We study the acceleration of an electric current-carrying and axially
symmetric string loop initially oscillating in the vicinity of a
Schwarzschild black hole embedded in an external asymptotically uniform
magnetic field. The plane of the string loop is orthogonal to the
magnetic field lines and the acceleration of the string loop occurs due
to the transmutation effect turning in the deep gravitational field the
internal energy of the oscillating strings to the energy of their
translational motion along the axis given by the symmetry of the black
hole spacetime and the magnetic field. We restrict our attention to the
motion of string loop with energy high enough, when it can overcome the
gravitational attraction and escape to infinity. We demonstrate that for
the current-carrying string loop the transmutation effect is enhanced by
the contribution of the interaction between the electric current of the
string loop and the external magnetic field and we give conditions that
have to be fulfilled for an efficient acceleration. The Schwarzschild
black hole combined with the strong external magnetic field can
accelerate the current-carrying string loop up to the velocities close
to the speed of light v˜c. Therefore, the string loop
transmutation effect can potentially well serve as an explanation for
acceleration of highly relativistic jets observed in microquasars and
active galactic nuclei.
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We study charged-fluid toroidal structures surrounding a nonrotating charged black hole immersed in a large-scale, asymptotically uniform magnetic field. In continuation of our former study on electrically charged matter in approximation of zero conductivity, we demonstrate the existence of orbiting structures in permanent rigid rotation in the equatorial plane and charged clouds hovering near the symmetry axis. We constrain the range of parameters that allow stable configurations and derive the geometrical shape of equipressure surfaces. Our simplified analytical study suggests that these regions of stability may be relevant for trapping electrically charged particles and dust grains in some areas of the black hole magnetosphere and thus important in some astrophysical situations.
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We study charged-fluid toroidal structures surrounding a nonrotating
charged black hole immersed in a large-scale, asymptotically uniform
magnetic field. In continuation of our former study on electrically
charged matter in approximation of zero conductivity, we demonstrate the
existence of orbiting structures in permanent rigid rotation in the
equatorial plane and charged clouds hovering near the symmetry axis. We
constrain the range of parameters that allow stable configurations and
derive the geometrical shape of equipressure surfaces. Our simplified
analytical study suggests that these regions of stability may be
relevant for trapping electrically charged particles and dust grains in
some areas of the black hole magnetosphere and thus important in some
astrophysical situations.
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We discuss photon and test-particle orbits in the Kehagias-Sfetsos (KS) metric of Hořava's gravity. For any value of the Hořava parameter ω, there are values of the gravitational mass M for which the metric describes a naked singularity, and this is always accompanied by a vacuum "antigravity sphere" on whose surface a test particle can remain at rest (in a zero angular momentum geodesic), and inside which no circular geodesics exist. The observational appearance of an accreting KS naked singularity in a binary system would be that of a quasistatic spherical fluid shell surrounded by an accretion disk, whose properties depend on the value of M, but are always very different from accretion disks familiar from the Kerr-metric solutions. The properties of the corresponding circular orbits are qualitatively similar to those of the Reissner-Nordström naked singularities. When event horizons are present, the orbits outside the Kehagias-Sfetsos black hole are qualitatively similar to those of the Schwarzschild metric.
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The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final downselection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supranuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m2 effective area, 2-30 keV, 240 eV spectral resolution, 1° collimated field of view) and a Wide Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.
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We discuss photon and test-particle orbits in the Kehagias-Sfetsos (KS)
metric of Hořava's gravity. For any value of the Hořava
parameter ω, there are values of the gravitational mass M for
which the metric describes a naked singularity, and this is always
accompanied by a vacuum "antigravity sphere" on whose surface a test
particle can remain at rest (in a zero angular momentum geodesic), and
inside which no circular geodesics exist. The observational appearance
of an accreting KS naked singularity in a binary system would be that of
a quasistatic spherical fluid shell surrounded by an accretion disk,
whose properties depend on the value of M, but are always very different
from accretion disks familiar from the Kerr-metric solutions. The
properties of the corresponding circular orbits are qualitatively
similar to those of the Reissner-Nordström naked singularities.
When event horizons are present, the orbits outside the Kehagias-Sfetsos
black hole are qualitatively similar to those of the Schwarzschild
metric.
Read More
The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3
Cosmic Vision framework and participated in the final downselection for
a launch slot in 2022-2024. Thanks to the unprecedented combination of
effective area and spectral resolution of its main instrument, LOFT will
study the behaviour of matter under extreme conditions, such as the
strong gravitational field in the innermost regions of accretion flows
close to black holes and neutron stars, and the supranuclear densities
in the interior of neutron stars. The science payload is based on a
Large Area Detector (LAD, 10 m2 effective area, 2-30 keV,
240 eV spectral resolution, 1° collimated field of view) and a Wide
Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source
location accuracy, 300 eV spectral resolution). The WFM is equipped with
an on-board system for bright events (e.g. GRB) localization. The
trigger time and position of these events are broadcast to the ground
within 30 s from discovery. In this paper we present the status of the
mission at the end of its Phase A study.
Read More
Astrophysical plasmas in the surrounding of compact objects and subject to intense gravitational and electromagnetic fields are believed to give rise to relativistic regimes. Theoretical and observational evidences suggest that magnetized plasmas of this type are collisionless and can persist for long times (e.g., with respect to a distant observer, coordinate, time), while exhibiting geometrical structures characterized by the absence of well-defined spatial symmetries. In this paper, the problem is posed whether such configurations can correspond to some kind of kinetic equilibrium. The issue is addressed from a theoretical perspective in the framework of a covariant Vlasov statistical description, which relies on the method of invariants. For this purpose, a systematic covariant variational formulation of gyrokinetic theory is developed, which holds without requiring any symmetry condition on the background fields. As a result, an asymptotic representation of the relativistic particle magnetic moment is obtained from its formal exact solution, in terms of a suitably defined invariant series expansion parameter (perturbative representation). On such a basis, it is shown that spatially non-symmetric kinetic equilibria can actually be determined, an example being provided by Gaussian-like distributions. As an application, the physical mechanisms related to the occurrence of a non-vanishing equilibrium fluid 4-flow are investigated.
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Based on the Newman-Janis algorithm, the Ayón-Beato-García spacetime metric [Phys. Rev. Lett. 80, 5056 (1998)] of the regular spherically symmetric, static, and charged black hole has been converted into rotational form. It is shown that the derived solution for rotating a regular black hole is regular and the critical value of the electric charge for which two horizons merge into one sufficiently decreases in the presence of the nonvanishing rotation parameter a of the black hole.
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We demonstrate that ultra-high-energy collisions of particles falling freely from rest at infinity can occur in the field of near-extreme Kehagias-Sfetsos naked singularities related to the Hořava gravity. However, the efficiency of escaping of created ultrarelativistic particles and the energy efficiency of the collisional process relative to distant observers are significantly lowered due to large gravitational redshift, which is substantially lower in comparison to those related to the collisions occurring close to the equatorial plane of near-extreme Kerr naked-singularity spacetimes. In the Kehagias-Sfetsos naked-singularity spacetimes, the energy efficiency relative to distant observers corresponds to the covariant energy of the colliding particles only. Finally, we demonstrate how the ultra-high-energy collisions are modified for charged particles if the Kehagias-Sfetsos naked singularities are immersed in a uniform magnetic field.
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