Epicyclic oscillations of test particles near marginally stable circular orbits around charged Kiselev black holes

    by Zdeněk Stuchlík· 01 září, 2021· in Fyzika· 0 comments
    The marginally stable circular orbits (MSCOs) of test particles in the spacetime exterior to a charged Kiselev black hole are investigated for three characteristic values of the equation of state parameter ωq, namely (i) ωq=-1 /3 , (ii) ωq=-1 , and (iii) ωq=-2 /3 , and for different values of the normalization factor α and electric charge Q of the black hole. It is found that the presence of the quintessence field shifts outward the innermost stable circular orbits (ISCOs) around the Kiselev black hole, having the same charge parameter Q , as compared to the ISCOs around a Riessner-Nordstrom black hole, while the effect of the quintessence field on the outermost stable circular orbits (OSCOs) is just opposite to that on the ISCOs. Further, the radii of the photon circular orbits are also calculated for different ranges of the parameters α and Q . It is observed that the photon orbits are also shifted outward as the value of α increases. The radial and latitudinal epicyclic motion of test particles, which can be related to the quasiperiodic oscillations of test particles slightly above the MSCOs in the vicinity of the charged Kiselev black hole, is analyzed for the three different values of ωq. It is seen that the azimuthal and latitudinal frequencies coincide, and the radial epicyclic frequency is different in dependence on the spacetime parameters. In the case of ωq=-1 /3 , the azimuthal and latitudinal frequencies depend on the radial position r of the particle, the charge Q , and the mass M of the black hole, and do not depend on the factor α . However, for ωq=-2 /3 and ωq=-1 , these two frequencies, along with the black hole parameters—i.e., M and Q and the radial position r —also depend on the factor α . The radial epicyclic frequency for all the values of ωq depends on M , Q , r , and also on the normalization factor α . We also compare the epicyclic frequencies with that for an uncharged black hole. With the increase of electric charge, the ISCO becomes closer to the central object, and one can observe epicyclic frequencies closer to the central object, which makes the epicyclic frequencies larger. The ISCO gets larger as α increases, and thus the epicyclic frequencies can be observed away from the central object and would be smaller as compared to the case of a pure Riessner-Nordstrom black hole without quintessence. As the effect of the parameters Q and α on the OSCOs is just opposite to that on the ISCOs, the epicyclic frequencies near the OSCOs behave the other way around.
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    Epicyclic Oscillations around Simpson–Visser Regular Black Holes and Wormholes

    by Zdeněk Stuchlík· 01 srpna, 2021· in Fyzika· 0 comments
    We study epicyclic oscillatory motion along circular geodesics of the Simpson–Visser meta-geometry describing in a unique way regular black-bounce black holes and reflection-symmetric wormholes by using a length parameter l. We give the frequencies of the orbital and epicyclic motion in a Keplerian disc with inner edge at the innermost circular geodesic located above the black hole outer horizon or on the our side of the wormhole. We use these frequencies in the epicyclic resonance version of the so-called geodesic models of high-frequency quasi-periodic oscillations (HF QPOs) observed in microquasars and around supermassive black holes in active galactic nuclei to test the ability of this meta-geometry to improve the fitting of HF QPOs observational data from the surrounding of supermassive black holes. We demonstrate that this is really possible for wormholes with sufficiently high length parameter l.
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    General relativistic rotational energy extraction from black holes-accretion disk systems

    by Zdeněk Stuchlík· 01 července, 2021· in Fyzika· 0 comments
    The determination of mass and spin parameters of the black holes (BHs) is crucial in the analysis of the merger of BHs and BHs formation and evolution, including accretion. Here we constrain the BH spin with the evaluation of the dimensionless parameter ξ representing the total rotational energy extracted versus the mass of the BH, following procedure introduced by Daly (2009 Astrophys. J. 691 L72-6) that is independent from the details of the specific extraction process. The energy extraction can power an outflow which can be then observed. We relate the energy extraction to the accreting configurations and the accretion processes occurring in a cluster of agglomerate corotating and counter-rotating tori orbiting one central Kerr SMBH, associating ξ to the characteristics of the accretion processes. We relate the regions of tori parameters to features of the energy extraction processes, binding ξ to properties of light surfaces by using the bundles developed by Pugliese and Quevedo (2019 Eur. Phys. J. C 79 209), relating measures in different regions of the spacetimes. We evaluate properties of the BH accretions disks, and correlate spacetimes prior and after their transition due to the energy extraction. Light surfaces are related to the generators of Killing horizons, proving limiting frequency of the stationary observers of the geometries. We consider the photon limiting curves of the stationary observers as constraints for various processes regulated by these frequencies, to relate different BH states, prior and after the energy extraction, investigating regions close to the BH horizons and rotational axis. From methodological view-point we used a naked singularity-BH correspondence defined with metric bundles to predict observational characteristics of the BH-accretion disk system. The analysis points relevant BH spins a ≈ 0.94M, a ≈ 0.7M and a ≈ 0.3M. We show the relation between the rotational law of the tori, the characteristic frequency of the bundle and the relativistic velocity defining the von Zeipel surfaces. The inferior limit on the formation of corotating tori is ℓ/a ⩾ 2, for counter-rotating tori ℓ/a ⩽ -22/5 (ℓ is the fluids specific angular momentum).
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    Neutrino trapping in extremely compact Tolman VII spacetimes

    by Zdeněk Stuchlík· 01 června, 2021· in Fyzika· 0 comments
    Extremely compact objects trap gravitational waves or neutrinos, assumed to move along null geodesics in the trapping regions. The trapping of neutrinos was extensively studied for spherically symmetric extremely compact objects constructed under the simplest approximation of the uniform energy density distribution, with radius located under the photosphere of the external spacetime; in addition, uniform emissivity distribution of neutrinos was assumed in these studies. Here we extend the studies of the neutrino trapping for the case of the extremely compact Tolman VII objects representing the simplest generalization of the internal Schwarzschild solution with uniform distribution of the energy density, and the correspondingly related distribution of the neutrino emissivity that is thus again proportional to the energy density; radius of such extremely compact objects can overcome the photosphere of the external Schwarzschild spacetime. In dependence on the parameters of the Tolman VII spacetimes, we determine the "local" and "global" coefficients of efficiency of the trapping and demonstrate that the role of the trapping is significantly stronger than in the internal Schwarzschild spacetimes. Our results indicate possible influence of the neutrino trapping in cooling of neutron stars.
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    General relativistic effects in neutron star electrodynamics

    by Zdeněk Stuchlík· 01 června, 2021· in Fyzika· 0 comments
    The paper explores general relativistic (GR) effects in electromagnetic fields of the rotating neutron star. The star has been assumed as a perfect conductor with infinity electric conductivity, i.e., σ →∞ . The analytical form of general relativistic Maxwell's equations for the electromagnetic fields has been derived in the presence of gravity. It is shown that six components of the electromagnetic fields can be expressed in terms of two profile functions. It has been shown that the Lense-Thirring term plays an important role in the generation of the multipole electromagnetic fields. We obtain that the rotation of the quadrupole magnetic field can create the dipole electric field. Moreover, we have also shown that GR effects are reasonably large for the highest order of electromagnetic multipole. Finally, as a test of our results, we investigate the effect of the Lense-Thirring term on the luminosity of magnetodipolar radiations.
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    Geodesic Structure of the Accelerated Stephani Universe

    by Zdeněk Stuchlík· 01 června, 2021· in Fyzika· 0 comments
    For the spherically symmetric Stephani cosmological model with an accelerated expansion, we investigate the main scenarios of the test particle and photon motion. We show that a comoving observer sees an appropriate picture. In the case of purely radial motion, the radial velocity decreases slightly with time due to the universe expansion. Both particles and photons spiral out of the center when the radial coordinate is constant. In the case of the motion with arbitrary initial velocity, the observable radial distance to the test particle can increase under negative observable radial velocity.
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    Dynamical stability of the modified Tolman VII solution

    by Zdeněk Stuchlík· 01 května, 2021· in Fyzika· 0 comments
    Studies of neutron stars are at their peak after the multimessenger observation of the binary merger event GW170817, which strongly constrains the stellar parameters like tidal deformability, masses, and radii. Although current and future observations will provide stronger limits on the neutron stars parameters, knowledge of explicit interior solutions to Einstein's equations, which connect observed parameters with the internal structure, are crucial to have a satisfactory description of the interior of these compact objects. A well-known exact solution, which has shown a relatively good approximation to a neutron star, is the Tolman VII solution. In order to provide a better fitting for the energy density profile, with the realistic equations of state for neutron stars, recently, Jiang and Yagi proposed a modified version of this model, which introduces an additional parameter α , reflecting the interplay of the quadratic and the newly added quartic term in the energy density profile. Here we study the dynamical stability of this modified Tolman VII solution using the theory of infinitesimal and adiabatic radial oscillations developed by Chandrasekhar. For this purpose, we determine values of the critical adiabatic index, for the onset of instability, considering configurations with varying compactness and α . We found that the new models are stable against radial oscillations for a considerable range of values of compactness and the new parameter α , thus supporting their applicability as a physically plausible approximation of realistic neutron stars.
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    Dynamical motion of matter around a charged black hole

    by Zdeněk Stuchlík· 01 dubna, 2021· in Fyzika· 0 comments
    We investigate the dynamical motion of test particles and accretion processes around spherically symmetric charged black hole. We first show the derivation of the Paczyňski-Wiita (PW) potential for any spherically symmetric spacetime and present analytical form of the PW potential in the Reissner-Nordström (RN) spacetime and the RN-type spacetime. We study the dependence of the characteristic orbits of test (neutral and charged) particles around a charged black hole from two main parameters, namely, charge of RN black hole and the interaction of charged particle with the external electromagnetic field of the black hole and present graphical trajectories of test particles around the black hole. It is shown that with increasing the absolute value of interaction parameter, |qQ|, the innermost stable circular orbit (ISCO) radius for charged particle always increases. It is also shown that the energy efficiency for a charged particles can reach up to 60% in the presence of an external charged black hole, while for neutral particles it can reach up to 8% for neutral particles. The capture cross-section of the photon is by the charged black and the optical appearance of the thin accretion disk around the charged black. Finally, we have studied the flux energy and the thermal spectrum of the accretion disk consisting of the polytropic gas, around the charged black hole.
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    Kerr-de Sitter black hole revisited

    by Zdeněk Stuchlík· 01 dubna, 2021· in Fyzika· 0 comments
    We interpret the cosmological constant as the energy of the vacuum, and under a minimum amount of assumptions, we show that it is deformed in the vicinity of a black hole. This leads us to reexamine the Kerr-de Sitter solution. We provide a new solution, simpler and geometrically richer, which shows the impact of the rotation in form of a warped curvature. We carry out a detailed and exact study on the new black hole solution, and we conclude with a conjecture regarding the possible impact of our results on alternative theories.
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    Polytropic spheres modelling dark matter haloes of dwarf galaxies

    by Zdeněk Stuchlík· 01 března, 2021· in Fyzika· 0 comments
    Context. Dwarf galaxies and their dark matter (DM) haloes have velocity curves of a different character than those in large galaxies. These velocity curves are modelled by a simple pseudo-isothermal model containing only two parameters, which do not give us insight into the physics of the DM halo.
    Aims: We seek to obtain some insight into the physical conditions in DM haloes of dwarf galaxies by using a simple physically based model of DM haloes.
    Methods: To treat the diversity of the dwarf galaxy velocity profiles in a unifying framework, we applied polytropic spheres characterised by the polytropic index n and the relativistic parameter σ as a model of dwarf-galaxy DM haloes and matched the velocity of circular geodesics of the polytropes to the velocity curves observed in the dwarf galaxies from the LITTLE THINGS ensemble.
    Results: We introduce three classes of the LITTLE THINGS dwarf galaxies in relation to the polytrope models due to the different character of the velocity profile. The first class corresponds to polytropes that have n < 1 with linearly increasing velocity along the whole profile, the second class has 1 < n < 2 and the velocity profile becomes flat in the external region, the third class has n > 2, and the velocity profile reaches a maximum and demonstrates a decline in the external region. The σ parameter has to be strongly non-relativistic (σ < 10-8) for all dwarf galaxy models; this parameter varies for the models of each class, but these variations have negligible influence on the character of the velocity profile.
    Conclusions: Our results indicate a possibility that at least two different kinds of DM are behind the composition of DM haloes. The matches of the observational velocity curves are of the same quality as those obtained by the pseudo-isothermal, core-like models of dwarf galaxy DM haloes.
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