684 research outputs found
Basic properties of toroidal structures in Kerr--de Sitter backgrounds
Perfect fluid tori with uniform distribution of the specific angular momentum
orbiting the Kerr-de Sitter black holes or naked singularities are studied.
Closed equipotential surfaces corresponding to stationary toroidal discs are
allowed only in the spacetimes admitting stable circular geodesics. The last
closed surface crosses itself in the cusp(s) enabling outflow(s) of matter from
the torus due to the violation of hydrostatic equilibrium. The repulsive
cosmological constant, , implies the existence of the outer cusp
(with a stabilizing effect on the tori because of "excretion", i.e., outflow of
matter from the torus into the outer space) and the strong collimation of open
equipotential surfaces along the rotational axis. Both the effects take place
nearby the so-called static radius where the gravitational attraction is just
balanced by the cosmic repulsion. The plus-family discs (which are always
corotating in the black-hole backgrounds but can be counterrotating, even with
negative energy of the fluid elements, in some naked singularity backgrounds)
are thicker and more extended than the minus-family ones (which are always
counterrotating in all backgrounds). If the parameters of naked-singularity
spacetimes are very close to the parameters of extreme black-hole spacetimes,
the family of possible disc-like configurations includes members with two
isolated discs where the inner one is always a counterrotating accretion disc.
Mass estimates for tori with nonrelativistic adiabatic equation of state give
limits on their central mass-density, for which the approximation of test fluid
is adequate.Comment: Updated version of the Section talk at Albert Einstein Century
International Conference at Palais de l'Unesco, Paris, France, 18-23 July,
2005; to appear in the Proceedings; AIP style files included; 8 page
Optical effects related to Keplerian discs orbiting Kehagias-Sfetsos naked singularities
We demonstrate possible optical signatures of the Kehagias-Sfetsos naked
singularity spacetimes representing spherically symmetric vacuum solution of
the modified Ho\v{r}ava gravity. In such spacetimes, accretion structures
significantly different from those present in the standard black hole
spacetimes occur due to the "antigravity" effect causing existence of an
internal static sphere surrounded by Keplerian discs. We focus our attention on
the optical effects related to the Keplerian accretion discs, constructing the
optical appearance of the Keplerian discs, the spectral continuum due to their
thermal radiation, and spectral profiled lines generated in the innermost parts
of such discs. The KS naked singularity signature is strongly encoded in the
characteristics of predicted optical effects, especially in the case of the
spectral continuum and spectral lines profiled by the strong gravity of the
spacetimes, due to the region of the vanishing of the angular velocity gradient
influencing the effectivity of the viscosity mechanism. We can conclude that
optical signatures of the Kehagias-Sfetsos naked singularities can be well
distinguished from the signatures of the standard black holes
Photon motion in Kerr-de Sitter spacetimes
We study general non-equatorial motion of photons in the Kerr-de Sitter black
hole and naked singularity spacetimes. The motion is governed by the impact
parameter related to axial symmetry of the spacetime, and impact parameter
related to its hidden symmetry. Appropriate 'effective potentials'
governing the latitudinal and radial motion are introduced and their behaviour
is examined by 'Chinese boxes' technique giving regions allowed for the motion
in terms of the impact parameters. Limits on the impact parameter and
are established in dependence on the spacetime parameters . The
motion can be of orbital type (crossing the equatorial plane, ) and
vortical type (tied above or bellow the equatorial plane, ). It is shown
that for negative values of the reality conditions imposed on the
latitudinal motion yield stronger constraints on the parameter than that
following from the reality condition of motion in the radial direction, which,
e. g., exclude possibility of existence of vortical motion of constant radius.
Several other consequences are determined and a classification of Kerr-de
Sitter spacetimes reflecting the behaviour of the effective potentials is
given.Comment: 33 pages,84 figure
Silhouette and spectral line profiles in the special modification of the Kerr black hole geometry generated by quintessential fields
We study optical effects in quintessential Kerr black hole spacetimes
corresponding to the limiting case of the equation-of-state parameter
of the quintessence. In dependence on the dimensionless
quintessential field parameter , we determine the black hole silhouette and
the spectral line profiles of Keplerian disks generated in this special
quintessential Kerr geometry, representing an extension of the general
modifications of the Kerr geometry introduced recently by Ghasemi-Nodehi and
Bambi \cite{Gha-Bam:2016:EPJC:}. We demonstrate that due to the influence of
the parameter , the silhouette is almost homogeneously enlarged, and the
spectral line profiles are redshifted with almost conserved shape
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