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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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    Mass of the active galactic nucleus black hole XMMUJ134736.6+173403

    by Zdeněk Stuchlík· Únor 01, 2019· in Fyzika· 0 comments
    A recent study of the X-ray source XMMUJ134736.6+173403 has revealed a strong quasi-periodic modulation in the X-ray flux. The observation of two quasiperiodic oscillations (QPOs) that occur on a daily timescale and exhibit a 3:1 frequency ratio strongly supports the evidence for the presence of an active galactic nucleus black hole (AGN BH). Assuming an orbital origin of QPOs, we calculated the upper and lower limit on AGN BH mass M and found M ≈ 107 - 109 M☉. When we compare this to mass estimates of other sources, XMMUJ134736.6+173403 appears to be the most massive source with commensurable QPO frequencies, and its mass represents the current observational upper limit on AGN BH mass based on QPO observations. We note that it will be crucial for the falsification of particular resonance models of QPOs whether only a single QPO with a frequency that completes the harmonic sequence 3 : 2 : 1 is found in this source, or if a new different pair of QPOs with frequencies in the 3 : 2 ratio is found. The former case would agree with the prediction of the 3 : 2 epicyclic resonance model and BH mass M ≈ (5a2 + 8a + 8)×107 M☉, where a is a dimensionless BH spin.
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    Accretion in strong field gravity with eXTP

    by Zdeněk Stuchlík· Únor 01, 2019· in Fyzika· 0 comments
    In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced "spectral- timing-polarimetry" techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process. X-spinmeasurements
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    Relativistic and Newtonian fluid tori with electric charge

    by Zdeněk Stuchlík· Leden 01, 2019· in Fyzika· 0 comments
    We discuss the effects of electric charging on the equilibrium configurations of magnetized, rotating fluid tori around black holes of different mass. In the context of gaseous/dusty tori in galactic nuclei, the central black hole dominates the gravitational field and it remains electrically neutral, while the surrounding material acquires some electric charge and exhibits non-negligible self-gravitational effect on the torus structure. The structure of the torus is influenced by the balance between the gravitational and electromagnetic forces. A cusp may develop even in Newtonian tori due to the charge distribution.
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    Shadow of the regular Bardeen black holes and comparison of the motion of photons and neutrinos

    by Zdeněk Stuchlík· Leden 01, 2019· in Fyzika· 0 comments
    The aim of the present research is the analysis of the photon motion in the regular spacetimes arising as solutions of the Einstein gravity coupled with a non-linear electrodynamics (NED). The photons no longer follow the null geodesic of the background spacetime, but the null geodesics of an effective geometry where the electromagnetic non- linearity is directly reflected in addition to the spacetime geometry. Motion of photons is compared to the motion of neutrinos that are not directly affected by the non-linearities of a non-Maxwellian electromagnetic field, and follow null geodesics of the background spacetime. We determine shadows of the regular Bardeen black holes, representing a special solution of the general relativity coupled with NED related to a magnetic charge, both for photons and neutrinos, and compare them to the shadow of the related Reissner-Nordstrom black holes. We demonstrate that the direct NED effects give clear signature of the presence of the regular black holes, on the level going up to 20% that is detectable by recent observational techniques. We also demonstrate strong influence of the NED effects on deflection angle of photons moving in the Bardeen spacetimes, and on the time delay of the motion of photons and neutrinos in vicinity of the black hole horizon.
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    Echoes of compact objects: New physics near the surface and matter at a distance

    by Zdeněk Stuchlík· Leden 01, 2019· in Fyzika· 0 comments
    It is well known that a hypothetical compact object that looks like an Einsteinian (Schwarzschild or Kerr) black hole everywhere except a small region near its surface should have the ringdown profile predicted by the Einstein theory at early and intermediate times, but modified by the so-called echoes at late times. A similar phenomenon appears when one considers an Einsteinian black hole and a shell of matter placed at some distance from it, so that astrophysical estimates could be made for the allowed mass of the black hole environment. While echoes for both systems have been extensively studied recently, no such analysis has been done for a system featuring phenomena simultaneously, that is, echoes due to new physics near the surface/event horizon and echoes due to matter at some distance from the black hole. Here, following Damour and Solodukhin [Phys. Rev. D 76, 024016 (2007), 10.1103/PhysRevD.76.024016] and Cardoso et al. [Phys. Rev. Lett. 116, 171101 (2016), 10.1103/PhysRevLett.116.171101], we consider a traversable wormhole obtained by identifying two Schwarzschild metrics with the same mass M at the throat, which is near the Schwarzschild radius, and add a nonthin shell of matter at a distance. This allows us to understand how the echoes of the surface of the compact object are affected by the astrophysical environment at a distance. The straightforward calculations for the time-domain profiles of such a system support the expectations that if the echoes are observed, they should most probably be ascribed to some new physics near the event horizon rather than some "environmental" effect.
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    Constraining the charge of the Galactic centre black hole

    by Zdeněk Stuchlík· Prosinec 01, 2018· in Fyzika· 0 comments
    In this contribution, we summarize our results concerning the observational constraints on the electric charge associated with the Galactic centre black hole - Sgr A*. According to the no-hair theorem, every astrophysical black hole, including supermassive black holes, is characterized by at most three classical, externally observable parameters - mass, spin, and the electric charge. While the mass and the spin have routinely been measured by several methods, the electric charge has usually been neglected, based on the arguments of efficient discharge in astrophysical plasmas. From a theoretical point of view, the black hole can attain charge due to the mass imbalance between protons and electrons in fully ionized plasmas, which yields about $sim 10^8,{rm C}$ for Sgr A*. The second, induction mechanism concerns rotating Kerr black holes embedded in an external magnetic field, which leads to electric field generation due to the twisting of magnetic field lines. This electric field can be associated with the induced Wald charge, for which we calculate the upper limit of $sim 10^{15},{rm C}$ for Sgr A*. Although the maximum theoretical limit of $sim 10^{15},{rm C}$ is still 12 orders of magnitude smaller than the extremal charge of Sgr A*, we analyse a few astrophysical consequences of having a black hole with a small charge in the Galactic centre. Two most prominent ones are the effect on the X-ray bremsstrahlung profile and the effect on the position of the innermost stable circular orbit.
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    Einstein-Klein-Gordon by gravitational decoupling

    by Zdeněk Stuchlík· Listopad 01, 2018· in Fyzika· 0 comments
    We investigate how a spherically symmetric scalar field can modify the Schwarzschild vacuum solution when there is no exchange of energy- momentum between the scalar field and the central source of the Schwarzschild metric. This system is described by means of the gravitational decoupling by Minimal Geometric Deformation (MGD- decoupling), which allows us to show that, under the MGD paradigm, the Schwarzschild solution is modified in such a way that a naked singularity appears.
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    Spherical photon orbits in the field of Kerr naked singularities

    by Zdeněk Stuchlík· Listopad 01, 2018· in Fyzika· 0 comments
    For the Kerr naked singularity (KNS) spacetimes, we study properties of spherical photon orbits (SPOs) confined to constant Boyer-Lindquist radius r. Some new features of the SPOs are found, having no counterparts in the Kerr black hole (KBH) spacetimes, especially stable orbits that could be pure prograde/retrograde, or with turning point in the azimuthal direction. At r>1 (r<1) the covariant photon energy E> 0 (E< 0), at r=1 there is E= 0. All unstable orbits must have E> 0. It is shown that the polar SPOs can exist only in the spacetimes with dimensionless spin a < 1.7996. Existence of closed SPOs with vanishing total change of the azimuth is demonstrated. Classification of the KNS and KBH spacetimes in dependence on their dimensionless spin a is proposed, considering the properties of the SPOs. For selected types of the KNS spacetimes, typical SPOs are constructed, including the closed paths. It is shown that the stable SPOs intersect the equatorial plane in a region of stable circular orbits of test particles, depending on the spin a. Relevance of this intersection for the Keplerian accretion discs is outlined and observational effects are estimated.
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    Black holes by gravitational decoupling

    by Zdeněk Stuchlík· Listopad 01, 2018· in Fyzika· 0 comments
    We investigate how a spherically symmetric fluid modifies the Schwarzschild vacuum solution when there is no exchange of energy- momentum between the fluid and the central source of the Schwarzschild metric. This system is described by means of the gravitational decoupling realised via the minimal geometric deformation approach, which allows us to prove that the fluid must be anisotropic. Several cases are then explicitly shown.
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