Science topics

X-ray Binary Stars

Neutron Stars

Clusters of Galaxies

X-ray surveys /

X-ray Binary Stars

Cataclysmic Variables (CVs) are compact, short period, interacting binaries with a white dwarf as the primary star. Our research interest is mainly focused on systems that harbour a strongly magnetic white dwarf. Here the field is large enough (1-230 MegaGauss) to greatly modify the mass transfer from the mass donating secondary. Unlike nonmagnetic CVs the infalling material is radially channelled to a compact region, where most of the kinetic energy is released as X-ray and cyclotron emission. We use spectral and temporal information from X-ray missions like XMM-Newton to derive an accurate understanding of the complex accretion zone. In addition, we try to resolve the structure of the mass flow and the heated white dwarf involving indirect imaging techniques such as Doppler tomography, eclipse mapping and light curve modelling.

Isolated Neutron Stars

Isolated Neutron stars are a new class of soft X-ray emitters discovered by ROSAT. They show some unique properties that attract a lot of follow-up studies over the last decade: a very soft, thermal X-ray spectrum with very little interstellar absorption, no obvious sign of magnetospheric emission, and no association with a supernova remnant. Despite the expection to find thousands of those objects in the ROSAT all-sky survey, to date only seven could be uniquely identified, now termed the Magnificent Seven. Our group was involved in the discovery of two of them, RBS1223 and RBS1556, which were identified as bright X-ray source with no optical counterpart down to a magnitude of 25.6 in the ROSAT Bright Survey. The great interest in those sources arose from the fact, that these stars offer the unique opportunity to study a neutron's star surface uncontaminated from any other emission. There is hope, that those stars help to derive constraints on the equation-of-state of ultradense matter.

Clusters of galaxies

Galaxy clusters are the largest gravitationally bound objects known in the Univere that may contain several tens to hundreds of galaxies. Their evolution strongly depends on various cosmoligical parameters and so they are ideal objects to stress cosmological models. As this dependency gets stronger in the higher-redshift regime, we try to to do both, establishing a well-defined sample a low-redshift and extend to objects beyond redshift z=1, which is roughly a distance of 8.5 billion lightyears. The XMM-Newton-archive provides a rich resource to search for these extended objects and performed surveys based on that archive. Low redshift objects may be identified and the redshifts measured from catalogue work (Takey, Schwope, and Lamer 2011, 2013, and 2014), whereas for high-redshift work follow-up observations involving the VLT and the LBT need to be performed.

 

X-ray surveys

With X-ray surveys at different flux levels one builds up statistically complete samples of X-ray emitters, a necessary prerequisite for e.g. evolutionary studies. Surveys also uncover the rare and exotic objects, worth to be followed-up individually. In the ROSAT era, we were involved in or leading surveys for bright sources at high latitudes, in the era of XMM-Newton we take part in the XMM-SSC (Survey Science Centre) which created the most massive X-ray catalogue to date with more than 500000 entries. Many identification programs are based on this catalogue and earlier, pre-mature versions.Surveys with AIP-leadership or involvement are: