Using known frequencies of the twin peak high-frequency quasiperiodic oscillations (HF QPOs) and known mass M of the central black hole, the black-hole dimensionless spin a can be determined assuming a concrete version of the resonance model. However, large range of observationally limited values of the black hole mass implies a low precision of the spin estimates. We discuss the possibility of higher precision of the black hole spin a measurements in the framework of multi-resonance model inspired by observations of more than two HF QPOs in some black hole sources. We determine the spin and mass dependence of the twin peak frequencies with a general rational ratio n:m assuming a non-linear resonance of oscillations with the epicyclic and Keplerian frequencies or their combinations. In the multi-resonant model, the twin peak resonances are combined properly to give the observed frequency set. We focus on the special case of duplex frequencies, when the top, bottom, or mixed frequency is common at two different radii where the resonances occur giving triple frequency sets.
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We test observational consequences of primordial Kerr superspinars indicated by string theories. We demonstrate that Kerr superspinars can serve as ultra-high energy accelerators and explore specific optical phenomena related to accretion discs orbiting them.
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The black hole mass and spin estimates assuming various specific models of the 3 : 2 high frequency quasi-periodic oscillations (HF QPOs) have been carried out in Török et al. (2005, 2011). Here we briefly summarize some current points. Spectral fitting of the spin a ≡ cJ/GM 2 in the microquasar GRS 1915 + 105 reveals that this system can contain a near extreme rotating black hole (e.g., McClintock et al., 2011). Confirming the high value of the spin would have significant consequences for the theory of the HF QPOs. The estimate of a > 0.9 is almost inconsistent with the relativistic precession (RP), tidal disruption (TD), and the warped disc (WD) model. The epicyclic resonance (Ep) and discoseismic models assuming the c- and g- modes are instead favoured. However, consideration of all three microquasars that display the 3 : 2 HF QPOs leads to a serious puzzle because the differences in the individual spins, such as a = 0.9 compared to a = 0.7, represent a generic problem almost for any unified orbital 3:2 QPO model.
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We study non-geodesic corrections to the quasicircular motion of charged test particles in the field of magnetized slowly rotating neutron stars. The gravitational field is approximated by the Lense-Thirring geometry, the magnetic field is of the standard dipole character. Using a fully-relativistic approach we determine influence of the electromagnetic interaction (both attractive and repulsive) on the quasicircular motion. We focus on the behaviour of the orbital and epicyclic frequencies of the motion. Components of the four-velocity of the orbiting charged test particles are obtained by numerical solution of equations of motion, the epicyclic frequencies are obtained by using the standard perturbative method. The role of the combined effect of the neutron star magnetic field and its rotation in the character of the orbital and epicyclic frequencies is discussed.
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Twin peak quasi-periodic oscillations (QPOs) appear in the X-ray
power-density spectra of several accreting low-mass neutron star (NS)
binaries. Observations of the peculiar Z-source Circinus X-1 display
unusually low QPO frequencies. Using these observations, we have
previously considered the relativistic precession (RP) twin peak QPO
model to estimate the mass of the central NS in Circinus X-1. We have
shown that such an estimate results in a specific mass-angular-momentum
(M - j) relation rather than a single preferred combination of M and j.
Here we confront our previous results with another binary, the atoll
source 4U 1636-53 that displays the twin peak QPOs at very high
frequencies, and extend the consideration to various twin peak QPO
models. In analogy to the RP model, we find that these imply their own
specific M - j relations. We explore these relations for both sources
and note differences in the χ2 behavior that represent a
dichotomy between high- and low-frequency sources. Based on the RP
model, we demonstrate that this dichotomy is related to a strong
variability of the model predictive power across the frequency plane.
This variability naturally comes from the radial dependence of
characteristic frequencies of orbital motion. As a consequence, the
restrictions on the models resulting from observations of low-frequency
sources are weaker than those in the case of high-frequency sources.
Finally we also discuss the need for a correction to the RP model and
consider the removing of M - j degeneracies, based on the twin peak
QPO-independent angular momentum estimates.
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Resonant Switch (RS) model of twin peak high-frequency quasi-periodic
oscillations (HF QPOs) assumes switch of twin oscillations at a resonant
point where frequencies of the upper and lower oscillations
νU and νL become commensurable and the twin
oscillations change from one pair of the oscillating modes
(corresponding to a specific model of HF QPOs) to some other pair due to
non-linear resonant phenomena. The RS model is used to determine range
of allowed values of spin a and mass M of the neutron star located in
the atoll source 4U 1636-53 where two resonant points are observed at
frequency ratios νU:νL=3:2, 5:4. We consider
the standard specific models of the twin oscillations based on the
orbital and epicyclic geodetical frequencies. The resonant points are
determined by the energy switch effect exhibited by the vanishing of the
amplitude difference of the upper and lower oscillations. The predicted
ranges of the neutron star parameters are strongly dependent on the twin
modes applied in the RS model. We demonstrate that for some of the
oscillatory modes used in the RS model the predicted parameters of the
neutron star are unacceptable. Among acceptable RS models the most
promising are those combining the Relativistic Precession and the Total
Precession frequency relations or their modifications.
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Twin peak quasi-periodic oscillations (QPOs) appear in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. Observations of the peculiar Z-source Circinus X-1 display unusually low QPO frequencies. Using these observations, we have previously considered the relativistic precession (RP) twin peak QPO model to estimate the mass of the central NS in Circinus X-1. We have shown that such an estimate results in a specific mass-angular-momentum (M - j) relation rather than a single preferred combination of M and j. Here we confront our previous results with another binary, the atoll source 4U 1636-53 that displays the twin peak QPOs at very high frequencies, and extend the consideration to various twin peak QPO models. In analogy to the RP model, we find that these imply their own specific M - j relations. We explore these relations for both sources and note differences in the χ2 behavior that represent a dichotomy between high- and low-frequency sources. Based on the RP model, we demonstrate that this dichotomy is related to a strong variability of the model predictive power across the frequency plane. This variability naturally comes from the radial dependence of characteristic frequencies of orbital motion. As a consequence, the restrictions on the models resulting from observations of low-frequency sources are weaker than those in the case of high-frequency sources. Finally we also discuss the need for a correction to the RP model and consider the removing of M - j degeneracies, based on the twin peak QPO-independent angular momentum estimates.
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Resonant Switch (RS) model of twin peak high-frequency quasi-periodic oscillations (HF QPOs) assumes switch of twin oscillations at a resonant point where frequencies of the upper and lower oscillations νU and νL become commensurable and the twin oscillations change from one pair of the oscillating modes (corresponding to a specific model of HF QPOs) to some other pair due to non-linear resonant phenomena. The RS model is used to determine range of allowed values of spin a and mass M of the neutron star located in the atoll source 4U 1636-53 where two resonant points are observed at frequency ratios νU:νL=3:2, 5:4. We consider the standard specific models of the twin oscillations based on the orbital and epicyclic geodetical frequencies. The resonant points are determined by the energy switch effect exhibited by the vanishing of the amplitude difference of the upper and lower oscillations. The predicted ranges of the neutron star parameters are strongly dependent on the twin modes applied in the RS model. We demonstrate that for some of the oscillatory modes used in the RS model the predicted parameters of the neutron star are unacceptable. Among acceptable RS models the most promising are those combining the Relativistic Precession and the Total Precession frequency relations or their modifications.
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We investigate the role of the tidal charge in orbital models of
high-frequency quasiperiodic oscillations (QPOs) observed in neutron
star binary systems. We show how the standard relativistic precession
(RP) model modified by the tidal charge fits the observational data,
giving estimates of the allowed values of the tidal charge and the brane
tension based on the processes going in the vicinity of neutron stars.
We compare our strong field regime restrictions with those given in the
weak field limit of solar system experiments.
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Using known frequencies of the twin peak quasiperiodic oscillations
(QPOs) and the known mass of the central black hole, the black hole
dimensionless spin a can be determined, assuming a concrete version of
the orbital resonance model. However, because of large range of
observationally limited values of the black hole mass, its spin can be
estimated with a low precision only. Higher precision of the black hole
dimensionless spin measurement is possible in the framework of
multi-resonance model of QPOs inspired by complex high-frequency QPO
patterns observed in some black hole and neutron star systems.
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