There is an explanation of the rotation curves in the periphery of spiral galaxies based on MOdified Newtonian Dynamics (MOND). Considering the motion of Magellanic Clouds in the gravitational field of Milky Way, we compare predictions of the CDM halo model with the cosmic repulsion term included to those obtained in the framework of the MOND theory. Our results demonstrate that the predictions of the CDM halo and MOND models differ very substantially, especially in the case of the Large Magellanic Cloud motion.
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General relativity combined with a non-linear electrodynamics enables to find regular black hole solutions. The best known solution of this kind is described by the Bardeen spacetime with spacetime parameters giving gravitational mass m and magnetic charge g. For ratio g/m large enough, the Bardeen spacetime describes a no-horizon regular solution. Here we demonstrate properties of the Bardeen spacetimes by the embedding diagrams of the equatorial plane of the ordinary geometry, and the optical geometry enabling reflection of properties of test particle motion.
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We consider equatorial motion of test particles around a rotating Kerr naked singularity in the Randall-Sundrum braneworld scenario and its implications for the properties of Keplerian accretion disks. We demonstrate existence of some unexpected phenomena related to properties of spacetimes having positive braneworld tidal charges. This new phenomenon can be an interesting explanation for extremely high energy cosmic radiation.
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We study properties of the magnetized toroidal structures orbiting the Kerr superspinars predicted by the string theory. We demonstrate specific features of the unmagnetized perfect fluid tori created in the deep potential well near the surface of the superspinars, enabling clear distinction between Kerr superspinars and black holes. Then we consider the effect of the magnetization of the perfect fluid tori and shift of their properties induced by the presence of the magnetic field.
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We have previously applied several models of high-frequency quasi-periodic oscillations (HF QPOs) to estimate the spin of the central Kerr black hole in the three Galactic microquasars, GRS 1915+105, GRO J1655-40, and XTE J1550-564. Here we explore the alternative possibility that the central compact body is a super-spinning object (or a naked singularity) with the external space-time described by Kerr geometry with a dimensionless spin parameter a ≡ cJ/GM2> 1. We calculate the relevant spin intervals for a subset of HF QPO models considered in the previous study. Our analysis indicates that for all but one of the considered models there exists at least one interval of a> 1 that is compatible with constraints given by the ranges of the central compact object mass independently estimated for the three sources. For most of the models, the inferred values of a are several times higher than the extreme Kerr black hole value a = 1. These values may be too high since the spin of superspinars is often assumed to rapidly decrease due to accretion when a ≫ 1. In this context, we conclude that only the epicyclic and the Keplerian resonance model provides estimates that are compatible with the expectation of just a small deviation from a = 1.
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We study the acceleration of an electric current-carrying and axially
symmetric string loop initially oscillating in the vicinity of a
Schwarzschild black hole embedded in an external asymptotically uniform
magnetic field. The plane of the string loop is orthogonal to the
magnetic field lines and the acceleration of the string loop occurs due
to the transmutation effect turning in the deep gravitational field the
internal energy of the oscillating strings to the energy of their
translational motion along the axis given by the symmetry of the black
hole spacetime and the magnetic field. We restrict our attention to the
motion of string loop with energy high enough, when it can overcome the
gravitational attraction and escape to infinity. We demonstrate that for
the current-carrying string loop the transmutation effect is enhanced by
the contribution of the interaction between the electric current of the
string loop and the external magnetic field and we give conditions that
have to be fulfilled for an efficient acceleration. The Schwarzschild
black hole combined with the strong external magnetic field can
accelerate the current-carrying string loop up to the velocities close
to the speed of light v˜c. Therefore, the string loop
transmutation effect can potentially well serve as an explanation for
acceleration of highly relativistic jets observed in microquasars and
active galactic nuclei.
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We demonstrate possible optical signatures of the Kehagias-Sfetsos (KS)
naked singularity spacetimes representing a spherically symmetric vacuum
solution of the modified Hořava gravity. In such spacetimes,
accretion structures significantly different from those present in
standard black hole spacetimes occur due to the
‘antigravity’ effect, which causes 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 the 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 cases where the spectral continuum and spectral lines are
profiled by the strong gravity of the spacetimes due to the vanishing
region of the angular velocity gradient influencing the effectiveness of
the viscosity mechanism. We can conclude that optical signatures of KS
naked singularities can be well distinguished from the signatures of
standard black holes.
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We study the acceleration of an electric current-carrying and axially symmetric string loop initially oscillating in the vicinity of a Schwarzschild black hole embedded in an external asymptotically uniform magnetic field. The plane of the string loop is orthogonal to the magnetic field lines and the acceleration of the string loop occurs due to the transmutation effect turning in the deep gravitational field the internal energy of the oscillating strings to the energy of their translational motion along the axis given by the symmetry of the black hole spacetime and the magnetic field. We restrict our attention to the motion of string loop with energy high enough, when it can overcome the gravitational attraction and escape to infinity. We demonstrate that for the current-carrying string loop the transmutation effect is enhanced by the contribution of the interaction between the electric current of the string loop and the external magnetic field and we give conditions that have to be fulfilled for an efficient acceleration. The Schwarzschild black hole combined with the strong external magnetic field can accelerate the current-carrying string loop up to the velocities close to the speed of light v∼c. Therefore, the string loop transmutation effect can potentially well serve as an explanation for acceleration of highly relativistic jets observed in microquasars and active galactic nuclei.
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We demonstrate possible optical signatures of the Kehagias-Sfetsos (KS) naked singularity spacetimes representing a spherically symmetric vacuum solution of the modified Hořava gravity. In such spacetimes, accretion structures significantly different from those present in standard black hole spacetimes occur due to the 'antigravity' effect, which causes 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 the 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 cases where the spectral continuum and spectral lines are profiled by the strong gravity of the spacetimes due to the vanishing region of the angular velocity gradient influencing the effectiveness of the viscosity mechanism. We can conclude that optical signatures of KS naked singularities can be well distinguished from the signatures of standard black holes.
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We study charged-fluid toroidal structures surrounding a nonrotating
charged black hole immersed in a large-scale, asymptotically uniform
magnetic field. In continuation of our former study on electrically
charged matter in approximation of zero conductivity, we demonstrate the
existence of orbiting structures in permanent rigid rotation in the
equatorial plane and charged clouds hovering near the symmetry axis. We
constrain the range of parameters that allow stable configurations and
derive the geometrical shape of equipressure surfaces. Our simplified
analytical study suggests that these regions of stability may be
relevant for trapping electrically charged particles and dust grains in
some areas of the black hole magnetosphere and thus important in some
astrophysical situations.
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