177 research outputs found
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
Plio-Pleistocene floras of the Vildštejn Formation in the Cheb Basin, Czech Republic – a floristic and palaeoenvironmental review
Tolman-Oppenheimer-Volkoff equations in presence of the Chaplygin gas: stars and wormhole-like solutions
We study static solutions of the Tolman--Oppenheimer--Volkoff equations for
spherically symmetric objects (stars) living in a space filled with the
Chaplygin gas. Two cases are considered. In the normal case all solutions
(excluding the de Sitter one) realize a three-dimensional spheroidal geometry
because the radial coordinate achieves a maximal value (the "equator"). After
crossing the equator, three scenarios are possible: a closed spheroid having a
Schwarzschild-type singularity with infinite blue-shift at the "south pole", a
regular spheroid, and a truncated spheroid having a scalar curvature
singularity at a finite value of the radial coordinate. The second case arises
when the modulus of the pressure exceeds the energy density (the phantom
Chaplygin gas). There is no more equator and all solutions have the geometry of
a truncated spheroid with the same type of singularity. We consider also static
spherically symmetric configurations existing in a universe filled with the
phantom Chaplygin gas only. In this case two classes of solutions exist:
truncated spheroids and solutions of the wormhole type with a throat. However,
the latter are not asymptotically flat and possess curvature singularities at
finite values of the radial coordinate. Thus, they may not be used as models of
observable compact astrophysical objects.Comment: A reference added, matches the version published in Physical Review
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
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
A two-mass expanding exact space-time solution
In order to understand how locally static configurations around
gravitationally bound bodies can be embedded in an expanding universe, we
investigate the solutions of general relativity describing a space-time whose
spatial sections have the topology of a 3-sphere with two identical masses at
the poles. We show that Israel junction conditions imply that two spherically
symmetric static regions around the masses cannot be glued together. If one is
interested in an exterior solution, this prevents the geometry around the
masses to be of the Schwarzschild type and leads to the introduction of a
cosmological constant. The study of the extension of the Kottler space-time
shows that there exists a non-static solution consisting of two static regions
surrounding the masses that match a Kantowski-Sachs expanding region on the
cosmological horizon. The comparison with a Swiss-Cheese construction is also
discussed.Comment: 15 pages, 5 figures. Replaced to match the published versio
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
Observations of a GX 301-2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM
The accretion-powered X-ray pulsar GX 301-2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late December 2018, with contiguous observations by the NICER X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin-up of the pulsar discovered with the Fermi GBM. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15-35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of (27+38-27)% (90% confidence limit) averaged over all pulse phases. We discuss the spin-up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur
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