In the framework of the brany models, rotating black holes are described by the Kerr metric with a tidal charge representing the influence of the non-local gravitational (tidal) effects of the bulk space Weyl tensor onto the black hole spacetime. Here we study the influence of the tidal charge onto profiled spectral lines generated by radiating tori orbiting in vicinity of a rotating black hole. We show that with lowering the negative tidal charge of the black hole, the profiled line becomes to be flatter and wider.
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In the framework of the brany models, rotating black holes are described by the Kerr metric with a tidal charge representing the influence of the shear effects of the bulk space Weyl tensor onto the black hole spacetime. Here we study the influence of the tidal charge onto some optical phenomena in rotating black hole spacetimes. The photon motion is given in terms of constants of the geodetical motion related to the spacetime symmetries and escape photon cones are given for special families of locally non-rotating, circular geodetical and radially freely-falling observers. The shadow of a rotating black hole and the shape of an equatorial thin accretion disk are given and classified in terms of the black hole rotational and tidal parameters.
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The "Extended Orbital Resonance Model", i.e., the idea of oscillations induced by the hump of the orbital velocity profile (related to the locally non-rotating frames - LNRF), which are proposed to excite the oscillations of Keplerian discs around near-extreme Kerr black holes with epicyclic frequencies, is used to estimate the mass and spin of three near-extreme Kerr black hole candidates GRS 1915+105, XTE J1650-500, and NGC 5408 X-1. The hump-induced oscillations are characterized by the so-called "humpy frequency", and a non-linear resonant coupling between these and epicyclic oscillations is expected. It it shown that the quasiperiodic variability (QPOs) observed in these sources can be matched with the proposals of the model, giving for the mass and spin of their black holes values consistent with the other observationally-established estimates.
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Humpy radial profiles of the LNRF-related orbital velocity was found for a circular motion of test particles and test perfect fluid orbiting near-extreme Kerr black holes and Kerr naked singularities. Preliminary results of an analogical study of the circular motion of test particles and fluid in the Kerr-(anti-)de Sitter spacetimes are presented.
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Trapping of neutrinos in brany extremely compact stars is studied, using the simplest model with massless neutrinos and uniform-density star. The influence of bulk tension on the trapping is given for two solutions, namely with the Reissner-Nordström-type of geometry described by a single brany parameter, and the second one determined by the energy density of the star and the brane tension.
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High-frequency QPOs in neutron-star binary systems could be explained by models based on parametric or forced resonance between oscillation modes of the accretion disc around the neutron star (with frequencies related to the Keplerian and epicyclic frequencies of the disc) and the relative rotational motion between the disc material and either the neutron star or the binary companion. Using Newtonian theory, we discuss the possibility of forced resonant phenomena being excited by means of gravitational perturbations coming from surface features on the neutron star and from the companion star. For each potential perturbation source, we have determined the Fourier decomposition of the gravitational perturbing forces acting on disc elements in the radial and vertical directions. The analysis presented for the binary companion can be applied also to black hole systems but the surface features would not be present in that case. The oscillations induced by the binary partner are of a different character from those which would be induced by a mountain or by the accretion columns. In the case of symmetric accretion columns, the excitation frequency in the radial direction is twice that in the vertical direction and higher order modes could be relevant for parts of the disc very close to the neutron star. In this inner region, the influence of the accretion columns is greater than that of the binary companion for producing forced oscillations in both the radial and vertical directions; in the intermediate part, the vertical oscillations are induced by the accretion columns while the radial oscillations are excited by the binary partner (if appropriate conditions are fulfilled); sufficiently far from the neutron star, the binary companion has the greater effect. The limits on the magnitude of neutron star deformations given by the possibility to excite observable QPOs coincide with restrictions coming from (non-)observations of gravitational waves by LIGO.
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Rotating black holes in the brany universe of the Randall-Sundrum type with infinite additional dimension are described by the Kerr geometry with a tidal charge b representing the interaction of the brany black hole and the bulk spacetime. We investigate the role of the tidal charge in the orbital resonance model of quasiperiodic oscillations (QPOs) in black hole systems. The orbital Keplerian frequency ν_{K} and the radial and vertical epicyclic frequencies ν_{r}, ν_{θ} of the equatorial, quasicircular geodetical motion are discussed, and the local maxima of their radial profiles related to Keplerian accretion discs are given, assuming the inner edge of the disc located at loci of the innermost stable circular geodesic. The resonant conditions are given for possible direct (parametric) resonances of the oscillations with the radial and vertical epicyclic frequencies and for some trapped oscillations of the warped discs with resonant combinational frequencies involving the Keplerian and radial epicyclic frequencies. It is shown, how the tidal charge could influence matching of the observational data indicating the 3:2 frequency ratio observed in GRS 1915+105 microquasar with prediction of the orbital resonance model. The "magic" dimensionless black hole spin enabling presence of strong resonant phenomena at the radius, where ν_{K}:ν_{θ}:ν_{r} = 3:2:1, is determined in dependence on the tidal charge. Such strong resonances could be relevant even in sources with highly scattered resonant frequencies, as those expected in Sgr A*.
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Using known frequencies of the accretion disc twin peak quasiperiodic oscillations (QPOs) and the known mass of the central black hole, the black hole dimensionless spin a can be determined, assuming a concrete version of the orbital resonance model. However, because of large range of observationally limited values of the black hole mass, its spin can be estimated with a low precision only. Higher precision of the black hole spin measurement is possible in the framework of multi-resonance model of QPOs inspired by complex high-frequency QPO patterns observed in some black hole and neutron star systems. In the simple orbital resonance models we determine the spin and mass dependence of the twin peak frequencies for non-linear resonances of oscillations with the epicyclic and Keplerian frequencies or their combinations in the case of a general rational frequency ratio n : m, n > m. In the multi-resonant model, the twin peak resonances are combined properly to give the observed frequency set. The multi-resonant model is proposed in three distinct versions. In the first one, related probably to the neutron star binary systems, more instances of one resonance occur at more specific radii. In the second case, more resonances are sharing one specific radius, allowing for "cooperative" resonant phenomena in the field of black holes with a specific value of spin. In the third ("ugly") case, more resonances occur at more specific radii; we restrict our attention to the case of two such resonant radii. For special values of the spin, only triple-set of frequencies is observed because of coincidence of some frequencies, allowing determination of the spin from the triple frequency ratio set. The spin is determined precisely, but not uniquely as the same frequency set could be relevant for more than one concrete spin and combination of resonant oscillations.
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We introduce the gravitational potential for the pseudo-Newtonian description of the gravitational field around static and spherically symmetric black holes in the universe with the repulsive cosmological constant, described in terms of the general relativistic approach by the Schwarzschild-de Sitter geometry. In order to demonstrate the accuracy of the pseudo-Newtonian approach and its possible applications, we construct the effective potential for a test particle motion and compare its behaviour with its general relativistic counterpart. Our results indicate that the pseudo-Newtonian potential could be useful in applications of developed Newtonian theories of accretion disks in astrophysically interesting situations in large galactic structures for spacetimes with the cosmological parameter y = Λ M^2 / 3 ≤ 10^{-6}.
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Marginally stable perfect fluid tori with uniform distribution of specific angular momentum are determined in the Reissner-Nordström-de Sitter black-hole and naked-singularity spacetimes. Perfect fluid toroidal configurations are allowed only in the spacetimes admitting existence of stable circular geodesics. The configurations with equipotential surfaces crossing itself in a cusp allow accretion (inner cusp) and/or excretion (outer cusp) of matter from the toroidal configuration. The classification of the Reissner-Nordström-de Sitter spacetimes according to the properties of the marginally stable tori is given.
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