Twin-peak quasi-periodic oscillations (QPOs) are observed in the X-ray
power-density spectra of several accreting low-mass neutron star (NS)
binaries. In our previous work we have considered several QPO models. We
have identified and explored mass-angular-momentum relations implied by
individual QPO models for the atoll source 4U 1636-53. In this paper we
extend our study and confront QPO models with various NS equations of
state (EoS). We start with simplified calculations assuming Kerr
background geometry and then present results of detailed calculations
considering the influence of NS quadrupole moment (related to
rotationally induced NS oblateness) assuming Hartle-Thorne spacetimes.
We show that the application of concrete EoS together with a particular
QPO model yields a specific mass-angular-momentum relation. However, we
demonstrate that the degeneracy in mass and angular momentum can be
removed when the NS spin frequency inferred from the X-ray burst
observations is considered. We inspect a large set of EoS and discuss
their compatibility with the considered QPO models. We conclude that
when the NS spin frequency in 4U 1636-53 is close to 580Hz we can
exclude 51 from 90 of the considered combinations of EoS and QPO models.
We also discuss additional restrictions that may exclude even more
combinations. Namely, there are 13 EOS compatible with the observed twin
peak QPOs and the relativistic precession model. However, when
considering the low frequency QPOs and Lense-Thirring precession, only 5
EOS are compatible with the model.
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Spherically symmetric equilibrium configurations of perfect fluid
obeying a polytropic equation of state are studied in spacetimes with a
repulsive cosmological constant. The configurations are specified in
terms of three parameters---the polytropic index $n$, the ratio of
central pressure and central energy density of matter $sigma$, and the
ratio of energy density of vacuum and central density of matter
$lambda$. The static equilibrium configurations are determined by two
coupled first-order nonlinear differential equations that are solved by
numerical methods with the exception of polytropes with $n=0$
corresponding to the configurations with a uniform distribution of
energy density, when the solution is given in terms of elementary
functions. The geometry of the polytropes is conveniently represented by
embedding diagrams of both the ordinary space geometry and the optical
reference geometry reflecting some dynamical properties of the geodesic
motion. The polytropes are represented by radial profiles of energy
density, pressure, mass, and metric coefficients. For all tested values
of $n>0$, the static equilibrium configurations with fixed parameters
$n$, $sigma$, are allowed only up to a critical value of the
cosmological parameter
$lambda_{mathrm{c}}=lambda_{mathrm{c}}(n,sigma)$. In the case of
$n>3$, the critical value $lambda_{mathrm{c}}$ tends to zero for
special values of $sigma$. The gravitational potential energy and the
binding energy of the polytropes are determined and studied by numerical
methods. We discuss in detail the polytropes with an extension
comparable to those of the dark matter halos related to galaxies, i.e.,
with extension $ell > 100,mathrm{kpc}$ and mass $M >
10^{12},mathrm{M}_{odot}$. ...
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Spherically symmetric equilibrium configurations of perfect fluid obeying a polytropic equation of state are studied in spacetimes with a repulsive cosmological constant. The configurations are specified in terms of three parameters—the polytropic index n , the ratio of central pressure and central energy density of matter σ , and the ratio of energy density of vacuum and central density of matter λ . The static equilibrium configurations are determined by two coupled first-order nonlinear differential equations that are solved by numerical methods with the exception of polytropes with n =0 corresponding to the configurations with a uniform distribution of energy density, when the solution is given in terms of elementary functions. The geometry of the polytropes is conveniently represented by embedding diagrams of both the ordinary space geometry and the optical reference geometry reflecting some dynamical properties of the geodesic motion. The polytropes are represented by radial profiles of energy density, pressure, mass, and metric coefficients. For all tested values of n >0 , the static equilibrium configurations with fixed parameters n , σ , are allowed only up to a critical value of the cosmological parameter λc=λc(n ,σ ). In the case of n >3 , the critical value λc tends to zero for special values of σ . The gravitational potential energy and the binding energy of the polytropes are determined and studied by numerical methods. We discuss in detail the polytropes with an extension comparable to those of the dark matter halos related to galaxies, i.e., with extension ℓ>100 kpc and mass M >1 012 M⊙ . For such largely extended polytropes, the cosmological parameter relating the vacuum energy to the central density has to be larger than λ =ρvac/ρc∼10-9. We demonstrate that the extension of the static general relativistic polytropic configurations cannot exceed the so-called static radius related to their external spacetime, supporting the idea that the static radius represents a natural limit on the extension of gravitationally bound configurations in an expanding universe dominated by the vacuum energy.
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We study optical effects in quintessential Kerr black hole spacetimes corresponding to the limiting case of the equation-of-state parameter ω q=-1/3 of the quintessence. In dependence on the dimensionless quintessential field parameter c, we determine the black hole silhouette and the spectral line profiles of Keplerian disks generated in this special quintessential Kerr geometry, representing an extension of the general modifications of the Kerr geometry introduced recently by Ghasemi-Nodehi and Bambi (Eur. Phys. J. C 56:#290, 2016). We demonstrate that due to the influence of the parameter c, the silhouette is almost homogeneously enlarged, and the spectral line profiles are redshifted with almost conserved shape.
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In this work we have obtained the set of new exact solutions of the Einstein equations that generalize the known Lemaitre-Tolman-Bondi solution for the certain case of nonzero pressure under zero spatial curvature. These solutions are pretending to describe the black hole immersed in the nonstatic cosmological background and give a possibility to investigate the problems concerning the effects of the cosmological expansion in gravitationally bounded systems. They may also be used as a seed models in the problem of structure formation in the universe at the epoch of matter and radiation decoupling. It was shown that each of the solutions obtained contains either the Reissner-Nordstrom or the Schwarzschild black hole in the central region of the space. It is demonstrated that the approach of the mass function use in solving the Einstein equations allows clear physical interpretation of the resulting solutions that is of much benefit to any their further application.
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We show that the braneworld rotating Kerr-Newman black hole and naked
singularity spacetimes with both positive and negative braneworld tidal
charge parameters can be separated into 14 classes according to the
properties of circular geodesics governing the Keplerian accretion. We
determine the efficiency of the Keplerian accretion disks for all
braneworld Kerr-Newman spacetimes. We demonstrate the occurrence of an
infinitely deep gravitational potential in Kerr-Newman naked singularity
spacetimes having the braneworld dimensionless tidal charge b ∈(1
/4 ,1 ) and the dimensionless spin a ∈(2 √{b }-√{b (4 b
-1 ) } , 2 √{b }+√{b (4 b -1 ) }) , implying unbound
efficiency of the Keplerian accretion and the possibility of extracting
the whole naked singularity mass. Therefore, we call them braneworld
"mining-unstable" Kerr-Newman naked singularity spacetimes. Fundamental
restriction on the relevance of the extraordinary—but fully
classical—phenomenon of the mining instability is given by
validity of the assumption of geodesic motion of the accreting matter.
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We show that the braneworld rotating Kerr-Newman black hole and naked singularity spacetimes with both positive and negative braneworld tidal charge parameters can be separated into 14 classes according to the properties of circular geodesics governing the Keplerian accretion. We determine the efficiency of the Keplerian accretion disks for all braneworld Kerr-Newman spacetimes. We demonstrate the occurrence of an infinitely deep gravitational potential in Kerr-Newman naked singularity spacetimes having the braneworld dimensionless tidal charge b ∈(1 /4 ,1 ) and the dimensionless spin a ∈(2 √{b }-√{b (4 b -1 ) } , 2 √{b }+√{b (4 b -1 ) }) , implying unbound efficiency of the Keplerian accretion and the possibility of extracting the whole naked singularity mass. Therefore, we call them braneworld "mining-unstable" Kerr-Newman naked singularity spacetimes. Fundamental restriction on the relevance of the extraordinary—but fully classical—phenomenon of the mining instability is given by validity of the assumption of geodesic motion of the accreting matter.
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We study motion and collision of particles in the gravitational field of
rotating black hole immersed in quintessential dark energy characterized
with the quintessential parameter ωqin(-1;-1/3)
governing the equation of state of the dark energy, and the
dimensionless quintessential field parameter tilde{c}. We focus on the
acceleration of particles due to collisional processes and show how the
center of mass energy depends on the quintessential field parameter
tilde{c}. We also make comparison of the obtained results to the
collisional energetics of quintessential static black holes
demonstrating the crucial role of the rotation parameter a in the
particle acceleration. Finally we study the dependence of the maximal
value of the efficiency of energy extraction through Penrose process for
rotating black hole with quintessential field parameter tilde{c}. It is
found that quintessence field decreases the energy extraction efficiency
through Penrose process and when the parameter tilde{c} vanishes one can
get the standard value of the efficiency coefficient for the Kerr black
hole as η˜ 21 %.
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We study motion and collision of particles in the gravitational field of rotating black hole immersed in quintessential dark energy characterized with the quintessential parameter ωqin(-1;-1/3) governing the equation of state of the dark energy, and the dimensionless quintessential field parameter tilde{c}. We focus on the acceleration of particles due to collisional processes and show how the center of mass energy depends on the quintessential field parameter tilde{c}. We also make comparison of the obtained results to the collisional energetics of quintessential static black holes demonstrating the crucial role of the rotation parameter a in the particle acceleration. Finally we study the dependence of the maximal value of the efficiency of energy extraction through Penrose process for rotating black hole with quintessential field parameter tilde{c}. It is found that quintessence field decreases the energy extraction efficiency through Penrose process and when the parameter tilde{c} vanishes one can get the standard value of the efficiency coefficient for the Kerr black hole as η∼ 21 %.
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The Large Observatory For x-ray Timing (LOFT) is a mission concept which
was proposed to ESA as M3 and M4 candidate in the framework of the
Cosmic Vision 2015-2025 program. Thanks to the unprecedented combination
of effective area and spectral resolution of its main instrument and the
uniquely large field of view of its wide field monitor, LOFT will be
able to study the behaviour of matter in extreme conditions such as the
strong gravitational field in the innermost regions close to black holes
and neutron stars and the supra-nuclear densities in the interiors of
neutron stars. The science payload is based on a Large Area Detector
(LAD, >8m2 effective area, 2-30 keV, 240 eV spectral
resolution, 1 degree 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 current technical and
programmatic status of the mission.
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