In this Proceedings, the talks presented during workshops RAGtime 4 (14-16 October 2002, Opava, Czech Republic) and RAGtime 5 (13-15 October 2003, Opava, Czech Republic) are collected.
Read More
In all four microquasars which show twin peak kHz QPOs, the ratio of the two frequencies is 3:2. This rather strongly supports the suggestion by Abramowicz, M. A. and Kluzniak [W. Abramowicz, M. A. and Kluzniak, W., (2001). A precise determination of black hole spin in GRO J1655-40. Astronomy and Astrophysics, 374:L19] that twin peak kHz QPOs are due to a resonance between some modes of accretion disk oscillations. Detailed studies of this suggestion revealed that several such non-linear resonances are present in nearly Keplerian disks in strong gravity. Here, we fit to observations predictions of the resonance hypothesis for two particular types of non-linear resonances between vertical and radial epicyclic frequencies. For three microquasars with known masses, the fits give an accurate estimate of the spin.
Read More
Solutions of general relativistic field equations for static, spherically symmetric, equilibrium perfect-fluid configurations obeying the polytropic and adiabatic equation of state in the presence of a repulsive cosmological constant are discussed. The influence of the cosmological constant on the total mass of the configurations, their radius and the profiles of energy density, rest energy density, pressure and metric coefficients is studied and compared for the polytropic and adiabatic case. The static equilibrium configurations are allowed for σ<σ_{crit} (α<α_{crit}), where the critical values σ_{crit} (α_{crit}) of the relativity parameter σ (α) ≡ pcent/rhocent c^{2} of the polytropes (adiabates) depend on the cosmological constant and the polytropic index n of the equation of state and can be determined by a numerical procedure. The numerical results show that for sufficiently small values of the relativity parameter σ=α≪ σ_{crit}, the polytropic spheres are more compact than the adiabatic ones. Increase of the cosmological constant causes increase of both the radius and mass of the spheres and makes the profiles of the metric coefficients flatter. For large values of the relativity paramater, σ=α≲ σ_{crit}, the situation is more complex and depends also on the value of the polytropic parameter n. The mass of the adiabatic spheres can exceed the mass of the polytropes for n≳ 2. In the case of n=3, the adiabatic spheres can even be more compact than the polytropic ones. Generally, the role of the cosmological constant is supressed with both σ=α and n growing.
Read More
Definition of the inertial forces in the framework of the optical reference geometry is applied to the stationary and axially symmmetric Kerr-de Sitter spacetimes. The attention is restricted to the inertial forces acting on particles moving along circular orbits in the equatorial plane of these spacetimes. It is shown, where the gravitational force vanishes, and where the centrifugal force vanishes independently of velocities of test particles. The Coriolis force does not vanish for a non-zero velocity.
Read More
We consider appearance of isotropically radiating sources located at a sphere at the static radius of the Schwarzschild-de Sitter spacetimes to static observers in vicinity of the black hole horizon and the cosmological horizon and to radially moving observers. We expect these observers to follow geodesics starting from the static radius. It is shown that the observed flux diverges at both the horizons for both classes of observers. Nevertheless the frequency shift remains finite at the horizons for the radially moving observers.
Read More
The role of the observed relict vacuum energy on the fluctuations of CMBR going through cosmological matter condensations is studied in the framework of the Einstein-Strauss-de Sitter vakuola model. It is shown that refraction of light at the matching surface of the vakuola and the expanding Friedman universe can be very important during the accelerated expansion of the universe when the velocity of the matching surface relative to static Schwarzschildian observers becomes relativistic.
Read More
We consider basic properties of both the geometrically thin and thick accretion disks in the Kerr-de Sitter black-hole and naked-singularity spacetimes. The properties are determined by character of the equatorial circular geodesics of these spacetimes and by the equilibrium configurations of a perfect fluid rotating around their symmetry axis. Transformation of a Kerr-de Sitter naked singularity into an extreme black hole due to accretion in the thin disks is briefly discussed for both the plus-family and minus-family disks. It is shown that such a conversion leads to an abrupt instability of the innermost parts of the plus-family accretion disks that can have strong observational consequences.
Read More
Aschenbach [Aschenbach, B. (2004). Measuring mass and angular momentum of black holes with high-frequency quasi-periodic oscillations. Astronomy and Astrophysics, 425:1075-1082] has shown that in Kerr black-hole spacetimes with rotation parameter a>0.9953, the Keplerian orbital velocity measured in locally non-rotating frames (LNRF) has a positive radial gradient in a small region in the vicinity of the event horizon and proposed that excitation of oscillations in Keplerian thin discs can be related to this fact. Similarly, we show that in the equatorial plane of marginally stable thick discs (with uniformly distributed specific angular momentum ℓ(r,θ)=const) the orbital velocity relative to the LNRF has a positive radial gradient in the vicinity of black holes with a>0.99979. The change of sign of the velocity gradient occurs just above the centre of the thick toroidal discs, in the region where stable circular geodesics of the Kerr spacetime are allowed. Therefore, the same mechanism as in the Keplerian discs could trigger oscillations in thick discs, but the rotational parameter of the Kerr spacetime must be much closer to the extreme-hole state with a=1. The global character of the phenomenon is given in terms of topology changes of the von Zeipel surfaces (equivalent to equivelocity surfaces in the tori with ℓ (r,θ)=const). Toroidal von Zeipel surfaces exist around the circle corresponding to the minimum of the equatorial LNRF velocity profile, indicating a possibility of development of some vertical instabilities in those parts of marginally stable tori with positive gradient of the LNRF velocity.
Read More
In this Proceedings, the talks presented during workshops RAGtime 4
(14-16 October 2002, Opava, Czech Republic) and RAGtime 5 (13-15 October
2003, Opava, Czech Republic) are collected.
Read More
