We present results of the study of some astrophysical phenomena taking
place in the vicinity of superspinars, described by the Kerr naked
singularity spacetime, that are capable to explain optical and other
effects usually attributed to rotating black holes.
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Binary pulsars represent a good laboratory for testing field theories in
strong field regime. In such cases, the effects of non-linear terms in
these theories would be easily measured. In the present work, we discuss
an exact solution of Einstein's field equations, in a field representing
a binary compact object. The solution is a time-dependent one, which
reduces to Curzon field, in the static case. The equations of motion of
a test particle in the above mentioned field are formulated and solved.
These equations can be used to study the motion of a third body (or a
photon) in the field of a binary system.
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We present our recent studies of charged particles motion out of the
equatorial plane in strong gravitational and electromagnetic fields.
Within the general relativistic approach, we have demonstrated that the
interplay between gravitational and electromagnetic action may allow for
stable off-equatorial circular motion along the so-called halo orbits
near compact objects, such as rotating magnetic compact stars and Kerr
black holes immersed in an asymptotically uniform magnetic field of
external origin. Locations of halo orbits correspond to minima of the
two-dimensional effective potential, which exhibits several
qualitativelly different kinds of behaviour, reflecting the charge of
moving particles and orientation of the motion. Along with the study of
the halo motion itself, we have discussed the general motion in the
related off-equatorial potential lobes, demonstrating its chaoticness or
regularity in terms of the Poincaré surfaces of sections and
recurrence plots. A possible outlook of this study is to build a single
test particles model of putative circumpulsar discs consisting of
charged dust particles.
Institute of Physics and Astronomical Institute have been operated under
the projects MSM 4781305903 and AV 0Z10030501, and further supported by
the Centre for Theoretical Astrophysics LC06014 in the Czech Republic.
JK, VK and ZS thank the Czech Science Foundation (ref. P209/10/P190,
205/07/0052, 202/09/0772). OK acknowledges the doctoral student program
of the Czech Science Foundation (205/09/H033).
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We study optical phenomena related to the appearance of Keplerian
accretion discs orbiting Kerr superspinars predicted by string theory.
The superspinar exterior is described by standard Kerr naked singularity
geometry breaking the black hole limit on the internal angular momentum
(spin). We construct local photon escape cones for a variety of orbiting
sources that enable us to determine the superspinars silhouette in the
case of distant observers. We show that the superspinar silhouette
depends strongly on the assumed edge where the external Kerr spacetime
is joined to the internal spacetime governed by string theory and
significantly differs from the black hole silhouette. The appearance of
the accretion disc is strongly dependent on the value of the superspinar
spin in both their shape and frequency shift profile. Apparent extension
of the disc grows significantly with the growing spin, while the
frequency shift grows with the descending spin. This behaviour differs
substantially from the appearance of discs orbiting black holes enabling
thus, at least in principle, to distinguish clearly the Kerr
superspinars and black holes. In vicinity of a Kerr superspinar the
non-escaped photons have to be separated to those captured by the
superspinar and those being trapped in its strong gravitational field
leading to self-illumination of the disc that could even influence its
structure and cause self-reflection effect of radiation of the disc. The
amount of trapped photons grows with descending superspinar spin. We
thus can expect significant self-illumination effects in the field of
Kerr superspinars with near-extreme spin a ~ 1.
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The high-frequency quasi-periodic oscillations (HF QPOs) that appear in
the X-ray fluxes of low-mass X-ray binaries remain an unexplained
phenomenon. Among other ideas, it has been suggested that a non-linear
resonance between two oscillation modes in an accretion disc orbiting
either a black hole or a neutron star plays a role in exciting the
observed modulation. Several possible resonances have been discussed. A
particular model assumes resonances in which the disc-oscillation modes
have the eigenfrequencies equal to the radial and vertical epicyclic
frequencies of geodesic orbital motion. This model has been discussed
for black hole microquasar sources as well as for a group of neutron
star sources. Assuming several neutron (strange) star equations of state
and Hartle-Thorne geometry of rotating stars, we briefly compare the
frequencies expected from the model to those observed. Our comparison
implies that the inferred neutron star radius RNS is larger
than the related radius of the marginally stable circular orbit
rms for nuclear matter equations of state and spin
frequencies up to 800 Hz. For the same range of spin and a strange star
(MIT) equation of state, the inferrred radius is RNS ˜
rms. The “Paczyński modulation” mechanism
considered within the model requires that RNS <
rms. However, we find this condition to be fulfilled only for
the strange matter equation of state, masses below 1 M⊙,
and spin frequencies above 800 Hz. This result most likely falsifies the
postulation of the neutron star 3:2 resonant eigenfrequencies being
equal to the frequencies of geodesic radial and vertical epicyclic
modes. We suggest that the 3:2 epicyclic modes could stay among the
possible choices only if a fairly non-geodesic accretion flow is
assumed, or if a different modulation mechanism operates.
Read More
We present our recent studies of charged particles motion out of the equatorial plane in strong gravitational and electromagnetic fields. Within the general relativistic approach, we have demonstrated that the interplay between gravitational and electromagnetic action may allow for stable off-equatorial circular motion along the so-called halo orbits near compact objects, such as rotating magnetic compact stars and Kerr black holes immersed in an asymptotically uniform magnetic field of external origin. Locations of halo orbits correspond to minima of the two-dimensional effective potential, which exhibits several qualitativelly different kinds of behaviour, reflecting the charge of moving particles and orientation of the motion. Along with the study of the halo motion itself, we have discussed the general motion in the related off-equatorial potential lobes, demonstrating its chaoticness or regularity in terms of the Poincaré surfaces of sections and recurrence plots. A possible outlook of this study is to build a single test particles model of putative circumpulsar discs consisting of charged dust particles. Institute of Physics and Astronomical Institute have been operated under the projects MSM 4781305903 and AV 0Z10030501, and further supported by the Centre for Theoretical Astrophysics LC06014 in the Czech Republic. JK, VK and ZS thank the Czech Science Foundation (ref. P209/10/P190, 205/07/0052, 202/09/0772). OK acknowledges the doctoral student program of the Czech Science Foundation (205/09/H033).
Read More
We study optical phenomena related to the appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by string theory. The superspinar exterior is described by standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable us to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with the growing spin, while the frequency shift grows with the descending spin. This behaviour differs substantially from the appearance of discs orbiting black holes enabling thus, at least in principle, to distinguish clearly the Kerr superspinars and black holes. In vicinity of a Kerr superspinar the non-escaped photons have to be separated to those captured by the superspinar and those being trapped in its strong gravitational field leading to self-illumination of the disc that could even influence its structure and cause self-reflection effect of radiation of the disc. The amount of trapped photons grows with descending superspinar spin. We thus can expect significant self-illumination effects in the field of Kerr superspinars with near-extreme spin a ~ 1.
Read More
The high-frequency quasi-periodic oscillations (HF QPOs) that appear in the X-ray fluxes of low-mass X-ray binaries remain an unexplained phenomenon. Among other ideas, it has been suggested that a non-linear resonance between two oscillation modes in an accretion disc orbiting either a black hole or a neutron star plays a role in exciting the observed modulation. Several possible resonances have been discussed. A particular model assumes resonances in which the disc-oscillation modes have the eigenfrequencies equal to the radial and vertical epicyclic frequencies of geodesic orbital motion. This model has been discussed for black hole microquasar sources as well as for a group of neutron star sources. Assuming several neutron (strange) star equations of state and Hartle-Thorne geometry of rotating stars, we briefly compare the frequencies expected from the model to those observed. Our comparison implies that the inferred neutron star radius RNS is larger than the related radius of the marginally stable circular orbit rms for nuclear matter equations of state and spin frequencies up to 800 Hz. For the same range of spin and a strange star (MIT) equation of state, the inferrred radius is RNS ∼ rms. The “Paczyński modulation” mechanism considered within the model requires that RNS < rms. However, we find this condition to be fulfilled only for the strange matter equation of state, masses below 1 M⊙, and spin frequencies above 800 Hz. This result most likely falsifies the postulation of the neutron star 3:2 resonant eigenfrequencies being equal to the frequencies of geodesic radial and vertical epicyclic modes. We suggest that the 3:2 epicyclic modes could stay among the possible choices only if a fairly non-geodesic accretion flow is assumed, or if a different modulation mechanism operates.
Read More
Accretion onto black holes and compact stars brings material in a zone
of strong gravitational and electromagnetic fields. We study dynamical
properties of motion of electrically charged particles forming a highly
diluted medium (a corona) in the regime of strong gravity and
large-scale (ordered) magnetic field. We start our work from a system
that allows regular motion, then we focus on the onset of chaos. To this
end, we investigate the case of a rotating black hole immersed in a
weak, asymptotically uniform magnetic field. We also consider a magnetic
star, approximated by the Schwarzschild metric and a test magnetic field
of a rotating dipole. These are two model examples of systems permitting
energetically bound, off-equatorial motion of matter confined to the
halo lobes that encircle the central body. Our approach allows us to
address the question of whether the spin parameter of the black hole
plays any major role in determining the degree of the chaoticness. To
characterize the motion, we construct the recurrence plots (RPs) and we
compare them with Poincaré surfaces of section. We describe the
RPs in terms of the recurrence quantification analysis, which allows us
to identify the transition between different dynamical regimes. We
demonstrate that this new technique is able to detect the chaos onset
very efficiently and provide its quantitative measure. The chaos
typically occurs when the conserved energy is raised to a sufficiently
high level that allows the particles to traverse the equatorial plane.
We find that the role of the black hole spin in setting the chaos is
more complicated than initially thought.
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We study the motion of charged test particles around a Kerr black hole
immersed in the asymptotically uniform magnetic field, concluding that
off-equatorial stable orbits are allowed in this system. Being
interested in dynamical properties of these astrophysically relevant
orbits we employ rather novel approach based on the analysis of
recurrences of the system to the vicinity of its previous states. We use
recurrence plots (RPs) as a tool to visualize recurrences of the
trajectory in the phase space. Construction of RPs is simple and
straightforward regardless of the dimension of the phase space, which is
a major advantage of this approach when compared to the ``traditional''
methods of the numerical analysis of dynamical systems (for instance the
visual survey of Poincaré surfaces of section, evaluation of the
Lyapunov spectra etc.). We show that RPs and their quantitative measures
(obtained from recurrence quantification analysis-RQA) are powerful
tools to detect dynamical regime of motion (regular vs. chaotic) and
precisely locate the transitions between these regimes.
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