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    Shadow sizes around black holes as possible signatures of Horndeski gravity

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    The classical general relativity (GR) is remarkably confirmed in different astronomical tests. Recently, gravitational waves have been detected from binary black holes and binary neutron starsystems. Moreover, constraints on alternative theories of gravity have been obtained, so thatgraviton mass bounds have been found such as m_g < 7.7 × 10^{-23} eV. It means that theories of massive gravity,if they are realized in nature, have to be very close to GR.In spite of a great progress of GR now there exist a number of alternative theories of gravity,such as f(R) theories, theories of massive gravity, scalar- tensor theories etc.Scalar-tensor theories proposed by Hordeski are among popular alternatives for classical general relativity (GR).There are no-hair theorems in such a theory, however, there exist hairy black hole solutions under some assumptions.for instantce, Babichev et al. (2017) constructed a set of hairy static black hole solutions (for quarticHordeski Lagrangian of scalar-tensor theory of gravity). We show that analytical expressions for shadow sizeobtained for Reissner - Nordström metric (with electric and tidal charges) may be usedto compare theoretical predictions and observations for asymptotically flat black holes with a scalar hair which mimics an electric charge.We present analytical expressions for shadow size for static black hole solutions with de-Sitter and anti-de-Sitter asymptotics.The relations could be used for comparison of the theoretical models with observational data obtained with the Event Horizon Telescope for the black hole at the Galactic Center or in galaxy M87.
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    Shadows around supermassive black holes as a tool to test GR and alternative theories of gravity

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    We obtained analytical expression for shadows around supermassive black holes for a wide class of spherically symmetric black hole solutions, such a Reissner - Nordstrom with a tidal charge, dyonic Reissner - Nordstrom - (anti) de-Sitter, black hole in Horndeski gravity. Therefore, observations of such shadows could be indicator of an alternative theory of gravity. We also found properties of shadows around Kerr black holes. which could be observed in the future with the Event Horizon Telescope and projected space-ground interferometer Millimetron.
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    The distinguishing signature of magnetic Penrose process

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    In this Letter, we wish to point out that the distinguishing feature of magnetic Penrose process (MPP) is its super high-efficiency exceeding 100 per cent (which was established in mid 1980s for discrete particle accretion) of extraction of rotational energy of a rotating black hole electromagnetically for a magnetic field of milli Gauss order. Another similar process, which is also driven by the electromagnetic field, is Blandford-Znajek mechanism (BZ) that could be envisaged as high magnetic field limit MPP as it requires threshold magnetic field of order 104 G. Recent simulation studies of fully relativistic magnetohydrodynamic (MHD) flows have borne out super high-efficiency signature of the process for high magnetic field regime; viz BZ. We would like to make a clear prediction that similar simulation studies of MHD flows for low magnetic field regime, where BZ would be inoperative, would also have superefficiency.
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    Comment on „Construction of regular black holes in general relativity“

    by Zdeněk Stuchlík· Červenec 01, 2018· in Fyzika· 0 comments
    We claim that the physical parameters of the constructed black hole solutions in general relativity (GR) coupled to nonlinear electrodynamics (NED) by Zhong-Ying Fan and Xiaobao Wang in the Paper [Phys. Rev. D 94, 124027 (2016), 10.1103/PhysRevD.94.124027] are misinterpreted, despite the formalism being correct. We argue that because of these misinterpretations, the derived black hole solutions and the Lagrangian densities presented in that paper are slightly inconsistent. In this comment, we present complete black hole solutions of the given Lagrangian densities which correct the interpretation of the physical parameters of the constructed black hole solutions and lead to the correct treatment and weak field limits of the physical parameters of the constructed solutions.
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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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    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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    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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    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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    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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    Light escape cones in local reference frames of Kerr-de Sitter black hole spacetimes and related black hole shadows

    by Zdeněk Stuchlík· Březen 01, 2018· in Fyzika· 0 comments
    We construct the light escape cones of isotropic spot sources of radiation residing in special classes of reference frames in the Kerr-de Sitter (KdS) black hole spacetimes, namely in the fundamental class of `non-geodesic' locally non-rotating reference frames (LNRFs), and two classes of `geodesic' frames, the radial geodesic frames (RGFs), both falling and escaping, and the frames related to the circular geodesic orbits (CGFs). We compare the cones constructed in a given position for the LNRFs, RGFs, and CGFs. We have shown that the photons locally counter-rotating relative to LNRFs with positive impact parameter and negative covariant energy are confined to the ergosphere region. Finally, we demonstrate that the light escaping cones govern the shadows of black holes located in front of a radiating screen, as seen by the observers in the considered frames. For shadows related to distant static observers the LNRFs are relevant.
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