The X-ray reflection features of irradiated accretion disks around black holes enable us to probe the effects of strong gravity and learn the black-hole properties. We investigate the reflection signs, i.e. the iron K-line and the Comptonized hump, which arise by reprocessing of radiation on the surface of an accretion disc, and how they are affected by the spin of a rotating black hole.
We develop models for the polarization signature of the radiation coming from the vicinity of accreting black holes. In the optical/UV range, polarimetry observations and modeling have already proven to be a very useful tool to investigate active galaxies by tracing geometrical and dynamical properties of structures surrounding their nuclei.
Accreting black holes in binary systems often exhibit quasi-periodic oscillations (QPOs) of the observed X-rays. Sometime the frequency of these oscillations is very high (kilohertz) and they occur at two distinct peaks. QPO properties differ between sources, however, it appears that they keep a fixed frequency ratio of small rational numbers. The origin of this phenomenon is currently unknown.
The short-term variability of active galactic nuclei is often being linked with a presence of hot spots residing on the surface of an accretion disc. We apply the theory of random point processes to model the observed signal from an ensemble of randomly generated spots and to reproduce typical features that are found by fourier-analysing X-ray lightcurves from galactic centres.
Do extremely rotating black holes power relativistic jets? A compelling answer may be beyond our reach yet for some time. For sure, magnetic fields play an important role in astrophysics. Near rotating compact objects, neutron stars and black holes, the field lines are wildly deformed by rapidly moving plasma and strong gravitational fields.
Dense star clusters surround nuclei of galaxies, including the centre of our own Milky Way. Studying the rapid motion of stars within the central arcsecond and their interactions with surrounding environment provides an essential tool to determine the mass of the central supermassive black hole in the Galaxy.
One of the promising sources of gravitational waves are extreme mass ratio inspirals. They are believed to routinely occur in the centers of galaxies, where comparably lighter compact objects inspiral into supermassive black holes due to gravitational radiation reaction. Gravitational-wave signals from these systems are expected to be detected by the space-based observatory LISA.
