101 research outputs found
Are eikonal quasinormal modes linked to the unstable circular null geodesics?
In Phys. Rev. D 79, 064016 (2009) it was claimed that quasinormal modes which
any stationary, spherically symmetric and asymptotically flat black hole emits
in the eikonal regime are determined by the parameters of the circular null
geodesic: the real and imaginary parts of the quasinormal mode are multiples of
the frequency and instability timescale of the circular null geodesics
respectively. We shall consider asymptotically flat black hole in the
Einstein-Lovelock theory, find analytical expressions for gravitational
quasinormal modes in the eikonal regime and analyze the null geodesics.
Comparison of the both phenomena shows that the expected link between the null
geodesics and quasinormal modes is violated in the Einstein-Lovelock theory.
Nevertheless, the correspondence exists for a number of other cases and here we
formulate its actual limits.Comment: 8 pages, revtex, refereed version, accepted for publication in
Physics Letters
Equatorial circular orbits in the Kerr-de Sitter spacetimes
Equatorial motion of test particles in the Kerr-de Sitter spacetimes is
considered. Circular orbits are determined, their properties are discussed for
both the black-hole and naked-singularity spacetimes, and their relevance for
thin accretion discs is established.Comment: 24 pages, 19 figures, REVTeX
Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models
The strong gravitational field of neutron stars in the brany universe could
be described by spherically symmetric solutions with a metric in the exterior
to the brany stars being of the Reissner-Nordstrom type containing a brany
tidal charge representing the tidal effect of the bulk spacetime onto the star
structure. We investigate the role of the tidal charge in orbital models of
high-frequency quasiperiodic oscillations (QPOs) observed in neutron star
binary systems. We focus on the relativistic precession model. We give the
radial profiles of frequencies of the Keplerian (vertical) and radial epicyclic
oscillations. We show how the standard relativistic precession model modified
by the tidal charge fits the observational data, giving estimates of the
allowed values of the tidal charge and the brane tension based on the processes
going in the vicinity of neutron stars. We compare the strong field regime
restrictions with those given in the weak-field limit of solar system
experiments.Comment: 26 pages, 6 figure
Role of electric charge in shaping equilibrium configurations of fluid tori encircling black holes
Astrophysical fluids may acquire non-zero electrical charge because of strong
irradiation or charge separation in a magnetic field. In this case,
electromagnetic and gravitational forces may act together and produce new
equilibrium configurations, which are different from the uncharged ones.
Following our previous studies of charged test particles and uncharged perfect
fluid tori encircling compact objects, we introduce here a simple test model of
a charged perfect fluid torus in strong gravitational and electromagnetic
fields. In contrast to ideal magnetohydrodynamic models, we consider here the
opposite limit of negligible conductivity, where the charges are tied
completely to the moving matter. This is an extreme limiting case which can
provide a useful reference against which to compare subsequent more complicated
astrophysically-motivated calculations. To clearly demonstrate the features of
our model, we construct three-dimensional axisymmetric charged toroidal
configurations around Reissner-Nordstr\"om black holes and compare them with
equivalent configurations of electrically neutral tori.Comment: 14 pages, 7 figure
Optical reference geometry of the Kerr-Newman spacetimes
Properties of the optical reference geometry related to Kerr-Newman
black-hole and naked-singularity spacetimes are illustrated using embedding
diagrams of their equatorial plane. Among all inertial forces defined in the
framework of the optical geometry, just the centrifugal force plays a
fundamental role in connection to the embedding diagrams because it changes
sign at the turning points of the diagrams. The limits of embeddability are
given, and it is established which of the photon circular orbits hosted the by
Kerr-Newman spacetimes appear in the embeddable regions. Some typical embedding
diagrams are constructed, and the Kerr-Newman backgrounds are classified
according to the number of embeddable regions of the optical geometry as well
as the number of their turning points. Embedding diagrams are closely related
to the notion of the radius of gyration which is useful for analyzing fluid
rotating in strong gravitational fields.Comment: 28 pages, 17 figure
Equilibrium conditions of spinning test particles in Kerr-de Sitter spacetimes
Equilibrium conditions and spin dynamics of spinning test particles are
discussed in the stationary and axially symmetric Kerr-de Sitter black-hole or
naked-singularity spacetimes. The general equilibrium conditions are
established, but due to their great complexity, the detailed discussion of the
equilibrium conditions and spin dynamics is presented only in the simple and
most relevant cases of equilibrium positions in the equatorial plane and on the
symmetry axis of the spacetimes. It is shown that due to the combined effect of
the rotation of the source and the cosmic repulsion the equilibrium is spin
dependent in contrast to the spherically symmetric spacetimes. In the
equatorial plane, it is possible at the so-called static radius, where the
gravitational attraction is balanced by the cosmic repulsion, for the spinless
particles as well as for spinning particles with arbitrarily large
azimuthal-oriented spin or at any radius outside the ergosphere with a
specifically given spin orthogonal to the equatorial plane. On the symmetry
axis, the equilibrium is possible at any radius in the stationary region and is
given by an appropriately tuned spin directed along the axis. At the static
radii on the axis the spin of particles in equilibrium must vanish
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