Dynamics of Compact Objects

What Are Compact Objects?

Black holes and neutron stars, both "dead" remnants of massive stars, contain so much mass within a small surface area that they are called compact objects. In this case, Einstein's theory of general relativity is required to properly describe the structure and behavior of these stars.

Black holes are purely general relativistic objects which form if a massive amount of fluid (usually a neutron star) encounters a gravitational instability which causes a catastrophic collapse. During the infall, signals emitted from the star will become more and more redshifted when viewed from a faraway stationary observer as a consequence of the laws of general relativity, until the redshift is so severe that nothing can is visible anymore. Then, a new black hole has formed, and anything surrounding it can only enter, but not leave its surface, which is therefore called the event horizon.

Neutron stars, on the other hand, are less compact than black holes, but still quite extreme objects. More than one solar mass of material is contained within an object of about 25 km diameter, which makes the density at the center several times the density of an atomic nucleus. The star is mostly bound by gravitation but stabilized by the strong force acting between the nucleons.

Magnetars are a special variant of neutron stars which are endowed with particularly strong magnetic fields. They comprise a model for the so-called soft gamma repeaters and anomalous X-ray pulsars, and are particularly interesting since they sometimes produce very violent outbursts of activity.

Dynamics of Compact Objects and Gravitational Waves

Both black holes and neutron stars are dynamical objects and have a spectrum of normal modes, which can be combined to describe oscillations and waves emitted from the object. These waves are nothing else but excitations of the spacetime, traveling through space until they pass by our own planet.

Detection of these gravitational waves would gives us important information about the qualities and internal structure of compact objects, which are often difficult to observe because they are small and comparatively dark. For this reason, a number of gravitational wave detectors have been constructed. So far, however, the only evidence for gravitational radiation is indirect, since the signals are extremely weak when they arrive Earth.

My Activities

The general relativistic magnetohydrodynamics codes Thor and Horizon I have authored are designed to study the dynamics of compact objects, to predict their normal modes of oscillation, and to assess gravitational wave signatures from them.

Currently, as part of the Sonderforschungsbereich/Transregio 7, we are investigating core magnetic field dynamics in magnetars to understand when and how the fields are unstable to perturbations, and how much gravitational radiation is emitted in these events. This research directly couples into gravitational wave observation, but also into fundamental questions of neutron star dynamics and their relation to the internal structure of the stars. Since many of these events occur on long (compared to the fluid flow and light crossing) timescales, large-scale three-dimensional simulations are required to simulate these processes. The application of GPU computing in the Horizon code has opened important new possibilities to study these objects.