152 research outputs found
Influence of the relict cosmological constant on accretion discs
Surprisingly, the relict cosmological constant has a crucial influence on
properties of accretion discs orbiting black holes in quasars and active
galactic nuclei. We show it by considering basic properties of both the
geometrically thin and thick accretion discs in the Kerr-de Sitter black-hole
(naked-singularity) spacetimes. Both thin and thick discs must have an outer
edge allowing outflow of matter into the outer space, located nearby the so
called static radius, where the gravitational attraction of a black hole is
balanced by the cosmological repulsion. Jets produced by thick discs can be
significantly collimated after crossing the static radius. Extension of discs
in quasars is comparable with extension of the associated galaxies, indicating
a possibility that the relict cosmological constant puts an upper limit on
extension of galaxies.Comment: 15 pages, 4 figures, invited pape
Forces in Kerr spacetimes with a repulsive cosmological constant
Forces defined in the framework of optical reference geometry are introduced
in the case of stationary and axially symmetric Kerr black-hole and
naked-singularity spacetimes with a repulsive cosmological constant. Properties
of the forces acting on test particles moving along circular orbits in the
equatorial plane are discussed, whereas it is shown where the gravitational
force vanishes and changes its orientation and where the centrifugal force
vanishes and changes its orientation independently of the velocity of test
particles related to the optical geometry; the Coriolis force does not vanish
for the velocity being non-zero. The spacetimes are classified according to the
number of circular orbits where the gravitational and centrifugal forces
vanish
Current-carrying string loops in black-hole spacetimes with a repulsive cosmological constant
Current-carrying string loop dynamics in Schwarzschild-de Sitter spacetimes
characterized by the cosmological parameter {\lambda}=1/3{\Lambda}M^2 is
investigated. With attention concentrated to the axisymmetric motion of string
loops it is shown that the resulting motion is governed by the presence of an
outer tension barrier and an inner angular momentum barrier that are influenced
by the black hole gravitational field given by the mass M and the cosmic
repulsion given by the cosmological constant {\Lambda}. The gravitational
attraction could cause capturing of the string having low energy by the black
hole or trapping in its vicinity; with high enough energy, the string can
escape (scatter) to infinity. The role of the cosmic repulsion becomes
important in vicinity of the so-called static radius where the gravitational
attraction is balanced by the cosmic repulsion-it is demonstrated both in terms
of the effective potential of the string motion and the basin boundary method
reflecting its chaotic character, that a potential barrier exists along the
static radius behind which no trapped oscillations may exist. The trapped
states of the string loops, governed by the interplay of the gravitating mass M
and the cosmic repulsion, are allowed only in Schwarzschild-de Sitter
spacetimes with the cosmological parameter {\lambda}<{\lambda}_trap 0.00497.
The trapped oscillations can extend close to the radius of photon circular
orbit, down to r_mt 3.3M.Comment: 21 pages, 22 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
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
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
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
Photon capture cones and embedding diagrams of the Ernst spacetime
The differences between the character of the Schwarzschild and Ernst
spacetimes are illustrated by comparing the photon capture cones, and the
embedding diagrams of the sections of the equatorial planes
of both the ordinary and optical reference geometry of these spacetimes. The
non-flat asymptotic character of the Ernst spacetime reflects itself in two
manifest facts: the escape photon cones correspond to purely outward radial
direction, and the embedding diagrams of both the ordinary and optical geometry
shrink to zero radius asymptotically. Using the properties of the embedding
diagrams, regions of these spacetimes which could have similar character are
estimated, and it is argued that they can exist for the Ernst spacetimes with a
sufficiently low strength of the magnetic field.Comment: 12 pages, 7 figure
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