Estimations of black hole spin in the three Galactic microquasars GRS 1915+105, GRO J1655-40, and XTE J1550-564 have been carried out based on spectral and timing X-ray measurements and various theoretical concepts. Among others, a non-linear resonance between axisymmetric epicyclic oscillation modes of an accretion disc around a Kerr black hole has been considered as a model for the observed high-frequency quasi-periodic oscillations (HF QPOs). Estimates of spin predicted by this model have been derived based on the geodesic approximation of the accreted fluid motion. Here we assume accretion flow described by the model of a pressure-supported torus and carry out related corrections to the mass-spin estimates. We find that for dimensionless black hole spin a ≡ cJ/GM2 ≲ 0.9, the resonant eigenfrequencies are very close to those calculated for the geodesic motion. Their values slightly grow with increasing torus thickness. These findings agree well with results of a previous study carried out in the pseudo-Newtonian approximation. The situation becomes different for a ≳ 0.9, in which case the resonant eigenfrequencies rapidly decrease as the torus thickness increases. We conclude that the assumed non-geodesic effects shift the lower limit of the spin, implied for the three microquasars by the epicyclic model and independently measured masses, from a ~ 0.7 to a ~ 0.6. Their consideration furthermore confirms compatibility of the model with the rapid spin of GRS 1915+105 and provides highly testable predictions of the QPO frequencies. Individual sources with a moderate spin (a ≲ 0.9) should exhibit a smaller spread of the measured 3:2 QPO frequencies than sources with a near-extreme spin (a ~ 1). This should be further examined using the large amount of high-resolution data expected to become available with the next generation of X-ray instruments, such as the proposed Large Observatory for X-ray Timing (LOFT).
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The neutral particle motion around rotating regular black hole that was
derived from the Ayón-Beato-García (ABG) black hole
solution by the Newman-Janis algorithm in the preceding paper (Toshmatov
et al., Phys. Rev. D, 89:104017, 2014) has been studied. The
dependencies of the ISCO (innermost stable circular orbits along
geodesics) and unstable orbits on the value of the electric charge of
the rotating regular black hole have been shown. Energy extraction from
the rotating regular black hole through various processes has been
examined. We have found expression of the center of mass energy for the
colliding neutral particles coming from infinity, based on the BSW
(Baňados-Silk-West) mechanism. The electric charge Q of rotating
regular black hole decreases the potential of the gravitational field as
compared to the Kerr black hole and the particles demonstrate less bound
energy at the circular geodesics. This causes an increase of efficiency
of the energy extraction through BSW process in the presence of the
electric charge Q from rotating regular black hole. Furthermore, we have
studied the particle emission due to the BSW effect assuming that two
neutral particles collide near the horizon of the rotating regular
extremal black hole and produce another two particles. We have shown
that efficiency of the energy extraction is less than the value 146.6 %
being valid for the Kerr black hole. It has been also demonstrated that
the efficiency of the energy extraction from the rotating regular black
hole via the Penrose process decreases with the increase of the electric
charge Q and is smaller in comparison to 20.7 % which is the value for
the extreme Kerr black hole with the specific angular momentum a= M.
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The neutral particle motion around rotating regular black hole that was derived from the Ayón-Beato-García (ABG) black hole solution by the Newman-Janis algorithm in the preceding paper (Toshmatov et al., Phys. Rev. D, 89:104017, 2014) has been studied. The dependencies of the ISCO (innermost stable circular orbits along geodesics) and unstable orbits on the value of the electric charge of the rotating regular black hole have been shown. Energy extraction from the rotating regular black hole through various processes has been examined. We have found expression of the center of mass energy for the colliding neutral particles coming from infinity, based on the BSW (Baňados-Silk-West) mechanism. The electric charge Q of rotating regular black hole decreases the potential of the gravitational field as compared to the Kerr black hole and the particles demonstrate less bound energy at the circular geodesics. This causes an increase of efficiency of the energy extraction through BSW process in the presence of the electric charge Q from rotating regular black hole. Furthermore, we have studied the particle emission due to the BSW effect assuming that two neutral particles collide near the horizon of the rotating regular extremal black hole and produce another two particles. We have shown that efficiency of the energy extraction is less than the value 146.6 % being valid for the Kerr black hole. It has been also demonstrated that the efficiency of the energy extraction from the rotating regular black hole via the Penrose process decreases with the increase of the electric charge Q and is smaller in comparison to 20.7 % which is the value for the extreme Kerr black hole with the specific angular momentum a= M.
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We study scalar, electromagnetic, and gravitational test fields in the
Hayward, Bardeen, and Ayón-Beato-García regular black hole
spacetimes and demonstrate that the test fields are stable in all of
these spacetimes. Using the sixth order WKB approximation of the linear
"axial" perturbative scheme, we determine dependence of the quasinormal
mode (QNM) frequencies on the characteristic parameters of the test
fields and the spacetime charge parameters of the regular black holes.
We give also the grey body factors, namely, the transmission and
reflection coefficients of scattered scalar, electromagnetic, and
gravitational waves. We show that the damping of the quasinormal modes
in regular black hole spacetimes is suppressed compared to the case of
Schwarzschild black holes, and increasing the charge parameter of the
regular black holes increases reflection and decreases the transmission
factor of the incident waves for each of the test fields.
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We study scalar, electromagnetic, and gravitational test fields in the Hayward, Bardeen, and Ayón-Beato-García regular black hole spacetimes and demonstrate that the test fields are stable in all of these spacetimes. Using the sixth order WKB approximation of the linear "axial" perturbative scheme, we determine dependence of the quasinormal mode (QNM) frequencies on the characteristic parameters of the test fields and the spacetime charge parameters of the regular black holes. We give also the grey body factors, namely, the transmission and reflection coefficients of scattered scalar, electromagnetic, and gravitational waves. We show that the damping of the quasinormal modes in regular black hole spacetimes is suppressed compared to the case of Schwarzschild black holes, and increasing the charge parameter of the regular black holes increases reflection and decreases the transmission factor of the incident waves for each of the test fields.
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The model of current-carrying string loop oscillations is tested to
explain the special set of frequencies related to the high-frequency
quasiperiodic oscillations (HF QPOs) observed recently in the low-mass
X-ray binary XTE J1701-407 containing a neutron star. The external
geometry of the neutron star is approximated by the Kerr geometry,
introducing errors not exceeding 10 % for slowly rotating massive
neutron stars. The frequencies of the radial and vertical string loop
oscillations are then governed by the mass and dimensionless spin of the
neutron star, and by the dimensionless parameter describing combined
effects of the string loop tension and its angular momentum. It is
explicitly demonstrated that the string-loop oscillation model can
explain the observed kHz frequencies for the neutron star parameters
restricted to the intervals and . However, the stringy parameter cannot
be the same for all the three HF QPO observations in the XTE J1701-407
source; the limits on the acceptable values of are given in dependence
on the spacetime parameters and.
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The model of current-carrying string loop oscillations is tested to explain the special set of frequencies related to the high-frequency quasiperiodic oscillations (HF QPOs) observed recently in the low-mass X-ray binary XTE J1701-407 containing a neutron star. The external geometry of the neutron star is approximated by the Kerr geometry, introducing errors not exceeding 10 % for slowly rotating massive neutron stars. The frequencies of the radial and vertical string loop oscillations are then governed by the mass and dimensionless spin of the neutron star, and by the dimensionless parameter describing combined effects of the string loop tension and its angular momentum. It is explicitly demonstrated that the string-loop oscillation model can explain the observed kHz frequencies for the neutron star parameters restricted to the intervals and . However, the stringy parameter cannot be the same for all the three HF QPO observations in the XTE J1701-407 source; the limits on the acceptable values of are given in dependence on the spacetime parameters and.
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We study acceleration of current-carrying string loops governed by
presence of an outer tension barrier and an inner angular momentum
barrier in the field of black holes. Relativistic current carrying
strings moving axisymmetrically along the axis of a black hole could in
a simplified way represent plasma that exhibits associated string-like
behavior. We demonstrate that string loops can be scattered near the
black hole horizon and the energy of string oscillations can be
efficiently converted to the energy of their linear motion. Such a
transmutation effect can potentially represent acceleration of jets in
active galactic nuclei and microquasars.
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Kerr naked singularities (superspinars) have to be efficiently converted
to a black hole due to accretion from Keplerian discs. In the final
stages of the conversion process the near-extreme Kerr naked
singularities (superspinars) provide a variety of extraordinary physical
phenomena. Such superspinning Kerr geometries can serve as an efficient
accelerator for extremely high-energy collisions enabling direct and
clear demonstration of the outcomes of the collision processes. We shall
discuss the efficiency and visibility of the ultra-high energy
collisions in the deepest parts of the gravitational well of
superspinning near-extreme Kerr geometries for the whole variety of
particles freely falling from infinity. We demonstrate that the ultra
high-energy processes can be obtained with no fine tuning of the motion
constants and the products of the collision can escape to infinity with
efficiency higher than in the case of the near-extreme black holes.
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We investigate possible signatures of a Kerr naked singularity
(superspinar) in the profiled spectral lines of radiation emitted by
monochromatically and isotropically radiating point sources forming a
Keplerian ring or disc around such compact object. We have found out
that the profiled spectral line of the radiating Keplerian ring can be
splitted into two parts due to the fact that there is no event horizon
in the naked singularity spacetimes.
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