186 research outputs found
Black hole spin inferred from 3:2 epicyclic resonance model of high-frequency quasi-periodic oscillations
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<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).Comment: 6 pages, 4 figures, accepted by Astronomy & Astrophysic
Electromagnetic Properties of Kerr-Anti-de Sitter Black Holes
We examine the electromagnetic properties of Kerr-anti-de Sitter (Kerr-AdS)
black holes in four and higher spacetime dimensions. Assuming that the black
holes may carry a test electric charge we show that the Killing one-form which
represents the difference between the timelike generators in the spacetime and
in the reference background can be used as a potential one-form for the
associated electromagnetic field. In four dimensions the potential one-form and
the Kerr-AdS metric with properly re-scaled mass parameter solve the
Einstein-Maxwell equations, thereby resulting in the familiar Kerr-Newman-AdS
solution. We solve the quartic equation governing the location of the event
horizons of the Kerr-Newman-AdS black holes and present closed analytic
expressions for the radii of the horizons. We also compute the gyromagnetic
ratio for these black holes and show that it corresponds to g=2 just as for
ordinary black holes in asymptotically flat spacetime. Next, we compute the
gyromagnetic ratio for the Kerr-AdS black holes with a single angular momentum
and with a test electric charge in all higher dimensions. The gyromagnetic
ratio crucially depends on the dimensionless ratio of the rotation parameter to
the curvature radius of the AdS background. At the critical limit, when the
boundary Einstein universe is rotating at the speed of light, it tends to g=2
irrespective of the spacetime dimension. Finally, we consider the case of a
five dimensional Kerr-AdS black hole with two angular momenta and show that it
possesses two distinct gyromagnetic ratios in accordance with its two
orthogonal 2-planes of rotation. In the special case of two equal angular
momenta, the two gyromagnetic ratios merge into one leading to g=4 at the
maximum angular velocities of rotation.Comment: Typos corrected; 31 pages, REVTe
Unstable fields in Kerr spacetimes
We show that both the interior region of a Kerr black
hole and the Kerr naked singularity admit unstable solutions of the
Teukolsky equation for any value of the spin weight. For every harmonic number
there is at least one axially symmetric mode that grows exponentially in time
and decays properly in the radial directions. These can be used as Debye
potentials to generate solutions for the scalar, Weyl spinor, Maxwell and
linearized gravity field equations on these backgrounds, satisfying appropriate
spatial boundary conditions and growing exponentially in time, as shown in
detail for the Maxwell case. It is suggested that the existence of the unstable
modes is related to the so called "time machine" region, where the axial
Killing vector field is time-like, and the Teukolsky equation, restricted to
axially symmetric fields, changes its character from hyperbolic to elliptic
The evolution of cosmic string loops in Kerr-de Sitter spacetimes
The equation of cosmic string loops in Kerr-de Sitter spacetimes is derived.
Having solved the equation numerically, we find that the loops can expand and
exist except for too small ones.Comment: 8 page
Coupling a Point-Like Mass to Quantum Gravity with Causal Dynamical Triangulations
We present a possibility of coupling a point-like, non-singular, mass
distribution to four-dimensional quantum gravity in the nonperturbative setting
of causal dynamical triangulations (CDT). In order to provide a point of
comparison for the classical limit of the matter-coupled CDT model, we derive
the spatial volume profile of the Euclidean Schwarzschild-de Sitter space glued
to an interior matter solution. The volume profile is calculated with respect
to a specific proper-time foliation matching the global time slicing present in
CDT. It deviates in a characteristic manner from that of the pure-gravity
model. The appearance of coordinate caustics and the compactness of the mass
distribution in lattice units put an upper bound on the total mass for which
these calculations are expected to be valid. We also discuss some of the
implementation details for numerically measuring the expectation value of the
volume profiles in the framework of CDT when coupled appropriately to the
matter source.Comment: 26 pages, 9 figures, updated published versio
Features of Motion Around Global Monopole in Asymptotically dS/AdS Spacetime
In this paper, we study the motion of test particle and light around the
Global Monopole in asymptotically dS/AdS spacetime. The motion of a test
particle and light in the exterior region of the global monopole in dS/AdS
spacetime has been investigated. Although the test particle's motion is quite
different from the case in asymptotically flat spacetime, the behaviors of
light(null geodesic) remain unchanged except a energy(frequency) shift. Through
a phase-plane analysis, we prove analytically that the existence of a periodic
solution to the equation of motion for a test particle will not be altered by
the presence of cosmological constant and the deficit angle, whose presence
only affects the position and type of the critical point on the phase plane. We
also show that the apparent capture section of the global monopole in dS/AdS
spacetime is quite different from that in flat spacetime.Comment: 15 pages, 4 PS figures, accepted for publication in Class. Quantum
Gra
Particle motion in the field of a five-dimensional charged black hole
In this paper, we have investigated the geodesics of neutral particles near a
five-dimensional charged black hole using a comparative approach. The effective
potential method is used to determine the location of the horizons and to study
radial and circular trajectories. This also helps us to analyze the stability
of radial and circular orbits. The radius of the innermost stable circular
orbits have also been determined. Contrary to the case of massive particles for
which, the circular orbits may have up to eight possible values of specific
radius, we find that the photons will only have two distinct values for the
specific radii of circular trajectories. Finally we have used the dynamical
systems analysis to determine the critical points and the nature of the
trajectories for the timelike and null geodesics.Comment: 15 pages, accepted for publication in Astrophysics and Space Scienc
Planetary Imaging Concept Testbed Using a Recoverable Experiment-Coronagraph (PICTURE C)
An exoplanet mission based on a high-altitude balloon is a next logical step in humanity's quest to explore Earthlike planets in Earthlike orbits orbiting Sunlike stars. The mission described here is capable of spectrally imaging debris disks and exozodiacal light around a number of stars spanning a range of infrared excesses, stellar types, and ages. The mission is designed to characterize the background near those stars, to study the disks themselves, and to look for planets in those systems. The background light scattered and emitted from the disk is a key uncertainty in the mission design of any exoplanet direct imaging mission, thus, its characterization is critically important for future imaging of exoplanets
Magnetized Particle Capture Cross Section for Braneworld Black Hole
Capture cross section of magnetized particle (with nonzero magnetic moment)
by braneworld black hole in uniform magnetic field is considered. The magnetic
moment of particle was chosen as it was done by \citet{rs99} and for the
simplicity particle with zero electric charge is chosen. It is shown that the
spin of particle as well as the brane parameter are to sustain the stability of
particles circularly orbiting around the black hole in braneworld i.e. spin of
particles and brane parameter try to prevent the capture by black hole.Comment: 7 pages, 4 figures, Accepted for publication in Astrophysics & Space
Scienc
Subanesthetic ketamine treatment promotes abnormal interactions between neural subsystems and alters the properties of functional brain networks
Acute treatment with subanesthetic ketamine, a non-competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist, is widely utilized as a translational model for schizophrenia. However, how acute NMDA receptor blockade impacts on brain functioning at a systems level, to elicit translationally relevant symptomatology and behavioral deficits, has not yet been determined. Here, for the first time, we apply established and recently validated topological measures from network science to brain imaging data gained from ketamine-treated mice to elucidate how acute NMDA receptor blockade impacts on the properties of functional brain networks. We show that the effects of acute ketamine treatment on the global properties of these networks are divergent from those widely reported in schizophrenia. Where acute NMDA receptor blockade promotes hyperconnectivity in functional brain networks, pronounced dysconnectivity is found in schizophrenia. We also show that acute ketamine treatment increases the connectivity and importance of prefrontal and thalamic brain regions in brain networks, a finding also divergent to alterations seen in schizophrenia. In addition, we characterize how ketamine impacts on bipartite functional interactions between neural subsystems. A key feature includes the enhancement of prefrontal cortex (PFC)-neuromodulatory subsystem connectivity in ketamine-treated animals, a finding consistent with the known effects of ketamine on PFC neurotransmitter levels. Overall, our data suggest that, at a systems level, acute ketamine-induced alterations in brain network connectivity do not parallel those seen in chronic schizophrenia. Hence, the mechanisms through which acute ketamine treatment induces translationally relevant symptomatology may differ from those in chronic schizophrenia. Future effort should therefore be dedicated to resolve the conflicting observations between this putative translational model and schizophrenia
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