data of polars. Spectral lines seen in the radiation emitted by the constituents of these systems are notably Doppler-shifted. The Doppler-shift is depending on the viewing angle of the system ('inclination') and is varying with orbital phase. Time-series of one-dimensional data (spectra) can be used to create a two-dimensional map of the intensity distribution in velocity-space (Doppler-tomography), or a map of the line-flux on the assumed surface of the mass-losing star (Roche-tomography) from the line profile changes ...