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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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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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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The axial electromagnetic (EM) perturbations of the black hole (BH)
solutions in general relativity coupled to nonlinear electrodynamics
(NED) were studied for both electrically and magnetically charged BHs,
assuming that the EM perturbations do not alter the spacetime geometry
in our preceding paper [Phys. Rev. D 97, 084058 (2018),
10.1103/PhysRevD.97.084058]. Here, as a continuation of that work, the
formalism for the polar EM perturbations of the BHs in general
relativity coupled to the NED is presented. We show that the quasinormal
modes (QNMs) spectra of polar EM perturbations of the electrically and
magnetically charged BHs in the NED are not isospectral, contrary to the
case of the standard Reissner-Nordström BHs in the classical linear
electrodynamics. It is shown by the detailed study of QNMs properties in
the eikonal approximation that the EM perturbations can be a powerful
tool to confirm that in the NED light ray does not follow the null
geodesics of the spacetime. By specifying the NED model and comparing
axial and polar EM perturbations of the electrically and magnetically
charged BHs, it is shown that QNM spectra of the axial EM perturbations
of magnetically (electrically) charged BH and polar EM perturbations of
the electrically (magnetically) charged BH are isospectral, i.e.,
ωmagax≈ωelpol
(ωmagpol≈ωelax).
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The exact axisymmetric and static solution of the Einstein equations
coupled to the axisymmetric and static gravitating scalar (or phantom)
field is presented. The spacetimes modified by the scalar field are
explicitly given for the so-called γ -metric and the Erez-Rosen metric
with quadrupole moment q , and the influence of the additional
deformation parameters γ* and q* generated by the
scalar field is studied. It is shown that the null energy condition is
satisfied for the phantom field, but it is not satisfied for the
standard scalar field. The test particle motion in both the modified γ
-metric and the Erez-Rosen quadrupole metric is studied; the circular
geodesics are determined, and near-circular trajectories are explicitly
presented for characteristic values of the spacetime parameters. It is
also demonstrated that the parameters γ* and q*
have no influence on the test particle motion in the equatorial plane.
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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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