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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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 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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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 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, ωrθϕ, 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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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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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 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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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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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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