301 research outputs found
Relativistic Dynamos in Magnetospheres of Rotating Compact Objects
The kinematic evolution of axisymmetric magnetic fields in rotating
magnetospheres of relativistic compact objects is analytically studied, based
on relativistic Ohm's law in stationary axisymmetric geometry. By neglecting
the poloidal flows of plasma in simplified magnetospheric models, we discuss
self-excited dynamos due to the frame-dragging effect (originally pointed out
by Khanna & Camenzind), and we propose alternative processes to generate
axisymmetric magnetic fields against ohmic dissipation. The first process
(which may be called induced excitation) is caused by the help of a background
uniform magnetic field in addition to the dragging of inertial frames. It is
shown that excited multipolar components of poloidal and azimuthal fields are
sustained as stationary modes, and outgoing Poynting flux converges toward the
rotation axis. The second one is self-excited dynamo through azimuthal
convection current, which is found to be effective if plasma rotation becomes
highly relativistic with a sharp gradient in the angular velocity. In this case
no frame-dragging effect is needed, and the coupling between charge separation
and plasma rotation becomes important. We discuss briefly the results in
relation to active phenomena in the relativistic magnetospheres.Comment: 16 pages, AASLaTeX macros v4.
Electromagnetic radiation due to naked singularity formation in self-similar gravitational collapse
Dynamical evolution of test fields in background geometry with a naked
singularity is an important problem relevant to the Cauchy horizon instability
and the observational signatures different from black hole formation. In this
paper we study electromagnetic perturbations generated by a given current
distribution in collapsing matter under a spherically symmetric self-similar
background. Using the Green's function method, we construct the formula to
evaluate the outgoing energy flux observed at the future null infinity. The
contributions from "quasi-normal" modes of the self-similar system as well as
"high-frequency" waves are clarified. We find a characteristic power-law time
evolution of the outgoing energy flux which appears just before naked
singularity formation, and give the criteria as to whether or not the outgoing
energy flux diverges at the future Cauchy horizon.Comment: 20 pages, 7 figures, references added to match the published versio
Distortion of Schwarzschild-anti-de Sitter black holes to black strings
Motivated by the existence of black holes with various topologies in
four-dimensional spacetimes with a negative cosmological constant, we study
axisymmetric static solutions describing any large distortions of
Schwarzschild-anti-de Sitter black holes parametrized by the mass . Under
the approximation such that is much larger than the anti-de Sitter radius,
it is found that a cylindrically symmetric black string is obtained as a
special limit of distorted spherical black holes. Such a prolonged distortion
of the event horizon connecting a Schwarzschild-anti-de Sitter black hole to a
black string is allowed without violating both the usual black hole
thermodynamics and the hoop conjecture for the horizon circumference.Comment: 13 pages, accepted for publication in Physical Review
Black Hole Magnetospheres Around Thin Disks Driving Inward and Outward Winds
We construct a simple model for stationary, axisymmetric black-hole
magnetospheres, in which the poloidal magnetic field is generated by a toroidal
electric current in a thin disk with the inner edge, by solving the vacuum
Maxwell equations in Schwarzschild background. In this work, to obtain a
concise analytical form of the magnetic stream function, we use the
approximation that the inner edge is far distant from the event horizon. The
global magnetospheric structure with the closed-loop and open field lines
threading the inner and outer parts of the disk is explicitly shown, claiming
that the model is useful as a starting point to study astrophysical problems
involving inward disk-driven winds to a black hole and outward ones to
infinity. The asymptotic shape of the field lines at the event horizon becomes
nearly cylindrical, while at infinity it becomes conical. The magnetic spot in
the disk connected with the black hole through the loop field lines occupies a
very narrow region with the ring area roughly equal to the horizon area. By
taking account of the existence of a uniform (external) magnetic field, we also
obtain the model for collimated open field lines. Then, it is found that the
magnetic connection between the black hole and the disk breaks down if the
uniform field is strong enough. Considering slow rotation of the magnetosphere
and angular momentum transfer by inward winds from the disk, the final
discussion is devoted to gradual disruption of the closed loops due to radial
accretion of disk plasma toward the black hole.Comment: 15 pages 4 figures accepted for publication in Ap
Asymptotic tails of massive scalar fields in Schwarzschild background
We investigate the asymptotic tail behavior of massive scalar fields in
Schwarzschild background. It is shown that the oscillatory tail of the scalar
field has the decay rate of at asymptotically late times, and the
oscillation with the period for the field mass is modulated by the
long-term phase shift. These behaviors are qualitatively similar to those found
in nearly extreme Reissner-Nordstr\"{o}m background, which are discussed in
terms of a resonant backscattering due to the space-time curvature.Comment: 21 pages, 2 figures, accepted for publication in Phys.Rev.
The double-Kerr equilibrium configurations involving one extreme object
We demonstrate the existence of equilibrium states in the limiting cases of
the double-Kerr solution when one of the constituents is an extreme object. In
the `extreme-subextreme' case the negative mass of one of the constituents is
required for the balance, whereas in the `extreme-superextreme' equilibrium
configurations both Kerr particles may have positive masses. We also show that
the well-known relation |J|=M^2 between the mass and angular momentum in the
extreme single Kerr solution ceases to be a characteristic property of the
extreme Kerr particle in a binary system.Comment: 12 pages, 3 figures, submitted to Class. Quantum Gra
The X-ray jet in the Crab Nebula: radical implications for pulsar theory?
The recent Chandra image of the Crab nebula shows a striking, axisymmetric
polar jet. It is shown that jets are formed in axisymmetric, magnetized pulsar
winds and that the jet luminosity scales relative to the total as
(\gamma_0\sigma_{eq})^{-4/3}, where \sigma_{eq} is the ratio of Poynting flux
to particle kinetic energy output at the equator at the base of the flow and
\gamma_0 the initial Lorentz factor of the flow. The results are applied to the
image of the Crab nebula, and the limit is set for the Crab pulsar of
\sigma_{eq} \leq 100. It is argued that conventional pulsar theory needs to be
reexamined in light of these limits.Comment: 13 page
Vacuum polarization of scalar fields near Reissner-Nordstr\"{o}m black holes and the resonance behavior in field-mass dependence
We study vacuum polarization of quantized massive scalar fields in
equilibrium at black-hole temperature in Reissner-Nordstr\"{o}m background. By
means of the Euclidean space Green's function we analytically derive the
renormalized expression at the event horizon with the area
. It is confirmed that the polarization amplitude
is free from any divergence due to the infinite red-shift
effect. Our main purpose is to clarify the dependence of on
field mass in relation to the excitation mechanism. It is shown for
small-mass fields with how the excitation of
caused by finite black-hole temperature is suppressed as increases, and it
is verified for very massive fields with that
decreases in proportion to with the amplitude equal to the
DeWitt-Schwinger approximation. In particular, we find a resonance behavior
with a peak amplitude at in the field-mass dependence of
vacuum polarization around nearly extreme (low-temperature) black holes. The
difference between Scwarzschild and nearly extreme black holes is discussed in
terms of the mass spectrum of quantum fields dominant near the event horizon.Comment: 24 pages, 1 figure Accepted in PR
Disk illumination by black hole superradiance of electromagnetic perturbations
Using the Kerr-Schild formalism to solve the Einstein-Maxwell equations, we
study energy transport due to time-dependent electromagnetic perturbations
around a Kerr black hole, which may work as a mechanism to illuminate a disk
located on the equatorial plane. For such a disk-hole system it is found that
the energy extraction from the hole can occur under the well-known
superradiance condition for wave frequency, even though the energy absorption
into the hole should be rather dominant near the polar region of the horizon.
We estimate the efficiency of the superradiant amplification of the disk
illumination. Further we calculate the time-averaged energy density
distribution to show explicitly the existence of a negative energy region near
the horizon and to discuss the possible generation of a hot spot on the disk.Comment: 11 pages, 4 figures, corrected typos, published in Physical Review
Hoop conjecture for colliding black holes : non-time-symmetric initial data
The hoop conjecture is well confirmed in momentarily static spaces, but it
has not been investigated systematically for the system with relativistic
motion. To confirm the hoop conjecture for non-time-symmetric initial data, we
consider the initial data of two colliding black holes with momentum and search
an apparent horizon that encloses two black holes. In testing the hoop
conjecture, we use two definitions of gravitational mass : one is the ADM mass
and the other is the quasi-local mass defined by Hawking. Although both
definitions of gravitational mass give fairly consistent picture of the hoop
conjecture, the hoop conjecture with the Hawking mass can judge the existence
of an apparent horizon for wider range of parameters of the initial data
compared to the ADM mass.Comment: 15pages, 4 figure
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