We study electrically charged perfect fluid toroidal structures
encircling a spherically symmetric gravitating object with Schwarzschild
spacetime geometry and endowed with a dipole magnetic field. The work
represents a direct continuation of our previous general-relativistic
studies of electrically charged fluid in the approximation of zero
conductivity, which formed tori around a Reissner-Nordström black hole
or a Schwarzschild black hole equipped with a test electric charge and
immersed in an asymptotically uniform magnetic field. After a general
introduction of the zero-conductivity charged fluid model, we discuss a
variety of possible topologies of the toroidal fluid configurations.
Along with the charged equatorial tori forming interesting coupled
configurations, we demonstrate the existence of the off-equatorial tori,
for which the dipole type of magnetic field seems to be necessary. We
focus on orbiting structures with constant specific angular momentum and
on those in permanent rigid rotation. We stress that the general
analytical treatment developed in our previous works is enriched here by
the integrated form of the pressure equations. To put our work into an
astrophysical context, we identify the central object with an
idealization of a nonrotating magnetic neutron star. Constraining ranges
of its parameters and also parameters of the circling fluid, we discuss
a possible relevance of the studied toroidal structures, presenting
along with their topology also pressure, density, temperature and charge
profiles.
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We study scalar, electromagnetic, axial, and polar gravitational
perturbations of the four-dimensional Reissner-Nordström-like black
holes with a tidal charge in the Randall-Sundrum braneworld in the first
approximation when the tidal perturbations are not taken into account.
The quasinormal modes of these perturbations have been studied in both
normal and eikonal regimes. Calculations have shown that the black holes
on the Randall-Sundrum brane are stable against all kinds of
perturbations. Moreover, we determine the greybody factor, giving
transmission and reflection of the scattered waves through the effective
potentials. It has been shown that the scalar perturbative fields are
the most favorite to reflect the wave as compared to the other fields.
With increasing value of the tidal charge, the ability of the all
perturbative potentials to reflect the waves decreases. Our calculations
in low- and high-frequency regimes have shown that black holes on the
braneworld always have a bigger absorption cross section of massless
scalar waves than the Schwarzschild and standard Reissner-Nordström
black holes.
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Shape and frequency-shift maps of direct and indirect images of
Keplerian discs orbiting in Bardeen and Ayòn-Beato-Garcìa (ABG) black
hole and no-horizon spacetimes are determined. Then profiles of spectral
lines generated in the innermost parts of the Keplerian discs in the
Bardeen and ABG spacetimes are constructed. The frequency-shift maps and
profiled spectral lines are compared to those generated in the field of
Schwarzschild black holes and possible observationally relevant
signatures of the regular black hole and no-horizon spacetimes are
discussed. We demonstrate that differences relative to the Schwarzschild
spacetimes are for the no-horizon spacetimes much more profound in
comparison to the regular black hole spacetimes and increase with
increasing charge parameter of the spacetime. The differences are
greater for small and large inclination angles than for intermediate
ones. For the no-horizon spacetimes, the differences enable us to
distinguish the Bardeen and ABG spacetime, if inclination angle to the
distant observer is known. We also show that contribution of the so-
called ghost images to the profiled lines increases with increasing
charge parameter of the spacetimes.
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We analyze the possibility that several instability points may be
formed, due to the Paczyński mechanism of violation of mechanical
equilibrium, in the orbiting matter around a supermassive Kerr black
hole. We consider a recently proposed model of a ringed accretion disk,
made up by several tori (rings) that can be corotating or counter-
rotating relative to the Kerr attractor due to the history of the
accretion process. Each torus is governed by the general relativistic
hydrodynamic Boyer condition of equilibrium configurations of rotating
perfect fluids. We prove that the number of the instability points is
generally limited and depends on the dimensionless spin of the rotating
attractor.
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We study the motion of charged particles in the field of a rotating
black hole immersed into an external asymptotically uniform magnetic
field, focusing on the epicyclic quasicircular orbits near the
equatorial plane. Separating the circular orbits into four qualitatively
different classes according to the sign of the canonical angular
momentum of the motion and the orientation of the Lorentz force, we
analyze the circular orbits using the so-called force formalism. We find
the analytical solutions for the radial profiles of velocity, specific
angular momentum, and specific energy of the circular orbits in
dependence on the black-hole dimensionless spin and the magnetic field
strength. The innermost stable circular orbits are determined for all
four classes of the circular orbits. The stable circular orbits with an
outward-oriented Lorentz force can extend to radii lower than the radius
of the corresponding photon circular geodesic. We calculate the
frequencies of the harmonic oscillatory motion of the charged particles
in the radial and vertical directions related to the equatorial circular
orbits and study the radial profiles of the radial, ωr;
vertical, ωθ; and orbital,
ωϕ, frequencies, finding significant differences
in comparison to the epicyclic geodesic circular motion. The most
important new phenomenon is the existence of toroidal charged particle
epicyclic motion with
ωr˜ωθ≫ωϕ
that could occur around retrograde circular orbits with an
outward-oriented Lorentz force. We demonstrate that for the rapidly
rotating black holes the role of the "Wald induced charge" can be
relevant.
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Shape and frequency-shift maps of direct and indirect images of
Keplerian discs orbiting in Bardeen and
Ayòn-Beato-Garcìa (ABG) black hole and no-horizon
spacetimes are determined. Then profiles of spectral lines generated in
the innermost parts of the Keplerian discs in the Bardeen and ABG
spacetimes are constructed. The frequency-shift maps and profiled
spectral lines are compared to those generated in the field of
Schwarzschild black holes and possible observationally relevant
signatures of the regular black hole and no-horizon spacetimes are
discussed. We demonstrate that differences relative to the Schwarzschild
spacetimes are for the no-horizon spacetimes much more profound in
comparison to the regular black hole spacetimes and increase with
increasing charge parameter of the spacetime. The differences are
greater for small and large inclination angles than for intermediate
ones. For the no-horizon spacetimes, the differences enable us to
distinguish the Bardeen and ABG spacetime, if inclination angle to the
distant observer is known. We also show that contribution of the
so-called ghost images to the profiled lines increases with increasing
charge parameter of the spacetimes.
Read More
We analyze the possibility that several instability points may be
formed, due to the Paczyński mechanism of violation of mechanical
equilibrium, in the orbiting matter around a supermassive Kerr black
hole. We consider a recently proposed model of a ringed accretion disk,
made up by several tori (rings) that can be corotating or
counter-rotating relative to the Kerr attractor due to the history of
the accretion process. Each torus is governed by the general
relativistic hydrodynamic Boyer condition of equilibrium configurations
of rotating perfect fluids. We prove that the number of the instability
points is generally limited and depends on the dimensionless spin of the
rotating attractor.
Read More
We study the motion of charged particles in the field of a rotating
black hole immersed into an external asymptotically uniform magnetic
field, focusing on the epicyclic quasicircular orbits near the
equatorial plane. Separating the circular orbits into four qualitatively
different classes according to the sign of the canonical angular
momentum of the motion and the orientation of the Lorentz force, we
analyze the circular orbits using the so-called force formalism. We find
the analytical solutions for the radial profiles of velocity, specific
angular momentum, and specific energy of the circular orbits in
dependence on the black-hole dimensionless spin and the magnetic field
strength. The innermost stable circular orbits are determined for all
four classes of the circular orbits. The stable circular orbits with an
outward-oriented Lorentz force can extend to radii lower than the radius
of the corresponding photon circular geodesic. We calculate the
frequencies of the harmonic oscillatory motion of the charged particles
in the radial and vertical directions related to the equatorial circular
orbits and study the radial profiles of the radial, ωr;
vertical, ωθ; and orbital, ωϕ, frequencies,
finding significant differences in comparison to the epicyclic geodesic
circular motion. The most important new phenomenon is the existence of
toroidal charged particle epicyclic motion with
ωr̃ωθ≫ωϕ that could occur around
retrograde circular orbits with an outward-oriented Lorentz force. We
demonstrate that for the rapidly rotating black holes the role of the
"Wald induced charge" can be relevant.
Read More
Frequencies of the three quasi-periodic oscillation (QPO) modes observed
simultaneously in the accreting black hole GRO J1655-40 are compared
with the predictions of models. Models in which all three QPO signals
are produced at the same radius are considered: these include different
versions of relativistic precession, epicyclic resonance, tidal
disruption, and warped disc models. Models that were originally proposed
to interpret only the twin high-frequency QPOs are generalized here to
interpret also the low-frequency QPO in terms of relativistic nodal
precession. Inferred values of the black hole mass and spin from each
QPO model are compared with the mass measured from optical observations
and the spin inferred from X-ray spectroscopy techniques. We find that
along with the relativistic precession model predicting M = (5.3 ± 0.1)
M☉,a = 0.286 ± 0.004, the so-called total precession model (M
= (5.5 ± 0.1) M☉,a = 0.276 ± 0.003), and the resonance
epicyclic model with beat frequency (M = (5.1 ± 0.1) M☉,a =
0.274 ± 0.003) also satisfy the optical mass test.
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In this work the model is constructed to describe the black hole
enclosed in the dust cosmological background in case of zero spatial
curvature. This model is based on our exact solution of the class of LTB
inhomogeneous solutions. We considered the properties of the model and
built the R-T-structure of the resulting space-time. It was shown that
central region includes the Schwarzchild-like black hole. We derived the
equations of motion of the test particle from the point of view of the
observer comoving with cosmological expansion. We found analytical
expressions for observable orbital and radial velocities of the particle
and plotted the surface profile of the total velocity in this case. In
comoving coordinate frame it is impossible to study the questions
concerning the black hole horizon but one can observe the local motion
of the particles influenced by the cosmological expansion.
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