Homepage of Matthias Vigelius

Gravitational radiation from accreting millisecond-pulsars

Observed spin frequency distribution of the fastest rotating neutron stars suggests that these objects are sources of gravitational waves: One possible mechanism to create a time-dependent quadrupole moment and therefore gravitational radiation is the formation of polar magnetic mountains. Such mountains are created when accreted material is confined at the poles by the magnetic tension of the stellar field. It is also widely believed that the observed reduction of the magnetic field of millisecond pulsars can be connected to the accretion phase during which the pulsar is spun up. A wide variety of reduction mechanisms have been proposed, including burial of the stellar field by magnetic mountains. Payne and Melatos have developed a self-consistent model of magnetic mountains. The mountains effectively screen the magnetic dipole moment, reducing it by 90% after 10^-4 Msun have been added, and produce an associated reduced mass quadrupole moment of ~5x10^37 g cm^2 which is the correct size to explain the observed spin distribution. Magnetic mountains are therefore sources of gravitational waves. They could be among the first object ever detected. In order to increase the detection sensitivity of continous sources by coherent integration, a detailed knowledge of the waveform is required. We are refining the magnetic mountain model to provide such an in-depth knowledge.

Bow shock simulations from pulsars

Bow shocks are ubiquitious phenomenae. We see them, for example, in the wind of young OB stars, in the solar wind, or in pulsar winds. Pulsars generally move supersonically through the ISM. While doing so, they emit a highly relativistic electron-positron wind. The interaction of the pulsar wind with the ISM results in a multilayer bow shock structure. We performed hydrodynamical simulations of such pulsar winds in order to explain some of the distinctive features found in the bow shock around PSR J2124-3358. We considered different situations commonly encountered, such as anisotropic wind emission, an ISM density gradient or the pulsar running into an extended ridge of material (a "wall"). We found that the peculiar features of PSR J2124-3358 (a kink and a curved X-ray tail) can be explained by the pulsar travelling into a wall and jet-like anisotropic wind emission.

Interaction of jets with galactic wind shells

In addition to the large energy output in the radio band, high z (~2) radio galaxies are usually associated with an extended Ly-alpha halo, often exceeding the size of the radio emission region. However, for radio galaxies smaller than ~50 kpc, Ly-alpha radiation is effectively absorbed, mostly in the blue wing. These absorbers can be associated with the radio emitting galaxy (van Ojik et al. 1997). Krause, 2005b, proposed a model to explain these associated absorbers. In the early stage of galactic evolution, stellar winds and supernova explosions inject gas and energy in the interstellar medium. This galactic wind results in a spherical expanding bow shock. The Lyman-alpha emission is due to gas ionised either by stellar radiation or collisional excitation. If the cooling time scale is comparable with the propagation time scale, the material behind the bow shock will cool and form a dense shell, effectively absorbing Ly-alpha emission at a narrow velocity range. Later (~80 Myr), accretion of the supermassive black hole in the galactic center will feed a jet. This jet will finally destroy the absorbing shell. This part of the project examines, how effective a light jet can destroy the wind shell and how this evolution affects the optical spectrum.

Structure of Gravastars

Gravastars are a new static spherical symmetric solution of the field equations of the General Theory of Relativity which can be considered as an alternative to the classical Black Hole concept. They consist of an inner layer with vacuum energy and a shell of relativistic matter located at the classical Schwarzschild radius. We examined numerically the structure of such configurations and considers several aspects of their stability. It can be shown, that the original solution can be extended to a wide family of similar configurations covering the whole parameter spectrum which is compatible with current theories and observations. However, the stability of these objects needs to be subject of further research, since the usual procedures cannot be applied without modifications.

Visualization of three-dimensional data

Visualizing the results is an important part of the scientific work, especially when it comes to publications and presentations. Basically, my data consists of scalar fields (such as plasma density) and vector fields (such as magnetic flux) given in three-dimensional polar spherical coordinates. I use IDL

for the analysis part, including the computation of isosurfaces, the integration of field lines etc. The data can then be viewed live thanks to IDL's object graphics capability. In order to produce pretty pictures I export the graphics objects -- surface mesh and polylines -- and postprocess them with the open-source renderer POVRAY.