An exact solution of the Lemaître-Tolman-Bondi class is investigated as a possible model of the Schwarzschild-like black hole embedded in a nonstatic dust-filled universe for the three types of spatial curvature. The solution is obtained in comoving coordinates by means of the mass function method. It is shown that the central part of space contains a Schwarzschild-like black hole. The R-T structure of the resulting spacetime is built. It is shown that the solution includes both the Schwarzschild and Friedmann solutions as its natural limits. The geodesic equations for test particles are analyzed. The particle observable velocities are found. The trajectories of the test particles are built from the point of view of both comoving and distant observers. For the distant observer, the results coincide with the Schwarzschild picture within a second-order accuracy near the symmetry center.
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There is strong observational evidence that many active galactic nuclei (AGNs) harbour super-massive black holes (SMBHs), demonstrating multi-accretion episodes during their life-time. In such AGNs, corotating and counterrotating tori, or strongly misaligned disks, as related to the central Kerr SMBH spin, can report traces of the AGNs evolution. Here we concentrate on aggregates of accretion disks structures, ringed accretion disks (RADs) orbiting a central Kerr SMBH, assuming that each torus of the RADs is centered in the equatorial plane of the attractor, tori are coplanar and axi-symmetric. Many of the RAD aspects are governed mostly by the spin of the Kerr geometry. We classify Kerr black holes (BHs) due to their dimensionless spin, according to possible combinations of corotating and counterrotating equilibrium or unstable (accreting) tori composing the RADs. The number of accreting tori in RADs cannot exceed n = 2. We present list of 14 characteristic values of the Kerr BH dimensionless spin a governing the classification in whole the black hole range , uniquely constrained by the RAD properties. The spin values are remarkably close providing an accurate characterization of the Kerr attractors based on the RAD properties. RAD dynamics is richer in the spacetimes of high spin values. One of the critical predictions states that a RAD tori couple formed by an outer accreting corotating and an inner accreting counterrotating torus is expected to be observed only around slowly spinning (a < 0.46M) BHs. The analysis strongly binds the fluid and BH characteristics providing indications on the situations where to search for RADs observational evidences. Obscuring and screening tori, possibly evident as traces in x-ray spectrum emission, are strongly constrained, eventually ruling out many assumptions used in the current investigations of the screening effects. We expect relevance of our classification of Kerr spacetimes in relation to astrophysical phenomena arising in different stages of AGNs life that could be observed by the planned x-ray satellite observatory ATHENA (Advanced Telescope for High ENergy Astrophysics).
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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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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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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 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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We discuss in the framework of general relativity the role of the dark energy represented by the cosmological constant, restricted due to cosmological tests, in the polytropic models of dark matter halos. The internal spacetime of the polytropic spheres governs circular geodesic orbits that can be compared with the velocity curves observed in large galaxies, indicating the possibility to use for the halo model both non-relativistic very extended and diluted polytropes, or relativistic polytropes with nearly critical value of the relativistic parameter sigma = p_mathrm{c}/varrho_mathrm{c} enabling extremely large polytrope extension, limited efficiently by the influence of the dark energy to agree with extension of dark matter halos of large galaxies. We also show that the so-called trapping relativistic polytropes with extremely large extension allow for gravitational instability of their central parts leading to the creation of a supermassive black hole inside of such an extremely extended polytrope representing galactic halo.
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We discuss the photon motion in the framework of general relativity coupled to non-linear electrodynamics. Photons no longer follow the null-geodesics of the spacetime but rather null-geodesics of associated effective metric. Here we compare structure of circular geodesics and time-delays of neutrinos in Bardeen spacetimes with those of photons in effective geometry. We also discuss construction the Keplerian disks images in the Bardeen spacetimes and compare them with the images of Keplerian disks in RN spacetime.
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Charged fluids circling in strong central gravitational and ambinet magnetic fields, characteristic for compact objects backgrounds, can embody interesting configurations. In contrast to the widely considered neutral fluid structures imitating thick equatorial accretion discs with negligible loss of mass, when the fluid is properly charged, we can find it forming unique toroidal structures `levitating' above the equatorial plane and also those hovering near the symmetry axis. Along with analytical topological studies of these structures, we can also present an survey of their basic physical characteristics, such as pressure, density and temperature profiles.
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In many astrophysically relevant situations, radiation-reaction forces acting upon a charge cannot be ignored, and the question of the location and stability of circular orbits in such a regime arises. The motion of a point charge with radiation reaction in flat spacetime is described by the Lorenz-Dirac (LD) equation, while in curved spacetime it is described by the DeWitt-Brehme (DWB) equation containing the Ricci term and a tail term. We show that for the motion of elementary particles in vacuum metrics, the DWB equation can be reduced to the covariant form of the LD equation, which we use here. Generically, the LD equation is plagued by runaway solutions, so we discuss computational ways of avoiding this problem when constructing numerical solutions. We also use the first iteration of the covariant LD equation, which is the covariant Landau-Lifshitz equation, comparing the results of these two approaches and showing the smallness of the third-order Schott term in the ultrarelativistic case. We calculate the corresponding energy and angular momentum loss of a particle and study the damping of charged particle oscillations around an equilibrium radius. We find that, depending on the orientation of the Lorentz force, the oscillating charged particle either spirals down to the black hole or stabilizes the circular orbit by decaying its oscillations. The latter case leads to the interesting new result of the particle orbit shifting outwards from the black hole. We also discuss the astrophysical relevance of the presented approach and provide estimates of the main parameters of the model.
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