In a series of works - Török et al. (2010, 2012a) and Urbanec et al. (2010a) - we explored restrictions to neutron star properties that are implied by various models of twin-peak quasi-periodic oscillations. Here we sketch an attempt to confront the obtained mass-angular-momentum relations and limits on neutron star compactness with the parameters estimated by assuming various equations of state and the spin frequency of the atoll source 4U 1636-53.
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A Newtonian model of non-conductive, charged, perfect fluid tori orbiting in combined spherical gravitational and dipolar magnetic fields is presented and stationary, axisymmetric toroidal structures are analyzed. Matter in such tori exhibits a purely circulatory motion and the resulting convection carries charges into permanent rotation around the symmetry axis. As a main result, we demonstrate the possible existence of off-equatorial charged tori and equatorial tori with cusps that also enable outflows of matter from the torus in the Newtonian regime. These phenomena qualitatively represent a new consequence of the interplay between gravity and electromagnetism. From an astrophysical point of view, our investigation can provide insight into processes that determine the vertical structure of dusty tori surrounding accretion disks.
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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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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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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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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.
Read More
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.
Read More
