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 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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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 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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Radiation reaction acting on a charged particle moving at a stable
circular orbit of a magnetized black hole (BH) can lead to the shift of
the orbital radius outward from the BH. The effect causes an increase of
the energy and angular momentum of the particle measured by an observer
at rest at infinity. In this paper, we show that "widening" of such
orbits is independent of the field configuration, but it appears only in
the cases with the external Lorentz force acting outward from the BH.
This condition corresponds to qLB > 0, where q and L are the charge
and angular momentum of the particle, and B is intensity of the external
magnetic field. As examples of the orbital widening, we consider two
scenarios with an external homogeneous magnetic field and a magnetic
dipole field generated by a current loop around a Schwarzschild BH. We
show that the orbital widening is accompanied by quasi-harmonic
oscillations of the particle, which are considerably large in the
magnetic dipole fields. We also estimate the timescales of orbital
widening, from which it follows that the effect can be relevant in the
vicinity of stellar-mass BHs.
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Ringed accretion disks (RADs) are agglomerations of perfect-fluid tori
orbiting around a single central attractor that could arise during
complex matter inflows in active galactic nuclei. We focus our analysis
to axi-symmetric accretion tori orbiting in the equatorial plane of a
supermassive Kerr black hole; equilibrium configurations, possible
instabilities, and evolutionary sequences of RADs were discussed in our
previous works. In the present work we discuss special instabilities
related to open equipotential surfaces governing the material funnels
emerging at various regions of the RADs, being located between two or
more individual toroidal configurations of the agglomerate. These open
structures could be associated to proto-jets. Boundary limiting surfaces
are highlighted, connecting the emergency of the jet-like instabilities
with the black hole dimensionless spin. These instabilities are
observationally significant for active galactic nuclei, being related to
outflows of matter in jets emerging from more than one torus of RADs
orbiting around supermassive black holes.
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A test fluid composed of relativistic collisionless neutral particles in
the background of Kerr metric is expected to generate non-isotropic
equilibrium configurations in which the corresponding stress-energy
tensor exhibits pressure and temperature anisotropies. This arises as a
consequence of the constraints placed on single-particle dynamics by
Killing tensor symmetries, leading to a peculiar non-Maxwellian
functional form of the kinetic distribution function describing the
continuum system. Based on this outcome, in this paper the generation of
Kerr-like metric by collisionless N -body systems of neutral matter
orbiting in the field of a rotating black hole is reported. The result
is obtained in the framework of covariant kinetic theory by solving the
Einstein equations in terms of an analytical perturbative treatment
whereby the gravitational field is decomposed as a prescribed background
metric tensor described by the Kerr solution plus a self-field
correction. The latter one is generated by the uncharged fluid at
equilibrium and satisfies the linearized Einstein equations having the
non-isotropic stress-energy tensor as source term. It is shown that the
resulting self-metric is again of Kerr type, providing a mechanism of
magnification of the background metric tensor and its qualitative
features.
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We determine the class of axisymmetric and asymptotically flat black-
hole spacetimes for which the test Klein-Gordon and Hamilton-Jacobi
equations allow for the separation of variables. The known Kerr, Kerr-
Newman, Kerr-Sen and some other black-hole metrics in various theories
of gravity are within the class of spacetimes described here. It is
shown that although the black-hole metric in the Einstein-dilaton-Gauss-
Bonnet theory does not allow for the separation of variables (at least
in the considered coordinates), for a number of applications it can be
effectively approximated by a metric within the above class. This gives
us some hope that the class of spacetimes described here may be not only
generic for the known solutions allowing for the separation of
variables, but also a good approximation for a broader class of metrics,
which does not admit such separation. Finally, the generic form of the
axisymmetric metric is expanded in the radial direction in terms of the
continued fractions and the connection with other black-hole
parametrizations is discussed.
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The electromagnetic (EM) perturbations of the black hole solutions in
general relativity coupled to nonlinear electrodynamics (NED) are
studied for both electrically and magnetically charged black holes,
assuming that the EM perturbations do not alter the spacetime geometry.
It is shown that the effective potentials of the electrically and
magnetically charged black holes related to test perturbative NED EM
fields are related to the effective metric governing the photon motion,
contrary to the effective potential of the linear electrodynamic
(Maxwell) field that is related to the spacetime metric. Consequently,
corresponding quasinormal (QN) frequencies differ as well. As a special
case, we study new family of the NED black hole solutions which tend in
the weak field limit to the Maxwell field, giving the Reissner-Nordström
(RN) black hole solution. We compare the NED Maxwellian black hole QN
spectra with the RN black hole QN spectra.
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We construct the light escape cones of isotropic spot sources of
radiation residing in special classes of reference frames in the Kerr-de
Sitter (KdS) black hole spacetimes, namely in the fundamental class of
`non-geodesic' locally non-rotating reference frames (LNRFs), and two
classes of `geodesic' frames, the radial geodesic frames (RGFs), both
falling and escaping, and the frames related to the circular geodesic
orbits (CGFs). We compare the cones constructed in a given position for
the LNRFs, RGFs, and CGFs. We have shown that the photons locally
counter-rotating relative to LNRFs with positive impact parameter and
negative covariant energy are confined to the ergosphere region.
Finally, we demonstrate that the light escaping cones govern the shadows
of black holes located in front of a radiating screen, as seen by the
observers in the considered frames. For shadows related to distant
static observers the LNRFs are relevant.
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