Marginally stable perfect fluid tori with uniform distribution of
specific angular momentum are determined in the
Reissner-Nordström-(anti-)de Sitter black-hole and nakedsingularity
spacetimes. Toroidal configurations are allowed only in the spacetimes
admitting existence of stable circular geodesics. The configurations
with equipotential surfaces crossing itself in a cusp allow accretion
(inner cusp) and/or excretion (outer cusp) of matter from the toroidal
configuration.
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Relevant properties of test-particle equatorial circular orbits in the
Kerr-anti-de Sitter spacetimes, including their existence, stability and
orientation, are briefly discussed.
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High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultradense matter and to black hole masses and spins. A 10 m2-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme "Matter under extreme conditions", namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic silicon drift detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of ~12 m2 (more than an order of magnitude larger than any spaceborne predecessor) in the 2-30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of <260 eV, LOFT will yield unprecedented information on strongly curved spacetimes and matter under extreme conditions of pressure and magnetic field strength.
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We study motion of current-carrying string loops in the field of braneworld spherically symmetric black holes and naked singularities. The spacetime is described by the Reissner-Nordström geometry with tidal charge b reflecting the non-local tidal effects coming from the external dimension; both positive and negative values of the spacetime parameter b are considered. We restrict attention to the axisymmetric motion of string loops when the motion can be fully governed by an appropriately defined effective potential related to the energy and angular momentum of the string loops. In dependence on these two constants of the motion, the string loops can be captured, trapped, or can escape to infinity. In close vicinity of stable equilibrium points at the centre of trapped states the motion is regular. We describe how it is transformed to chaotic motion with growing energy of the string loop. In the field of naked singularities the trapped states located off the equatorial plane of the system exist and trajectories unable to cross the equatorial plane occur, contrary to the trajectories in the field of black holes where crossing the equatorial plane is always admitted. We concentrate our attention to the so called transmutation effect when the string loops are accelerated in the deep gravitational field near the black hole or naked singularity by transforming the oscillatory energy to the energy of the transitional motion. We demonstrate that the influence of the tidal charge can be substantial especially in the naked singularity spacetimes with b > 1 where the acceleration to ultrarelativistic velocities with Lorentz factor γ ~ 100 can be reached, being more than one order higher in comparison with those obtained in the black hole spacetimes.
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High-time-resolution X-ray observations of compact objects provide
direct access to strong-field gravity, to the equation of state of
ultradense matter and to black hole masses and spins. A 10
m2-class instrument in combination with good spectral
resolution is required to exploit the relevant diagnostics and answer
two of the fundamental questions of the European Space Agency (ESA)
Cosmic Vision Theme "Matter under extreme conditions", namely: does
matter orbiting close to the event horizon follow the predictions of
general relativity? What is the equation of state of matter in neutron
stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as
one of the four Cosmic Vision M3 candidate missions to undergo an
assessment phase, will revolutionise the study of collapsed objects in
our galaxy and of the brightest supermassive black holes in active
galactic nuclei. Thanks to an innovative design and the development of
large-area monolithic silicon drift detectors, the Large Area Detector
(LAD) on board LOFT will achieve an effective area of ~12 m2
(more than an order of magnitude larger than any spaceborne predecessor)
in the 2-30 keV range (up to 50 keV in expanded mode), yet still fits a
conventional platform and small/medium-class launcher. With this large
area and a spectral resolution of <260 eV, LOFT will yield
unprecedented information on strongly curved spacetimes and matter under
extreme conditions of pressure and magnetic field strength.
Read More
We study motion of current-carrying string loops in the field of
braneworld spherically symmetric black holes and naked singularities.
The spacetime is described by the Reissner-Nordström geometry with
tidal charge b reflecting the non-local tidal effects coming from the
external dimension; both positive and negative values of the spacetime
parameter b are considered. We restrict attention to the axisymmetric
motion of string loops when the motion can be fully governed by an
appropriately defined effective potential related to the energy and
angular momentum of the string loops. In dependence on these two
constants of the motion, the string loops can be captured, trapped, or
can escape to infinity. In close vicinity of stable equilibrium points
at the centre of trapped states the motion is regular. We describe how
it is transformed to chaotic motion with growing energy of the string
loop. In the field of naked singularities the trapped states located off
the equatorial plane of the system exist and trajectories unable to
cross the equatorial plane occur, contrary to the trajectories in the
field of black holes where crossing the equatorial plane is always
admitted. We concentrate our attention to the so called transmutation
effect when the string loops are accelerated in the deep gravitational
field near the black hole or naked singularity by transforming the
oscillatory energy to the energy of the transitional motion. We
demonstrate that the influence of the tidal charge can be substantial
especially in the naked singularity spacetimes with b > 1 where the
acceleration to ultrarelativistic velocities with Lorentz factor γ
~ 100 can be reached, being more than one order higher in comparison
with those obtained in the black hole spacetimes.
Read More
The LOFT mission concept is one of four candidates selected by ESA for the M3 launch opportunity as Medium Size missions of the Cosmic Vision programme. The launch window is currently planned for between 2022 and 2024. LOFT is designed to exploit the diagnostics of rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars, as well as the physical state of ultradense matter. These primary science goals will be addressed by a payload composed of a Large Area Detector (LAD) and a Wide Field Monitor (WFM). The LAD is a collimated (<1 degree field of view) experiment operating in the energy range 2-50 keV, with a 10 m2 peak effective area and an energy resolution of 260 eV at 6 keV. The WFM will operate in the same energy range as the LAD, enabling simultaneous monitoring of a few-steradian wide field of view, with an angular resolution of <5 arcmin. The LAD and WFM experiments will allow us to investigate variability from submillisecond QPO’s to yearlong transient outbursts. In this paper we report the current status of the project.
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The LOFT mission concept is one of four candidates selected by ESA for
the M3 launch opportunity as Medium Size missions of the Cosmic Vision
programme. The launch window is currently planned for between 2022 and
2024. LOFT is designed to exploit the diagnostics of rapid X-ray flux
and spectral variability that directly probe the motion of matter down
to distances very close to black holes and neutron stars, as well as the
physical state of ultradense matter. These primary science goals will be
addressed by a payload composed of a Large Area Detector (LAD) and a
Wide Field Monitor (WFM). The LAD is a collimated (<1 degree field of
view) experiment operating in the energy range 2-50 keV, with a 10
m2 peak effective area and an energy resolution of 260 eV at
6 keV. The WFM will operate in the same energy range as the LAD,
enabling simultaneous monitoring of a few-steradian wide field of view,
with an angular resolution of <5 arcmin. The LAD and WFM experiments
will allow us to investigate variability from submillisecond QPO’s
to yearlong transient outbursts. In this paper we report the current
status of the project.
Read More
Extremely compact stars (ECS) (having radius R < 3 GM/ c 2) contain captured null geodesics. Certain part of neutrinos produced in their interior will be trapped, influencing thus their neutrino luminosity and thermal evolution. The trapping effect has been previously investigated for the internal Schwarzschild spacetimes with the uniform distribution of energy density. Here, we extend our earlier study considering the influence of the cosmological constant Λ on the trapping phenomena. Our model for the interior of ECS is based on the internal Schwarzschild-(anti-)de Sitter (S(a)dS) spacetimes with uniform distribution of energy density matched to the external vacuum S(a)dS spacetime with the same cosmological constant. Assuming uniform and isotropic distribution of local neutrino emissivity we determine behavior of the trapping coefficients, i.e., "global" one representing influence on the neutrino luminosity and "local" one representing influence on the cooling process. We demonstrate that the repulsive (attractive) cosmological constant has tendency to enhance (damp) the trapping phenomena.
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Extremely compact stars (ECS) (having radius R < 3 GM/ c
2) contain captured null geodesics. Certain part of neutrinos
produced in their interior will be trapped, influencing thus their
neutrino luminosity and thermal evolution. The trapping effect has been
previously investigated for the internal Schwarzschild spacetimes with
the uniform distribution of energy density. Here, we extend our earlier
study considering the influence of the cosmological constant Λ on
the trapping phenomena. Our model for the interior of ECS is based on
the internal Schwarzschild-(anti-)de Sitter (S(a)dS) spacetimes with
uniform distribution of energy density matched to the external vacuum
S(a)dS spacetime with the same cosmological constant. Assuming uniform
and isotropic distribution of local neutrino emissivity we determine
behavior of the trapping coefficients, i.e., "global" one representing
influence on the neutrino luminosity and "local" one representing
influence on the cooling process. We demonstrate that the repulsive
(attractive) cosmological constant has tendency to enhance (damp) the
trapping phenomena.
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