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    Dark energy influencing polytropic spheres modelling dark matter halos

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    We discuss in the framework of general relativity the role of the dark energy represented by the cosmological constant, restricted due to cosmological tests, in the polytropic models of dark matter halos. The internal spacetime of the polytropic spheres governs circular geodesic orbits that can be compared with the velocity curves observed in large galaxies, indicating the possibility to use for the halo model both non- relativistic very extended and diluted polytropes, or relativistic polytropes with nearly critical value of the relativistic parameter sigma = p_mathrm{c}/varrho_mathrm{c} enabling extremely large polytrope extension, limited efficiently by the influence of the dark energy to agree with extension of dark matter halos of large galaxies. We also show that the so-called trapping relativistic polytropes with extremely large extension allow for gravitational instability of their central parts leading to the creation of a supermassive black hole inside of such an extremely extended polytrope representing galactic halo.
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    Photon motion in the framework of general relativity coupled to non-linear electrodynamics

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    We discuss the photon motion in the framework of general relativity coupled to non-linear electrodynamics. Photons no longer follow the null-geodesics of the spacetime but rather null-geodesics of associated effective metric. Here we compare structure of circular geodesics and time-delays of neutrinos in Bardeen spacetimes with those of photons in effective geometry. We also discuss construction the Keplerian disks images in the Bardeen spacetimes and compare them with the images of Keplerian disks in RN spacetime.
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    Off-equatorial accretion discs – imitation by circling charged fluids

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    Charged fluids circling in strong central gravitational and ambinet magnetic fields, characteristic for compact objects backgrounds, can embody interesting configurations. In contrast to the widely considered neutral fluid structures imitating thick equatorial accretion discs with negligible loss of mass, when the fluid is properly charged, we can find it forming unique toroidal structures `levitating' above the equatorial plane and also those hovering near the symmetry axis. Along with analytical topological studies of these structures, we can also present an survey of their basic physical characteristics, such as pressure, density and temperature profiles.
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    Radiation Reaction of Charged Particles Orbiting a Magnetized Schwarzschild Black Hole

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    In many astrophysically relevant situations, radiation-reaction forces acting upon a charge cannot be ignored, and the question of the location and stability of circular orbits in such a regime arises. The motion of a point charge with radiation reaction in flat spacetime is described by the Lorenz-Dirac (LD) equation, while in curved spacetime it is described by the DeWitt-Brehme (DWB) equation containing the Ricci term and a tail term. We show that for the motion of elementary particles in vacuum metrics, the DWB equation can be reduced to the covariant form of the LD equation, which we use here. Generically, the LD equation is plagued by runaway solutions, so we discuss computational ways of avoiding this problem when constructing numerical solutions. We also use the first iteration of the covariant LD equation, which is the covariant Landau-Lifshitz equation, comparing the results of these two approaches and showing the smallness of the third-order Schott term in the ultrarelativistic case. We calculate the corresponding energy and angular momentum loss of a particle and study the damping of charged particle oscillations around an equilibrium radius. We find that, depending on the orientation of the Lorentz force, the oscillating charged particle either spirals down to the black hole or stabilizes the circular orbit by decaying its oscillations. The latter case leads to the interesting new result of the particle orbit shifting outwards from the black hole. We also discuss the astrophysical relevance of the presented approach and provide estimates of the main parameters of the model.
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    Orbital widening due to radiation reaction around a magnetized black hole

    by Zdeněk Stuchlík· Červen 01, 2018· in Fyzika· 0 comments
    Radiation reaction acting on a charged particle moving at a stable circular orbit of a magnetized black hole (BH) can lead to the shift of the orbital radius outward from the BH. The effect causes an increase of the energy and angular momentum of the particle measured by an observer at rest at infinity. In this paper, we show that "widening" of such orbits is independent of the field configuration, but it appears only in the cases with the external Lorentz force acting outward from the BH. This condition corresponds to qLB > 0, where q and L are the charge and angular momentum of the particle, and B is intensity of the external magnetic field. As examples of the orbital widening, we consider two scenarios with an external homogeneous magnetic field and a magnetic dipole field generated by a current loop around a Schwarzschild BH. We show that the orbital widening is accompanied by quasi-harmonic oscillations of the particle, which are considerably large in the magnetic dipole fields. We also estimate the timescales of orbital widening, from which it follows that the effect can be relevant in the vicinity of stellar-mass BHs.
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    Magnification effect of Kerr metric by configurations of collisionless particles in non-isotropic kinetic equilibria

    by Zdeněk Stuchlík· Květen 01, 2018· in Fyzika· 0 comments
    A test fluid composed of relativistic collisionless neutral particles in the background of Kerr metric is expected to generate non-isotropic equilibrium configurations in which the corresponding stress-energy tensor exhibits pressure and temperature anisotropies. This arises as a consequence of the constraints placed on single-particle dynamics by Killing tensor symmetries, leading to a peculiar non-Maxwellian functional form of the kinetic distribution function describing the continuum system. Based on this outcome, in this paper the generation of Kerr-like metric by collisionless N -body systems of neutral matter orbiting in the field of a rotating black hole is reported. The result is obtained in the framework of covariant kinetic theory by solving the Einstein equations in terms of an analytical perturbative treatment whereby the gravitational field is decomposed as a prescribed background metric tensor described by the Kerr solution plus a self-field correction. The latter one is generated by the uncharged fluid at equilibrium and satisfies the linearized Einstein equations having the non-isotropic stress-energy tensor as source term. It is shown that the resulting self-metric is again of Kerr type, providing a mechanism of magnification of the background metric tensor and its qualitative features.
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    Proto-jet configurations in RADs orbiting a Kerr SMBH: symmetries and limiting surfaces

    by Zdeněk Stuchlík· Květen 01, 2018· in Fyzika· 0 comments
    Ringed accretion disks (RADs) are agglomerations of perfect-fluid tori orbiting around a single central attractor that could arise during complex matter inflows in active galactic nuclei. We focus our analysis to axi-symmetric accretion tori orbiting in the equatorial plane of a supermassive Kerr black hole; equilibrium configurations, possible instabilities, and evolutionary sequences of RADs were discussed in our previous works. In the present work we discuss special instabilities related to open equipotential surfaces governing the material funnels emerging at various regions of the RADs, being located between two or more individual toroidal configurations of the agglomerate. These open structures could be associated to proto-jets. Boundary limiting surfaces are highlighted, connecting the emergency of the jet-like instabilities with the black hole dimensionless spin. These instabilities are observationally significant for active galactic nuclei, being related to outflows of matter in jets emerging from more than one torus of RADs orbiting around supermassive black holes.
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    Electromagnetic perturbations of black holes in general relativity coupled to nonlinear electrodynamics

    by Zdeněk Stuchlík· Duben 01, 2018· in Fyzika· 0 comments
    The electromagnetic (EM) perturbations of the black hole solutions in general relativity coupled to nonlinear electrodynamics (NED) are studied for both electrically and magnetically charged black holes, assuming that the EM perturbations do not alter the spacetime geometry. It is shown that the effective potentials of the electrically and magnetically charged black holes related to test perturbative NED EM fields are related to the effective metric governing the photon motion, contrary to the effective potential of the linear electrodynamic (Maxwell) field that is related to the spacetime metric. Consequently, corresponding quasinormal (QN) frequencies differ as well. As a special case, we study new family of the NED black hole solutions which tend in the weak field limit to the Maxwell field, giving the Reissner-Nordström (RN) black hole solution. We compare the NED Maxwellian black hole QN spectra with the RN black hole QN spectra.
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    Axisymmetric black holes allowing for separation of variables in the Klein-Gordon and Hamilton-Jacobi equations

    by Zdeněk Stuchlík· Duben 01, 2018· in Fyzika· 0 comments
    We determine the class of axisymmetric and asymptotically flat black- hole spacetimes for which the test Klein-Gordon and Hamilton-Jacobi equations allow for the separation of variables. The known Kerr, Kerr- Newman, Kerr-Sen and some other black-hole metrics in various theories of gravity are within the class of spacetimes described here. It is shown that although the black-hole metric in the Einstein-dilaton-Gauss- Bonnet theory does not allow for the separation of variables (at least in the considered coordinates), for a number of applications it can be effectively approximated by a metric within the above class. This gives us some hope that the class of spacetimes described here may be not only generic for the known solutions allowing for the separation of variables, but also a good approximation for a broader class of metrics, which does not admit such separation. Finally, the generic form of the axisymmetric metric is expanded in the radial direction in terms of the continued fractions and the connection with other black-hole parametrizations is discussed.
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    Tori sequences as remnants of multiple accreting periods of Kerr SMBHs

    by Zdeněk Stuchlík· Březen 01, 2018· in Fyzika· 0 comments
    Super-massive black holes (SMBHs) hosted in active galactic nuclei (AGNs) can be characterized by multi-accreting periods as the attractors interact with the environment during their life-time. These multi- accretion episodes should leave traces in the matter orbiting the attractor. Counterrotating and even misaligned structures orbiting around the SMBHs would be consequences of these episodes. Our task in this work is to consider situations where such accretions occur and to trace their remnants represented by several toroidal accreting fluids, corotating or counterrotating relative to the central Kerr attractor, and created in various regimes during the evolution of matter configurations around SMBHs. We focus particularly on the emergence of matter instabilities, i.e., tori collisions, accretion onto the central Kerr black hole, or creation of jet-like structures (proto-jets). Each orbiting configuration is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluid. We prove that sequences of configurations and hot points, where an instability occurs, characterize the Kerr SMBHs, depending mainly on their spin-mass ratios. The occurrence of tori accretion or collision are strongly constrained by the fluid rotation with respect to the central black hole and the relative rotation with respect to each other. Our investigation provides characteristic of attractors where traces of multi-accreting episodes can be found and observed.
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