We study gravitational redshift of photons and neutrinos radiated by the
braneworld neutron or quark stars that are considered in the framework
of the simple model of the internal spacetime with uniform distribution
of energy density, and the external spacetime described by the
Reissner-Nordström geometry characterized by the braneworld
``tidal'' charge b. For negative tidal charges, the external spacetime
is of the black-hole type, while for positive tidal charges, the
external spacetime can be of both black-hole and naked-singularity type.
We consider also extremely compact stars allowing existence of trapped
null geodesics in their interior. We assume radiation of photons from
the surface at radius R, neutrinos from the whole compact star interior,
and their motion along radial null geodesics of the spacetime. In
dependency on the compact stars parameters b and R, the photon surface
redshift is related to the range of the neutrino internal redshift and
the signatures of the tidal charge and possible existence of extremely
compact stars are discussed. When both surface (photon) and internal
(neutrino) redshift are given by observations, both compact star
parameters R and b can be determined in the framework of our simple
model.
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Spectral fitting of the spin a ≡ cJ/GM2 in the
microquasar GRS 1915+105 estimate values higher than a = 0.98. However,
there are certain doubts about this (nearly) extremal number. Confirming
a high value of a > 0.9 would have significant concequences for the
theory of high-frequency quasiperiodic oscillations (HF QPOs). Here we
discuss its possible implications assuming several commonly used orbital
models of 3:2 HF QPOs. We show that the estimate of a > 0.9 is almost
inconsistent with two hot-spot (relativistic precession and tidal
disruption) models and the warped disc resonance model. In contrast, we
demonstrate that the epicyclic resonance and discoseismic models
assuming the c- and g-modes are favoured. We extend our discussion to
another two microquasars that display the 3:2 HF QPOs. The frequencies
of these QPOs scale roughly inversely to the microquasar masses, and the
differences in the individual spins, such as a = 0.9 compared to a =
0.7, represent a generic problem for most of the discussed geodesic 3:2
QPO models. To explain the observations of all the three microquasars by
one unique mechanism, the models would have to accommodate very large
non-geodesic corrections.
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We study gravitational redshift of photons and neutrinos radiated by the braneworld neutron or quark stars that are considered in the framework of the simple model of the internal spacetime with uniform distribution of energy density, and the external spacetime described by the Reissner-Nordström geometry characterized by the braneworld ``tidal'' charge b. For negative tidal charges, the external spacetime is of the black-hole type, while for positive tidal charges, the external spacetime can be of both black-hole and naked-singularity type. We consider also extremely compact stars allowing existence of trapped null geodesics in their interior. We assume radiation of photons from the surface at radius R, neutrinos from the whole compact star interior, and their motion along radial null geodesics of the spacetime. In dependency on the compact stars parameters b and R, the photon surface redshift is related to the range of the neutrino internal redshift and the signatures of the tidal charge and possible existence of extremely compact stars are discussed. When both surface (photon) and internal (neutrino) redshift are given by observations, both compact star parameters R and b can be determined in the framework of our simple model.
Read More
Spectral fitting of the spin a ≡ cJ/GM2 in the microquasar GRS 1915+105 estimate values higher than a = 0.98. However, there are certain doubts about this (nearly) extremal number. Confirming a high value of a > 0.9 would have significant concequences for the theory of high-frequency quasiperiodic oscillations (HF QPOs). Here we discuss its possible implications assuming several commonly used orbital models of 3:2 HF QPOs. We show that the estimate of a > 0.9 is almost inconsistent with two hot-spot (relativistic precession and tidal disruption) models and the warped disc resonance model. In contrast, we demonstrate that the epicyclic resonance and discoseismic models assuming the c- and g-modes are favoured. We extend our discussion to another two microquasars that display the 3:2 HF QPOs. The frequencies of these QPOs scale roughly inversely to the microquasar masses, and the differences in the individual spins, such as a = 0.9 compared to a = 0.7, represent a generic problem for most of the discussed geodesic 3:2 QPO models. To explain the observations of all the three microquasars by one unique mechanism, the models would have to accommodate very large non-geodesic corrections.
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Influence of cosmological constant on toroidal fluid configurations
around charged spherically symmetric black holes and naked singularities
is demostrated by study of perfect-fluid tori with uniform distribution
of specific angular momentum orbiting in the
Reissner-Nordström-(anti-)de Sitter spacetimes. Toroidal
configurations are allowed only in the spacetimes admitting existence of
stable circular geodesics. Configurations with marginally closed
equipotential (equipressure) surfaces crossing itself in a cusp allow
accretion (through the inner cusp) and/or excretion (through the outer
cusp) of matter from the toroidal configuration. Detailed classification
of the Reissner-Nordström-(anti-)de Sitter spacetimes according to
properties of the marginally stable tori is given. It is demonstrated
that in the Reissner-Nordström-de Sitter naked-singularity
spacetimes an interesting phenomenon of doubled tori can exist enabling
exchange of matter between two tori in both inward and outward
directions. In naked-singularity spacetimes the accretion onto the
central singularity is impossible due to existence of a potential
barrier.
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Exchange of dominance between twin kHz quasi-periodic oscillations
(QPOs) observed in some low-mass-X-ray-binaries (LMXBs) suggests the
possibility of a resonance between two oscillatory modes. We study the
behaviour of the effective gravitational potential around specific
resonant radii, and estimate the role of the higher-order terms
governing the non-linear, anharmonic forcing. We discuss the impact it
has on the mode amplitude in the linear and non-linear regimes. We also
discuss a related possibility of lowering of the neutron star mass
estimates from the highest observed QPO frequencies.
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Influence of cosmological constant on toroidal fluid configurations around charged spherically symmetric black holes and naked singularities is demostrated by study of perfect-fluid tori with uniform distribution of specific angular momentum orbiting in the Reissner-Nordström-(anti-)de Sitter spacetimes. Toroidal configurations are allowed only in the spacetimes admitting existence of stable circular geodesics. Configurations with marginally closed equipotential (equipressure) surfaces crossing itself in a cusp allow accretion (through the inner cusp) and/or excretion (through the outer cusp) of matter from the toroidal configuration. Detailed classification of the Reissner-Nordström-(anti-)de Sitter spacetimes according to properties of the marginally stable tori is given. It is demonstrated that in the Reissner-Nordström-de Sitter naked-singularity spacetimes an interesting phenomenon of doubled tori can exist enabling exchange of matter between two tori in both inward and outward directions. In naked-singularity spacetimes the accretion onto the central singularity is impossible due to existence of a potential barrier.
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Exchange of dominance between twin kHz quasi-periodic oscillations (QPOs) observed in some low-mass-X-ray-binaries (LMXBs) suggests the possibility of a resonance between two oscillatory modes. We study the behaviour of the effective gravitational potential around specific resonant radii, and estimate the role of the higher-order terms governing the non-linear, anharmonic forcing. We discuss the impact it has on the mode amplitude in the linear and non-linear regimes. We also discuss a related possibility of lowering of the neutron star mass estimates from the highest observed QPO frequencies.
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Current-carrying string loop dynamics in Schwarzschild-de Sitter
spacetimes characterized by the cosmological parameter
λ=(1)/(3)ΛM2 is investigated. With attention
concentrated to the axisymmetric motion of string loops it is shown that
the resulting motion is governed by the presence of an outer tension
barrier and an inner angular momentum barrier that are influenced by the
black hole gravitational field given by the mass M and the cosmic
repulsion given by the cosmological constant Λ. The gravitational
attraction could cause capturing of the string having low energy by the
black hole or trapping in its vicinity; with high enough energy, the
string can escape (scatter) to infinity. The role of the cosmic
repulsion becomes important in vicinity of the so-called static radius
where the gravitational attraction is balanced by the cosmic
repulsion—it is demonstrated both in terms of the effective
potential of the string motion and the basin boundary method reflecting
its chaotic character, that a potential barrier exists along the static
radius behind which no trapped oscillations may exist. The trapped
states of the string loops, governed by the interplay of the gravitating
mass M and the cosmic repulsion, are allowed only in Schwarzschild-de
Sitter spacetimes with the cosmological parameter
λ<λtrap˜0.00497. The trapped
oscillations can extend close to the radius of photon circular orbit,
down to rmt˜3.3M.
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Current-carrying string loop dynamics in Schwarzschild-de Sitter spacetimes characterized by the cosmological parameter λ=(1)/(3)ΛM2 is investigated. With attention concentrated to the axisymmetric motion of string loops it is shown that the resulting motion is governed by the presence of an outer tension barrier and an inner angular momentum barrier that are influenced by the black hole gravitational field given by the mass M and the cosmic repulsion given by the cosmological constant Λ. The gravitational attraction could cause capturing of the string having low energy by the black hole or trapping in its vicinity; with high enough energy, the string can escape (scatter) to infinity. The role of the cosmic repulsion becomes important in vicinity of the so-called static radius where the gravitational attraction is balanced by the cosmic repulsion—it is demonstrated both in terms of the effective potential of the string motion and the basin boundary method reflecting its chaotic character, that a potential barrier exists along the static radius behind which no trapped oscillations may exist. The trapped states of the string loops, governed by the interplay of the gravitating mass M and the cosmic repulsion, are allowed only in Schwarzschild-de Sitter spacetimes with the cosmological parameter λ<λtrap∼0.00497. The trapped oscillations can extend close to the radius of photon circular orbit, down to rmt∼3.3M.
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