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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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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We study oscillations of an electric current-carrying and axially-symmetric string loop in the vicinity of a Schwarzschild black hole embedded in an asymptotically uniform magnetic field. The radial profiles of frequencies of small oscillations of the string loop around stable equilibrium points are given for the radial and vertical harmonic modes that are relevant also in the quasi-periodic stages of the oscillations. Their properties in dependence on the uniform magnetic field intensity and angular momentum parameters of the string loops are determined. We examine the relevance of resonant phenomena of the radial and vertical string-loop oscillations at their frequency ratio 3:2. The oscillatory frequencies of the string loops are compared with the frequencies of high-frequency quasi-periodic oscillations (HF QPOs) observed in the microquasars GRS 1915+105, XTE 1550-564, GRO 1655-40 containing a black hole. We have demonstrated that the influence of the uniform magnetic field does not allow us to explain all the observed data for non-rotating black holes. Clearly, rotation of the black hole is necessary to explain the all the observed frequencies in the microquasars by the string loop oscillations.
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We study the non-monotonic Keplerian velocity profiles related to locally nonrotating frames (LNRF) in the field of near-extreme braneworld Kerr black holes and naked singularities in which the non-local gravitational effects of the bulk are represented by a braneworld tidal charge b and the 4D geometry of the spacetime structure is governed by the Kerr-Newman geometry. We show that positive tidal charge has a tendency to restrict the values of the black hole dimensionless spin a admitting existence of the non-monotonic Keplerian LNRF-velocity profiles; the non-monotonic profiles exist in the black hole spacetimes with tidal charge smaller than b = 0.41005 (and spin larger than a = 0.76808). With decreasing value of the tidal charge (which need not be only positive), both the region of spin allowing the non-monotonicity in the LNRF-velocity profile around braneworld Kerr black hole and the velocity difference in the minimum-maximum parts of the velocity profile increase implying growing astrophysical relevance of this phenomenon.
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The current-carrying string loops oscillating around a stable equilibrium position in the Kerr background are considered to explain the twin high-frequency quasiperiodic oscillations (HF QPOs) observed in the low-mass X-ray binary 4U 1636-53 containing a neutron star. The frequencies of the radial and vertical string loop oscillations are governed by the mass and spin parameters of the neutron star, and by the string parameter describing combined effects of its tension and angular momentum. The frequencies of the radial and vertical modes of the string loop oscillations can cover the large scatter of the twin HF QPO data observed in the 4U 1636-53 source, but the estimates of the mass M and spin a of the neutron star are rather high, M ∼2.65 M and a ∼ 0.45, while related to the theory of the neutron star structure. Therefore, the string loop oscillation model in the case of the 4U 1636-53 source requires a correction based on an electrically charged string loop interacting with the magnetic field of the neutron star.
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The imprints of the tidal-charge parameter b are determined for the spectral line profiles generated by radiation from the surface of optically thick perfect-fluid tori orbiting a spherically symmetric braneworld black holes. We assume that each point on the surface radiates isotropically at a fixed spectral line frequency. We give the direct and indirect image of a torus and the spectral line profile in dependence on the impact parameter for a large inclination angle to the distant observer when the relativistic effects are strongest. We give the map of the frequency shift across the surface and dependence of the maximal and minimal frequency shift of the radiation from the tori surface, giving thus a relevant information on the tidal charge parameter b.
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We present Newtonian model of non-conductive charged perfect fluid tori orbiting in combined spherical gravitational and dipolar magnetic fields, focusing on stationary, axisymmetric toroidal structures. Matter in such tori exhibits a purely circulatory motion and the resulting convection carries charges into permanent rotation around the symmetry axis. As a main result we demonstrate possible existence of off-equatorial charged tori and equatorial tori with cusps enabling outflows of matter from the torus also in Newtonian regime. From astrophysical point of view, our investigation can provide an insight into processes determining vertical structure of dusty tori surrounding accretion discs.
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We are investigating singularity structure of the rotating black hole and naked singularity spacetimes in the Randall-Sundrum second type (RSII) of the brane-world scenario. We will show that structure of this singularity is very similar to its classical counterpart, even in the cases of negative tidal charge, which is equivalent to the Kerr-Newman black hole with the complex charge Q (with zero real part). We also study behaviour of the ergosphere and will show that this region can exists under specific situation.
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Axisymmetric string loops oscillating around a stable equilibrium position in the Kerr background are applied to explain the special set of frequencies related to the high-frequency quasiperiodic oscillations observed in the low mass X-ray binary XTE J1701-462 containing a neutron star. Frequencies of the radial and vertical string loop oscillations are determined by the mass M and dimensionless spin a of the neutron star, and by dimensionless parameter ω describing combined effects of the string loop tension and its angular momentum. Equilibrium position of the string loop is given by its angular momentum and energy. The string-loop oscillation model can explain the observed kHz frequencies, but the stringy parameter ω cannot be the same for all the three HF QPO observations in the XTE J1701-462 source; the limits on the acceptable values of ω are given in dependence on the spacetime parameters M and a. However, the model implies restrict ion M > 3.3M on the neutron star mass that is too high to be compatible with the standard theory of neutron stars. A proper correction on the mass-limit can be generally introduced due to the electromagnetic interaction of an electrically charged string loop with magnetic field of the neutron star.
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Twin-peak quasiperiodic oscillations (QPOs) are observed in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. In our work we consider several QPO models and focus especially on the atoll source 4U 1636-53 with its large set of QPO measurements. We find that the considered models require the QPO excitation radii in 4U 1636-53 to be close to the inner-most stable circular orbit of the accretion disc. We explore and summarize mass-angular-momentum relations and limits on NS compactness implied by individual QPO models. We confront these relations with NS parameters given by various NS equations of state (EoS). The application of concrete EoS removes the degeneracy in the mass and angular momentum determined from the QPO models when the spin frequency is known. Moreover, the applied NS EoS are compatible only with some of the considered QPO models. In our work we compare simplified calculations that assume Kerr background geometry to the detailed calculations considering NS oblateness influence in Hartle-Thorne spacetimes.
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