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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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 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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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 present a Resonant Switch (RS) model of twin peak high-frequency
quasi-periodic oscillations (HF QPOs), assuming switch of twin
oscillations at a resonant point, where frequencies of the upper and
lower oscillations νU and νL become to be
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 enables to
determine range of allowed values of spin a and mass M of the neutron
star located at the atoll source 4U 1636-53 where two resonant points
are observed at frequency ratios νU : νL =
3:2, 5:4.
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We consider twin-peak quasi-periodic oscillations (QPOs) observed in the
accreting low-mass neutron star (NS) binaries and explore restrictions
to NS properties that are implied by various QPO models. For each model
and each source, the consideration results in a specific relation
between the NS mass M and the angular-momentum j rather than in their
single preferred combination. Furthermore, the inferred restrictions on
NS properties (or QPO models) are weaker for the low-frequency sources
than for the high-frequency sources.
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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.
Read More
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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