Phasenaufgelöste Spektrophotometrie des magnetischen kataklysmischen Veränderlichen EX Hya

Abstract

This thesis describes time-resolved observations of the eclipsing intermediate polar (IP) EX Hya at X-ray, UV, optical and near infrared wavelengths. IPs form a subclass of the cataclysmic variables (CVs). The latter are close binary stars with orbital periods of a few hours where a low-mass sun-like star loses mass to a white dwarf. During this process (which is called accretion) the transferred matter is heated to high temperatures, and consequently CVs appear as bright sources of radiation in a wide wavelength range. If the white dwarf has no magnetic field, accretion occurs via a so-called accretion disc onto the equatorial regions of the white dwarf. In IPs, however, the white dwarf has a magnetic field which is strong enough to break up the inner parts of the accretion disc and funnel the infalling matter onto regions close to the magnetic poles of the white dwarf, but which is too weak to synchronize the rotation (spin) of the white dwarf with the orbital motion of the binary system (as in the strongly magnetic polars). Consequently, IPs exhibit intensity modulations with both the orbital and the spin period. Therefore it is in principle possible to study the system geometry and the various accretion-induced radiation processes in the different regions of the system separately by means of time-resolved observations in various energy ranges.EX Hya is one of the most prominent and best studied IPs, shows a couple of remarkable properties and thus is one of the key systems for the understanding of this class of objects. However, in the literature there was so far no complete and consistent model for EX Hya. In this thesis, for the first time a detailed study of the system over the complete energy range from hard X-rays to the near infrared is carried out. From the data analysis a geometrically and energetically consistent model for EX Hya is derived which can explain all major observed properties of the system described in the literature. The model provides important new results concerning the distance of the system, the masses of the two components and the accretion rate. Furthermore, it raises suspicion that the method to determine the masses of white dwarfs in magnetic CVs from the observed hard X-ray radiation suffers from severe systematic uncertainties.