In this Proceedings, the talks presented during workshops Proceedings of
RAGtime 8/9: Workshops on black holes and neutron stars, 15-19/19-21
September 2006/2007, Hradec nad Moravicí, Opava, Czech Republic
are collected.
Relativistic Astrophysics Group (RAG) at the Institute of Physics, the
Faculty of Philosophy and Science of the Silesian University in Opava,
started a series of Workshops on Black Holes and Neutron Stars called
RAGtime in 1999. The purpose of the workshops was to provide an
opportunity for the presentation and discussion of recent developments
in the field of relativistic astrophysics related to accretion processes
onto black holes and neutron stars, and to general physical phenomena
connected to the properties of black holes and their vicinity, and the
internal structure of neutron stars or quark stars, as they were
obtained by collaborating research groups at the Silesian University in
Opava, the Faculty of Mathematics and Physics of Charles University in
Prague, the International School for Advanced Studies in Trieste, the
Institute of Astrophysics at University of Oxford, the Department of
Astrophysics of Göteborg University, the Institute of Physics at
the University of Bergen, the Institute of Astronomy of the Polish
Academy of Science, and other remarkable institutes.
The RAGtime workshops are also vitally important for students of
theoretical physics and/or astrophysics at the Silesian University in
Opava, because they have a unique opportunity to be regularly in direct
contact with the most recent results of relativistic astrophysics and
they also have a possibility to discuss problems with leading
astrophysicists of worldwide reputation like Marek Abramowicz, John
Miller, Jeff McClintock, Ron Remillard, Włodzimierz
Kluźniak, Shoji Kato, Luciano Rezzolla, Vladimír Karas,
Petr Hadrava and others.
We would like to thank all the authors for careful preparation of their
contributions. We are also indebted to the Ministry of Education of the
Czech Republic for providing financial support in the framework of the
Grant MSM 4781305903, and all other sponsors for their contributions to
the successful course of the last RAGtime meetings.
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Several models of the X-ray flux modulation observed in the low-mass
X-ray binaries (LMXBs), in particular the neutron star LMXBs, deal with
Keplerian and epicyclic frequencies of geodesic orbital motion. We
discuss non-geodesic corrections to the orbital and epicyclic
frequencies of charged test particles caused by presence of a neutron
star magnetic field. The magnetic field is considered to be generated by
an intrinsic static dipole magnetic moment of a neutron star represented
by the Schwarzschild geometry. We present fully general relativistic
formulae for the orbital and epicyclic frequencies, obtained using the
appropriate equations governing perturbations of the circular motion.
The most significant correction arises for the radial epicyclic
frequency. The zero point of the corrected radial epicyclic frequency
defines radius of the effective innermost stable circular orbit
"(EISCO)." The dipole magnetic field also violates equality of the
orbital and vertical epicyclic frequencies corresponding to the
spherical symmetry of the Schwarzschild geometry.
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In the framework of the brany cosmology, the influence of the bulk
spacetime on the black holes in the brane can be described by the so
called tidal charge, which has a character similar to the charge
parameter in the standard black-hole solutions, but can be both positive
and negative. We discuss the influence of the tidal charge on the test
particle and photon motion in the spherically symmetric spacetimes with
a nonzero cosmological constant using the analysis realised in the case
of Reissner-Nordström-(anti-)de Sitter spacetimes. We concentrate
on the properties of circular geodesics as they play an important role
in determining the brane properties of both thin and thick accretion
discs. Some implications of the tidal charge influence are outlined.
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Being inspired by existence of non-equatorial circular (halo) stable
orbits of charged particles in composite of gravitational, dipole
magnetic and co-rotational electric fields near some planets, we find
possible existence of these orbits in strong gravitational fields
described by the Kerr-Newman geometry. By using the general relativistic
inertial forces formalism combined with the effective potential
approach, we show that the stable halo orbits do exist there. But it
seems that they are out of the astrophysical importance, being hidden
under the inner black-hole horizon, or appearing in the naked
singularity spacetimes.
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In the framework of the brany models, rotating black holes are described
by the Kerr metric with a tidal charge representing the influence of the
non-local gravitational (tidal) effects of the bulk space Weyl tensor
onto the black hole spacetime. Here we study the influence of the tidal
charge onto profiled spectral lines generated by radiating tori orbiting
in vicinity of a rotating black hole. We show that with lowering the
negative tidal charge of the black hole, the profiled line becomes to be
flatter and wider.
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In the framework of the brany models, rotating black holes are described
by the Kerr metric with a tidal charge representing the influence of the
shear effects of the bulk space Weyl tensor onto the black hole
spacetime. Here we study the influence of the tidal charge onto some
optical phenomena in rotating black hole spacetimes. The photon motion
is given in terms of constants of the geodetical motion related to the
spacetime symmetries and escape photon cones are given for special
families of locally non-rotating, circular geodetical and radially
freely-falling observers. The shadow of a rotating black hole and the
shape of an equatorial thin accretion disk are given and classified in
terms of the black hole rotational and tidal parameters.
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The "Extended Orbital Resonance Model", i.e., the idea of oscillations
induced by the hump of the orbital velocity profile (related to the
locally non-rotating frames - LNRF), which are proposed to excite the
oscillations of Keplerian discs around near-extreme Kerr black holes
with epicyclic frequencies, is used to estimate the mass and spin of
three near-extreme Kerr black hole candidates GRS 1915+105, XTE
J1650-500, and NGC 5408 X-1. The hump-induced oscillations are
characterized by the so-called "humpy frequency", and a non-linear
resonant coupling between these and epicyclic oscillations is expected.
It it shown that the quasiperiodic variability (QPOs) observed in these
sources can be matched with the proposals of the model, giving for the
mass and spin of their black holes values consistent with the other
observationally-established estimates.
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Humpy radial profiles of the LNRF-related orbital velocity was found for
a circular motion of test particles and test perfect fluid orbiting
near-extreme Kerr black holes and Kerr naked singularities. Preliminary
results of an analogical study of the circular motion of test particles
and fluid in the Kerr-(anti-)de Sitter spacetimes are presented.
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Trapping of neutrinos in brany extremely compact stars is studied, using
the simplest model with massless neutrinos and uniform-density star. The
influence of bulk tension on the trapping is given for two solutions,
namely with the Reissner-Nordström-type of geometry described by a
single brany parameter, and the second one determined by the energy
density of the star and the brane tension.
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High-frequency QPOs in neutron-star binary systems could be explained by
models based on parametric or forced resonance between oscillation modes
of the accretion disc around the neutron star (with frequencies related
to the Keplerian and epicyclic frequencies of the disc) and the relative
rotational motion between the disc material and either the neutron star
or the binary companion.
Using Newtonian theory, we discuss the possibility of forced resonant
phenomena being excited by means of gravitational perturbations coming
from surface features on the neutron star and from the companion star.
For each potential perturbation source, we have determined the Fourier
decomposition of the gravitational perturbing forces acting on disc
elements in the radial and vertical directions. The analysis presented
for the binary companion can be applied also to black hole systems but
the surface features would not be present in that case.
The oscillations induced by the binary partner are of a different
character from those which would be induced by a mountain or by the
accretion columns. In the case of symmetric accretion columns, the
excitation frequency in the radial direction is twice that in the
vertical direction and higher order modes could be relevant for parts of
the disc very close to the neutron star. In this inner region, the
influence of the accretion columns is greater than that of the binary
companion for producing forced oscillations in both the radial and
vertical directions; in the intermediate part, the vertical oscillations
are induced by the accretion columns while the radial oscillations are
excited by the binary partner (if appropriate conditions are fulfilled);
sufficiently far from the neutron star, the binary companion has the
greater effect. The limits on the magnitude of neutron star deformations
given by the possibility to excite observable QPOs coincide with
restrictions coming from (non-)observations of gravitational waves by
LIGO.
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