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    Effective Geometry of the Bardeen Spacetimes: Gravitational Lensing and Frequency Mapping of Keplerian Disks

    by Zdeněk Stuchlík· Březen 01, 2019· in Fyzika· 0 comments
    We study the appearance of Keplerian accretion disks in order to demonstrate the influence of the nonlinear electrodynamics (NED) on the gravitational lensing and frequency shifting of the images of the Keplerian disks. We focus our attention on the Bardeen black hole backgrounds with magnetic charges that could be considered to be acceptable solutions for the Einstein gravitational equations combined with those representing an NED. Photons governing the appearance of the Keplerian disks follow null geodesics of the effective geometry related to the Bardeen spacetime. We compare the appearance governed by the effective geometry to those governed by the spacetime geometry itself, and to the appearance of Keplerian disks orbiting a related Reissner- Nordstrom black hole spacetime. We demonstrate a clear and very strong difference between the disk images determined by the effective geometry and the others, both in the shape and (especially) in the frequency mapping where it exhibits a difference of three orders.
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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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    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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    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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    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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    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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    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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    Massive nonminimally coupled scalar field in Reissner-Nordström spacetime: Long-lived quasinormal modes and instability

    by Zdeněk Stuchlík· Listopad 01, 2018· in Fyzika· 0 comments
    Here we show that the phenomenon of arbitrarily long-lived quasinormal modes (called quasiresonances) of a massive scalar field in the vicinity of a black hole is not an artifact of the test field approximation, but takes place also when the (derivative) coupling of a scalar field with the Einstein tensor is taken into consideration. We observe that at large coupling and high multipole numbers, the growing modes appear in the spectrum, which are responsible for the eikonal instability of the field. For small coupling, when the configuration is stable, there appear the purely imaginary quasinormal modes which are nonperturbative in the coupling constant. At the sufficiently small coupling the nonminimal scalar field is stable and the asymptotic late-time tails are not affected by the coupling term. The accurate calculations of quasinormal frequencies for a massive scalar field with the derivative coupling in the Reissner-Nordström black-hole background are performed with the help of the Frobenius method, time-domain integration and WKB expansion.
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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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    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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