String theory predicts the existence of extremely compact objects spinning faster than Kerr black holes. The spacetime exterior to such superspinars is described by Kerr naked singularity geometry breaking the black-hole limit on the internal angular momentum. We demonstrate that the conversion of Kerr superspinars into a near-extreme black hole due to an accretion counterrotating Keplerian disc is much more effective in comparison with the case of a corotating one since both the accreted rest mass necessary for conversion and the evolution time of conversion are by orders smaller for counterrotating discs. The conversion time of Kerr superspinars is given for several accretion regimes, and it is shown that the self-regulated accretion flow implies fastest evolution to the black-hole state. In the final stages of the conversion, Kerr superspinars can serve as very efficient particle accelerators in the region where the black-hole horizon forms.
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String theory predicts the existence of extremely compact objects
spinning faster than Kerr black holes. The spacetime exterior to such
superspinars is described by Kerr naked singularity geometry breaking
the black-hole limit on the internal angular momentum. We demonstrate
that the conversion of Kerr superspinars into a near-extreme black hole
due to an accretion counterrotating Keplerian disc is much more
effective in comparison with the case of a corotating one since both the
accreted rest mass necessary for conversion and the evolution time of
conversion are by orders smaller for counterrotating discs. The
conversion time of Kerr superspinars is given for several accretion
regimes, and it is shown that the self-regulated accretion flow implies
fastest evolution to the black-hole state. In the final stages of the
conversion, Kerr superspinars can serve as very efficient particle
accelerators in the region where the black-hole horizon forms.
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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.
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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.
Read More
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
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.
Read More
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.
Read More
