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    Quasinormal modes, scattering, and Hawking radiation in the vicinity of an Einstein-dilaton-Gauss-Bonnet black hole

    by Zdeněk Stuchlík· Červen 01, 2019· in Fyzika· 0 comments
    Classical (quasinormal) and quantum (Hawking) radiations are investigated for test fields in the background of a four dimensional, spherically symmetric and asymptotically flat black hole in the Einstein-dilaton-Gauss-Bonnet (EdGB) theory. The geometry of the EdGB black hole deviates from the Schwarzschild geometry only slightly. Therefore, here we observe that the quasinormal spectrum also deviates from its Schwarzschild limit at most moderately, allowing for a 9% decrease in the damping rate and up to a 6% decrease in the real oscillation frequency. However, the intensity of Hawking radiation of an electromagnetic and Dirac fields turned out to be much more sensitive characteristic than its quasinormal spectrum, allowing for a 57% and 48% increase of the energy emission rate respectively. The analytical formula for the eikonal regime of quasinormal modes is derived for test fields and it is shown that the correspondence between the eikonal quasinormal modes and null geodesics is indeed fulfilled for test fields, but is not expected for the gravitational one.
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    A causal Schwarzschild-de Sitter interior solution by gravitational decoupling

    by Zdeněk Stuchlík· Červen 01, 2019· in Fyzika· 0 comments
    We employ the minimal geometric deformation approach to gravitational decoupling (MGD-decoupling) in order to build an exact anisotropic version of the Schwarzschild interior solution in a space-time with cosmological constant. Contrary to the well-known Schwarzschild interior, the matter density in the new solution is not uniform and possesses subluminal sound speed. It therefore satisfies all standard physical requirements for a candidate astrophysical object.
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    Determination of chaotic behaviour in time series generated by charged particle motion around magnetized Schwarzschild black holes

    by Zdeněk Stuchlík· Červen 01, 2019· in Fyzika· 0 comments
    We study behaviour of ionized region of a Keplerian disk orbiting a Schwarzschild black hole immersed in an asymptotically uniform magnetic field. In dependence on the magnetic parameter B, and inclination angle θ of the disk plane with respect to the magnetic field direction, the charged particles of the ionized disk can enter three regimes: (1) regular oscillatory motion, (2) destruction due to capture by the magnetized black hole, (3) chaotic regime of the motion. In order to study transition between the regular and chaotic type of the charged particle motion, we generate time series of the solution of equations of motion under various conditions, and study them by non-linear (box counting, correlation dimension, Lyapunov exponent, recurrence analysis, machine learning) methods of chaos determination. We demonstrate that the machine learning method appears to be the most efficient in determining the chaotic region of the θ -r space. We show that the chaotic character of the ionized particle motion increases with the inclination angle. For the inclination angles θ ̃ 0 whole the ionized internal part of the Keplerian disk is captured by the black hole.
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    Anisotropic Tolman VII solution by gravitational decoupling

    by Zdeněk Stuchlík· Červen 01, 2019· in Fyzika· 0 comments
    Using the gravitational decoupling by the minimal geometric deformation approach, we build an anisotropic version of the well-known Tolman VII solution, determining an exact and physically acceptable interior two- fluid solution that can represent behavior of compact objects. Comparison of the effective density and density of the perfect fluid is demonstrated explicitly. We show that the radial and tangential pressure are different in magnitude giving thus the anisotropy of the modified Tolman VII solution. The dependence of the anisotropy on the coupling constant is also shown.
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    Epicyclic Oscillations in the Hartle─Thorne External Geometry

    by Zdeněk Stuchlík· Červen 01, 2019· in Fyzika· 0 comments
    The external Hartle─Thorne geometry, which describes the spacetime outside a slowly rotating compact star, is characterized by the gravitational mass M, angular momentum J, and quadrupole moment Q of the star and gives a convenient description, which, for the rotation frequencies of more than 95% of known pulsars, is sufficiently accurate for most purposes. We focus here on the motion of particles in these spacetimes, presenting a detailed systematic analysis of the frequency properties of radial and vertical epicyclic motion and of orbital motion. Our investigation is motivated by X-ray observations of binary systems containing a rotating neutron star that is accreting matter from its binary companion. In these systems, twin high-frequency quasi- periodic oscillations (QPOs) are sometimes observed with a frequency ratio approaching 3:2 or 5:4, and these may be explained by models involving the orbital and epicyclic frequencies of quasi-circular geodesic motion. In our analysis, we use realistic equations of state for the stellar matter and proceed in a self-consistent way, following the Hartle─Thorne approach in calculating both the corresponding values of Q, M, and J for the stellar model and the properties of the surrounding spacetime. Our results are then applied to a range of geodetical models for QPOs. A key feature of our study is that it implements the recently discovered universal relations among neutron- star parameters so that the results can be directly used for models with different masses M, radii R, and rotational frequencies f rot.
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    Anisotropic ultracompact Schwarzschild star by gravitational decoupling

    by Zdeněk Stuchlík· Květen 01, 2019· in Fyzika· 0 comments
    We employ the minimal geometric deformation approach to gravitational decoupling (MGD-decoupling) in order to generate an exact anisotropic and non-uniform version of the ultracompact Schwarzschild star, or 'gravastar', proposed by Mazur and Mottola. This new system represents an ultracompact configuration of radius $R_{S}=2cal{M}$ whose interior metric can be matched smoothly to a conformally deformed Schwarzschild exterior. Remarkably, the model satisfies some of the basic requirements to describe a stable stellar model, such as a positive density everywhere and decreasing monotonously from the centre, as well as a non-uniform and monotonic pressure.
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    Stable Schwarzschild stars as black-hole mimickers

    by Zdeněk Stuchlík· Květen 01, 2019· in Fyzika· 0 comments
    The Schwarzschild star is an ultracompact object beyond the Buchdahl limit, which has Schwarzschild geometry outside its surface and positive pressure in the external layer which vanishes at the surface. Recently it has been shown that the Schwarzschild star is stable against spherically-symmetric perturbations. Here we study arbitrary axial non- spherical perturbations, and show that the observable quasinormal modes can be as close to the Schwarzschild limit as one wishes, what makes the Schwarzschild star a very good mimicker of a black hole. The decaying time-domain profiles prove that the Schwarzschild star is stable against non-spherical perturbations as well. Another peculiar feature is the absence of echoes at the end of the ringdown. Instead we observe a non- oscillating mode which might belong to the class of algebraically special modes. At asymptotically late times, Schwarzschildian power-law tails dominate in the signal.
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    RADs energetics and constraints on emerging tori collisions around super-massive Kerr black holes

    by Zdeněk Stuchlík· Duben 01, 2019· in Fyzika· 0 comments
    We provide constraints on possible configurations and interactions of two coplanar tori orbiting a central Kerr black hole (BH), in dependence on its dimensionless spin. The two-tori configurations can be directly linked to the current models featuring the obscuration of galactic BH X-ray emission. The emergence of each torus instability phases is discussed and tori collision has been also investigated. The first simple evaluation of the center-of-mass energy proves that collision- energy-efficiency increases with the dimensionless BH spin. We explore the phenomenological aspects of the corotating and counterrotating tori by analyzing properties of the orbiting toroidal configurations related to the fluid enthalpy flux, the mass-flux, the mass-accretion-rates, and the cusp luminosity in the two cases of corotating and counterrotating fluids in dependence on the SMBH spin. The analysis resulted ultimately in a comparative investigation of the properties of corotating versus counterrotating tori, demonstrating that two accretion tori can orbit around the central Kerr attractor only under very specific conditions. Our results also demonstrate that the dynamics of the unstable phases of these double tori systems is significant for the high energy phenomena which could be observable in the X-ray emission and extremely energetic phenomena in active galactic nuclei and quasar.
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    Gravitational lensing around Kehagias-Sfetsos compact objects surrounded by plasma

    by Zdeněk Stuchlík· Duben 01, 2019· in Fyzika· 0 comments
    We study the optical properties of the Kehagias-Sfetsos (KS) compact objects, characterized by the "Hov{r}ava" parameter $omega_{_{KS}}$, in the presence of plasma, considering its homogeneous or power-law density distribution. The strong effects of both "Hov{r}ava" parameter $omega_{_{KS}}$ and plasma on the shadow cast by the KS compact objects are demonstrated. Using the weak field approximation, we investigate the gravitational lensing effect. Strong dependence of the deflection angle of the light on both the "Hov{r}ava" and plasma parameter is explicitly shown. The magnification of image source due to the weak gravitational lensing is given for both the homogeneous and inhomogeneous plasma.
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    Relaxations of perturbations of spacetimes in general relativity coupled to nonlinear electrodynamics

    by Zdeněk Stuchlík· Březen 01, 2019· in Fyzika· 0 comments
    Three well-known exact regular solutions of general relativity coupled to nonlinear electrodynamics (NED), namely the Maxwellian, Bardeen, and Hayward regular spacetimes, which can describe either a regular black hole or a geometry without horizons, have been considered. Relaxation times for the scalar, electromagnetic (EM) and gravitational perturbations of black holes and no-horizon spacetimes have been estimated in comparison with the ones of the Schwarzschild and Reissner- Nordström spacetimes. It has been shown that the considered geometries in general relativity coupled to the NED have never-vanishing circular photon orbits, and on account of this fact, these spacetimes always oscillate the EM perturbations with quasinormal frequencies. Moreover, we have shown that the EM perturbations in the eikonal regime can be a powerful tool to confirm (i) that the light rays do not follow null geodesics in the NED by the relaxation rates and (ii) if the underlying solution has a correct weak field limit to the Maxwell electrodynamics by the angular velocity of the circular photon orbit.
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