Estimates of the black hole mass M and the dimensionless spin a in the
microquasar GRO J1655-40 implied by strong gravity effects related to
the timing and spectral measurements are controversial, if the mass
restriction determined by the dynamics related to independent optical
measurements, M opt = (5.4 ± 0.3) M ⊙,
is applied. The timing measurements of twin high-frequency (HF)
quasiperiodic oscillations (QPOs) with the frequency ratio 3:2 and the
simultaneously observed low-frequency (LF) QPO imply spin in the range
ain (0.27{--}0.29) if models based on the frequencies of geodesic
epicyclic motion are used to fit the timing measurements, and the
correlated creation of the twin HF QPOs and the LF QPO at a common
radius is assumed. On the other hand, the spectral continuum method
implies ain (0.65{--}0.75), and the Fe-line-profile method implies ain
(0.94{--}0.98). This controversy can be cured if we abandon the
assumption of the occurrence of the twin HF QPOs and the simultaneously
observed LF QPO at a common radius. We demonstrate that the epicyclic
resonance model of the twin HF QPOs is able to predict the spin in
agreement with the Fe-profile method, but no model based on the geodesic
epicyclic frequencies can be in agreement with the spectral continuum
method. We also show that the non-geodesic string loop oscillation model
of twin HF QPOs predicts spin a > 0.3 under the optical measurement
limit on the black hole mass, which is in agreement with both the
spectral continuum and Fe-profile methods.
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The massless particle motion around rotating wormhole in the presence of
plasma environment has been studied. It has been shown that the presence
of the plasma decreases the inner radius of the circular orbits of
photons around rotating wormhole. The shadow cast by rotating wormhole
surrounded by inhomogeneous plasma with the radial power-law density has
been explored. It has been shown that the shape and size of the wormhole
shadow is distorted and changed depending on i) plasma parameters, ii)
wormhole rotation and iii) inclination angle between observer plane and
axis of rotation of wormhole. As an example we have considered an
inverse radial distribution of the plasma density and different types of
the wormhole solution.
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The Large Observatory For x-ray Timing (LOFT) is a mission concept which was proposed to ESA as M3 and M4 candidate in the framework of the Cosmic Vision 2015-2025 program. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument and the uniquely large field of view of its wide field monitor, LOFT will be able to study the behaviour of matter in extreme conditions such as the strong gravitational field in the innermost regions close to black holes and neutron stars and the supra-nuclear densities in the interiors of neutron stars. The science payload is based on a Large Area Detector (LAD, >8m2 effective area, 2-30 keV, 240 eV spectral resolution, 1 degree collimated field of view) and a Wide Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g., GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the current technical and programmatic status of the mission.
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Estimates of the black hole mass M and the dimensionless spin a in the microquasar GRO J1655-40 implied by strong gravity effects related to the timing and spectral measurements are controversial, if the mass restriction determined by the dynamics related to independent optical measurements, M opt = (5.4 ± 0.3) M ⊙, is applied. The timing measurements of twin high-frequency (HF) quasiperiodic oscillations (QPOs) with the frequency ratio 3:2 and the simultaneously observed low-frequency (LF) QPO imply spin in the range ain (0.27{--}0.29) if models based on the frequencies of geodesic epicyclic motion are used to fit the timing measurements, and the correlated creation of the twin HF QPOs and the LF QPO at a common radius is assumed. On the other hand, the spectral continuum method implies ain (0.65{--}0.75), and the Fe-line-profile method implies ain (0.94{--}0.98). This controversy can be cured if we abandon the assumption of the occurrence of the twin HF QPOs and the simultaneously observed LF QPO at a common radius. We demonstrate that the epicyclic resonance model of the twin HF QPOs is able to predict the spin in agreement with the Fe-profile method, but no model based on the geodesic epicyclic frequencies can be in agreement with the spectral continuum method. We also show that the non-geodesic string loop oscillation model of twin HF QPOs predicts spin a > 0.3 under the optical measurement limit on the black hole mass, which is in agreement with both the spectral continuum and Fe-profile methods.
Read More
The massless particle motion around rotating wormhole in the presence of plasma environment has been studied. It has been shown that the presence of the plasma decreases the inner radius of the circular orbits of photons around rotating wormhole. The shadow cast by rotating wormhole surrounded by inhomogeneous plasma with the radial power-law density has been explored. It has been shown that the shape and size of the wormhole shadow is distorted and changed depending on i) plasma parameters, ii) wormhole rotation and iii) inclination angle between observer plane and axis of rotation of wormhole. As an example we have considered an inverse radial distribution of the plasma density and different types of the wormhole solution.
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We study electrically charged perfect fluid toroidal structures
encircling a spherically symmetric gravitating object with Schwarzschild
spacetime geometry and endowed with a dipole magnetic field. The work
represents a direct continuation of our previous general-relativistic
studies of electrically charged fluid in the approximation of zero
conductivity, which formed tori around a Reissner-Nordström black
hole or a Schwarzschild black hole equipped with a test electric charge
and immersed in an asymptotically uniform magnetic field. After a
general introduction of the zero-conductivity charged fluid model, we
discuss a variety of possible topologies of the toroidal fluid
configurations. Along with the charged equatorial tori forming
interesting coupled configurations, we demonstrate the existence of the
off-equatorial tori, for which the dipole type of magnetic field seems
to be necessary. We focus on orbiting structures with constant specific
angular momentum and on those in permanent rigid rotation. We stress
that the general analytical treatment developed in our previous works is
enriched here by the integrated form of the pressure equations. To put
our work into an astrophysical context, we identify the central object
with an idealization of a nonrotating magnetic neutron star.
Constraining ranges of its parameters and also parameters of the
circling fluid, we discuss a possible relevance of the studied toroidal
structures, presenting along with their topology also pressure, density,
temperature and charge profiles.
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We study scalar, electromagnetic, axial, and polar gravitational
perturbations of the four-dimensional Reissner-Nordström-like black
holes with a tidal charge in the Randall-Sundrum braneworld in the first
approximation when the tidal perturbations are not taken into account.
The quasinormal modes of these perturbations have been studied in both
normal and eikonal regimes. Calculations have shown that the black holes
on the Randall-Sundrum brane are stable against all kinds of
perturbations. Moreover, we determine the greybody factor, giving
transmission and reflection of the scattered waves through the effective
potentials. It has been shown that the scalar perturbative fields are
the most favorite to reflect the wave as compared to the other fields.
With increasing value of the tidal charge, the ability of the all
perturbative potentials to reflect the waves decreases. Our calculations
in low- and high-frequency regimes have shown that black holes on the
braneworld always have a bigger absorption cross section of massless
scalar waves than the Schwarzschild and standard Reissner-Nordström
black holes.
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We study optical effects in quintessential Kerr black hole spacetimes
corresponding to the limiting case of the equation-of-state parameter
$omega_{q}=-1/3$ of the quintessence. In dependence on the
dimensionless quintessential field parameter $c$, we determine the black
hole silhouette and the spectral line profiles of Keplerian disks
generated in this special quintessential Kerr geometry, representing an
extension of the general modifications of the Kerr geometry introduced
recently by Ghasemi-Nodehi and Bambi cite{Gha-Bam:2016:EPJC:}. We
demonstrate that due to the influence of the parameter $c$, the
silhouette is almost homogeneously enlarged, and the spectral line
profiles are redshifted with almost conserved shape.
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We study electrically charged perfect fluid toroidal structures encircling a spherically symmetric gravitating object with Schwarzschild spacetime geometry and endowed with a dipole magnetic field. The work represents a direct continuation of our previous general-relativistic studies of electrically charged fluid in the approximation of zero conductivity, which formed tori around a Reissner-Nordström black hole or a Schwarzschild black hole equipped with a test electric charge and immersed in an asymptotically uniform magnetic field. After a general introduction of the zero-conductivity charged fluid model, we discuss a variety of possible topologies of the toroidal fluid configurations. Along with the charged equatorial tori forming interesting coupled configurations, we demonstrate the existence of the off-equatorial tori, for which the dipole type of magnetic field seems to be necessary. We focus on orbiting structures with constant specific angular momentum and on those in permanent rigid rotation. We stress that the general analytical treatment developed in our previous works is enriched here by the integrated form of the pressure equations. To put our work into an astrophysical context, we identify the central object with an idealization of a nonrotating magnetic neutron star. Constraining ranges of its parameters and also parameters of the circling fluid, we discuss a possible relevance of the studied toroidal structures, presenting along with their topology also pressure, density, temperature and charge profiles.
Read More
We study scalar, electromagnetic, axial, and polar gravitational perturbations of the four-dimensional Reissner-Nordström-like black holes with a tidal charge in the Randall-Sundrum braneworld in the first approximation when the tidal perturbations are not taken into account. The quasinormal modes of these perturbations have been studied in both normal and eikonal regimes. Calculations have shown that the black holes on the Randall-Sundrum brane are stable against all kinds of perturbations. Moreover, we determine the greybody factor, giving transmission and reflection of the scattered waves through the effective potentials. It has been shown that the scalar perturbative fields are the most favorite to reflect the wave as compared to the other fields. With increasing value of the tidal charge, the ability of the all perturbative potentials to reflect the waves decreases. Our calculations in low- and high-frequency regimes have shown that black holes on the braneworld always have a bigger absorption cross section of massless scalar waves than the Schwarzschild and standard Reissner-Nordström black holes.
Read More
