We discuss the possibility that high-frequency QPOs in neutron-star binary systems may result from forced resonant oscillations of matter in the innermost parts of the accretion disc, excited by gravitational perturbations coming from asymmetries of the neutron star or from the companion star. We find that neutron-star asymmetries could, in principle, be effective for inducing both radial and vertical oscillations of relevant amplitude while the binary companion might possibly produce significant radial oscillations but not vertical ones. Misaligned neutron-star quadrupole moments of a size advocated elsewhere for explaining limiting neutron star periods could be large enough also for the present purpose.
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We discuss the possibility that high-frequency QPOs in neutron-star
binary
systems may result from forced resonant oscillations of matter in the
innermost parts of the accretion disc, excited by gravitational
perturbations
coming from asymmetries of the neutron star or from the companion star.
We
find that neutron-star asymmetries could, in principle, be effective for
inducing both radial and vertical oscillations of relevant amplitude
while
the binary companion might possibly produce significant radial
oscillations
but not vertical ones. Misaligned neutron-star quadrupole moments of a
size
advocated elsewhere for explaining limiting neutron star periods could
be
large enough also for the present purpose.
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Fitting the observational data of the twin peak kHz quasiperiodic oscillations (QPO) from low mass X-ray binaries (LMXBs) by the relativistic precession model gives a substantially higher neutron star mass estimate, M~2 Msolar, than the ``canonical value'', M~1.4 Msolar. Using a fully general relativistic approach we discuss the non-geodesic corrections to the orbital and epicyclic frequencies of slightly charged circularly orbiting test particles caused by the presence of a neutron star magnetic field. We show that consideration of such non-geodesic corrections can bring down the neutron star mass estimate and improve the quality of twin peak QPO data fits based on relativistic precession frequency relations.
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Humpy LNRF-velocity profiles were found for both Keplerian thin discs and marginally stable thick discs orbiting almost extreme Kerr black holes, being located just above the innermost stable circular orbit. In thick marginally stable discs, the global character of this phenomenon is given by topology changes of equivelocity (von Zeipel) surfaces, as the standard cylindrical von Zeipel surfaces are transformed into toroidal surfaces existing around the circle corresponding to the local minimum of the equatorial humpy LNRF-velocity profile. These phenomena could indicate an instability in the disc triggering so-called "humpy" oscillations, the frequency of which is estimated by the maximum positive gradient of the humpy velocity profile in terms of the proper radial distance. The humpy oscillations could force epicyclic oscillations, if appropriate resonance conditions are satisfied.
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Basic properties of equipotential (equipressure) surfaces in test barotropic perfect fluid tori with uniform distribution of the specific angular momentum orbiting Kerr-de Sitter black holes are summarized. The central mass-densities of adiabatic non-relativistic tori, for which the approximation of test fluid is adequate, are given and compared with the typical densities of Giant Molecular Clouds.
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The Sturm-Liouville eigenvalue equation for eigenmodes of the radial oscillations is determined for spherically symmetric perfect fluid configurations in spacetimes with a nonzero cosmological constant and applied in the cases of configurations with uniform distribution of energy density and polytropic spheres. It is shown that a repulsive cosmological constant rises the critical adiabatic index and decreases the critical radius under which the dynamical instability occurs.
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Trapping of neutrinos in extremely compact stars containing trapped null geodesics is studied. We calculated the ratio of produced to trapped neutrinos in the simplest model of uniform density stars. This gives the upper limit on trapping coefficients in real objects.
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We developed realistic fully general relativistic computer code for simulation of optical projection in a strong, spherically symmetric gravitational field. Standard theoretical analysis of optical projection for an observer in the vicinity of a Schwarzschild black hole is extended to black hole spacetimes with a repulsive cosmological constant, i.e, Schwarzschild-de Sitter (SdS) spacetimes. Influence of the cosmological constant is investigated for static observers and observers radially free-falling from static radius. Simulation includes effects of gravitational lensing, multiple images, Doppler and gravitational frequency shift, as well as the amplification of intensity. The code generates images of static observers sky and a movie simulations for radially free-falling observers. Techniques of parallel programming are applied to get high performance and fast run of the simulation code.
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Results of investigation of the behaviour of inertial forces related to the optical reference geometry in the Kerr-de Sitter spacetimes and the features of the embedding diagrams of the geometry are summarized.
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