Zdenek Stuchlik

The orbital resonance model for twin peak kHz quasi periodic oscillations in microquasars

In all microquasars with double peak high frequency QPOs, the ratio of the frequencies is 3:2, which supports the suggestion that a non-linear resonance between two modes of oscillation in the accretion disk plays a role in exciting the observed modulations of the X-ray flux. We discuss evidence in favor of this interpretation and relate the black hole spin to the frequencies expected for various types of resonances that may occur in nearly Keplerian disks in strong gravity. For those microquasars where the mass of the central X-ray source is known, the black hole spin can be deduced from a comparison of the observed and expected frequencies.

OPTICAL PHENOMENA IN THE FIELD OF BRANEWORLD KERR BLACK HOLES

We study the influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The escape photon cones are determined for special families of locally non-rotating, circular geodetical and radially free falling observers. The silhouette of a rotating black hole, the shape of an equatorial thin accretion disk and time delay effect for direct and indirect images of a radiation hot spot orbiting the black hole are given and classified in terms of the black hole rotational and tidal parameters. It is shown that rising of negatively-valued tidal parameter, with rotational parameter fixed, generally strenghtens the relativistic effects and suppresses the rotation induced asymmetries in the optical phenomena.We study the influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The escape photon cones are determined for special families of locally non-rotating, circular geodetical and radially free falling observers. The silhuette of a rotating black hole, the shape of an equatorial thin accretion disk and time delay effect for direct and indirect images of a radiation hot spot orbiting the black hole are given and classified in terms of the black hole rotational and tidal parameters. It is shown that rising of negatively-valued tidal parameter, with rotational parameter fixed, generally strenghtens the relativistic effects and suppresses the rotation induced asymmetries in the optical phenomena.

Null geodesics in black hole metrics with non-zero cosmological constant

We study the radial motion along null geodesics in the Reissner-Nordström-de Sitter and Kerr-de Sitter space-times. We analyze the properties of the effective potential and we discuss circular orbits. We find that:1)the radius of circular photon orbits in the Reissner-Nordström-de Sitter space-times does not depend on the cosmological constant. We show also how this is related to properties of the optical reference geometry.2)For a specific range of the cosmological constant, photons with high impact parameter may travel radially between the cosmological horizon and the black hole horizon in the equatorial plane of the Kerr-de Sitter space-times.

Radial and vertical epicyclic frequencies of Keplerian motion in the field of Kerr naked singularities. Comparison with the black hole case and possible instability of naked-singularity accretion discs

Relativistic Keplerian orbital frequency (

ON MASS CONSTRAINTS IMPLIED BY THE RELATIVISTIC PRECESSION MODEL OF TWIN-PEAK QUASI-PERIODIC OSCILLATIONS IN CIRCINUS X-1

\nu_{\mathrm{K}}

Off-equatorial orbits in strong gravitational fields near compact objects?II: halo motion around magnetic compact stars and magnetized black holes

) and related epicyclic frequencies (radial

Quadrupole moments of rotating neutron stars and strange stars

\nu_{\mathrm{r}}

Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models

, vertical

Multi-resonance orbital model of high-frequency quasi-periodic oscillations: possible high-precision determination of black hole and neutron star spin

\nu_{\theta}

Confronting the models of 3:2 quasiperiodic oscillations with the rapid spin of the microquasar GRS 1915+105

) play an important role in the physics of accretion discs orbiting Kerr black holes – quasiperiodic oscillations observed in microquasars can be explained by associated resonant or trapping effects. Because of growing theoretical evidence of the possible existence of naked singularities, we discuss the behaviour of the fundamenal orbital frequencies for Keplerian motion in the field of Kerr naked singularities, primarily in order to find phenomena that could observationally distinguish a hypothetical naked singularity from black holes. Some astrophysically important consequences are sketched, namely the existence of strong resonant frequency for all Kerr naked singularities, with radial and vertical epicyclic frequencies equal and given by the relation