1,295 research outputs found
Evidence of Twisted flux-tube Emergence in Active Regions
Elongated magnetic polarities are observed during the emergence phase of
bipolar active regions (ARs). These extended features, called magnetic tongues,
are interpreted as a consequence of the azimuthal component of the magnetic
flux in the toroidal flux-tubes that form ARs. We develop a new systematic and
user-independent method to identify AR tongues. Our method is based on
determining and analyzing the evolution of the AR main polarity inversion line
(PIL). The effect of the tongues is quantified by measuring the acute angle [
tau] between the orientation of the PIL and the direction orthogonal to the AR
main bipolar axis. We apply a simple model to simulate the emergence of a
bipolar AR. This model lets us interpret the effect of magnetic tongues on
parameters that characterize ARs ( e.g. the PIL inclination and the tilt
angles, and their evolution). In this idealized kinematic emergence model, tau
is a monotonically increasing function of the twist and has the same sign as
the magnetic helicity. We systematically apply our procedure to a set of
bipolar ARs that were observed emerging in line-of-sight magnetograms over
eight years. For most of the cases studied, the tongues only have a small
influence on the AR tilt angle since tongues have a much lower magnetic flux
than the more concentrated main polarities. From the observed evolution of tau,
corrected for the temporal evolution of the tilt angle and its final value when
the AR is fully emerged, we estimate the average number of turns in the
subphotospherically emerging flux-rope. These values for the 41 observed ARs
are below unity, except for one. This indicates that subphotospheric flux-ropes
typically have a low amount of twist, i.e. highly twisted flux-tubes are rare.
Our results demonstrate that the evolution of the PIL is a robust indicator of
the presence of tongues and constrains the amount of twist in emerging
flux-tube
The transition from adiabatic inspiral to geodesic plunge for a compact object around a massive Kerr black hole: Generic orbits
The inspiral of a stellar mass compact object falling into a massive Kerr
black hole can be broken into three different regimes: An adiabatic inspiral
phase, where the inspiral timescale is much larger than the orbital period; a
late-time radial infall, which can be approximated as a plunging geodesic; and
a regime where the body transitions from the inspiral to plunge. In earlier
work, Ori and Thorne have outlined a method to compute the trajectory during
this transition for a compact object in a circular, equatorial orbit. We
generalize this technique to include inclination and eccentricity.Comment: 11 pages, 6 figures. Accepted by Phys. Rev. D. New version addresses
referee's comment
Gravitational radiation from a particle in circular orbit around a black hole. VI. Accuracy of the post-Newtonian expansion
A particle of mass moves on a circular orbit around a nonrotating black
hole of mass . Under the assumption the gravitational waves
emitted by such a binary system can be calculated exactly numerically using
black-hole perturbation theory. If, further, the particle is slowly moving,
then the waves can be calculated approximately analytically, and expressed in
the form of a post-Newtonian expansion. We determine the accuracy of this
expansion in a quantitative way by calculating the reduction in signal-to-noise
ratio incurred when matched filtering the exact signal with a nonoptimal,
post-Newtonian filter.Comment: 5 pages, ReVTeX, 1 figure. A typographical error was discovered in
the computer code used to generate the results presented in the paper. The
corrected results are presented in an Erratum, which also incorporates new
results, obtained using the recently improved post-Newtonian calculations of
Tanaka, Tagoshi, and Sasak
Absorption of mass and angular momentum by a black hole: Time-domain formalisms for gravitational perturbations, and the small-hole/slow-motion approximation
The first objective of this work is to obtain practical prescriptions to
calculate the absorption of mass and angular momentum by a black hole when
external processes produce gravitational radiation. These prescriptions are
formulated in the time domain within the framework of black-hole perturbation
theory. Two such prescriptions are presented. The first is based on the
Teukolsky equation and it applies to general (rotating) black holes. The second
is based on the Regge-Wheeler and Zerilli equations and it applies to
nonrotating black holes. The second objective of this work is to apply the
time-domain absorption formalisms to situations in which the black hole is
either small or slowly moving. In the context of this small-hole/slow-motion
approximation, the equations of black-hole perturbation theory can be solved
analytically, and explicit expressions can be obtained for the absorption of
mass and angular momentum. The changes in the black-hole parameters can then be
understood in terms of an interaction between the tidal gravitational fields
supplied by the external universe and the hole's tidally-induced mass and
current quadrupole moments. For a nonrotating black hole the quadrupole moments
are proportional to the rate of change of the tidal fields on the hole's world
line. For a rotating black hole they are proportional to the tidal fields
themselves.Comment: 36 pages, revtex4, no figures, final published versio
Black-hole binary simulations: the mass ratio 10:1
We present the first numerical simulations of an initially non-spinning
black-hole binary with mass ratio as large as 10:1 in full general relativity.
The binary completes approximately 3 orbits prior to merger and radiates about
0.415% of the total energy and 12.48% of the initial angular momentum in the
form of gravitational waves. The single black hole resulting from the merger
acquires a kick of about 66.7 km/s relative to the original center of mass
frame. The resulting gravitational waveforms are used to validate existing
formulas for the recoil, final spin and radiated energy over a wider range of
the symmetric mass ratio parameter eta=M1*M2/(M1+M2)^2 than previously
possible. The contributions of l > 2 multipoles are found to visibly influence
the gravitational wave signal obtained at fixed inclination angles.Comment: To match published versio
Killing vectors and anisotropy
We consider an action that can generate fluids with three unequal stresses
for metrics with a spacelike Killing vector. The parameters in the action are
directly related to the stress anisotropies. The field equations following from
the action are applied to an anisotropic cosmological expansion and an
extension of the Gott-Hiscock cosmic string
Gravitational radiation from infall into a black hole: Regularization of the Teukolsky equation
The Teukolsky equation has long been known to lead to divergent integrals
when it is used to calculate the gravitational radiation emitted when a test
mass falls into a black hole from infinity. Two methods have been used in the
past to remove those divergent integrals. In the first, integrations by parts
are carried out, and the infinite boundary terms are simply discarded. In the
second, the Teukolsky equation is transformed into another equation which does
not lead to divergent integrals. The purpose of this paper is to show that
there is nothing intrinsically wrong with the Teukolsky equation when dealing
with non-compact source terms, and that the divergent integrals result simply
from an incorrect choice of Green's function. In this paper, regularization of
the Teukolsky equation is carried out in an entirely natural way which does not
involve modifying the equation.Comment: ReVTeX, 23 page
Relativistic theory of tidal Love numbers
In Newtonian gravitational theory, a tidal Love number relates the mass
multipole moment created by tidal forces on a spherical body to the applied
tidal field. The Love number is dimensionless, and it encodes information about
the body's internal structure. We present a relativistic theory of Love
numbers, which applies to compact bodies with strong internal gravities; the
theory extends and completes a recent work by Flanagan and Hinderer, which
revealed that the tidal Love number of a neutron star can be measured by
Earth-based gravitational-wave detectors. We consider a spherical body deformed
by an external tidal field, and provide precise and meaningful definitions for
electric-type and magnetic-type Love numbers; and these are computed for
polytropic equations of state. The theory applies to black holes as well, and
we find that the relativistic Love numbers of a nonrotating black hole are all
zero.Comment: 25 pages, 8 figures, many tables; final version to be published in
Physical Review
Light-cone coordinates based at a geodesic world line
Continuing work initiated in an earlier publication [Phys. Rev. D 69, 084007
(2004)], we construct a system of light-cone coordinates based at a geodesic
world line of an arbitrary curved spacetime. The construction involves (i) an
advanced-time or a retarded-time coordinate that labels past or future light
cones centered on the world line, (ii) a radial coordinate that is an affine
parameter on the null generators of these light cones, and (iii) angular
coordinates that are constant on each generator. The spacetime metric is
calculated in the light-cone coordinates, and it is expressed as an expansion
in powers of the radial coordinate in terms of the irreducible components of
the Riemann tensor evaluated on the world line. The formalism is illustrated in
two simple applications, the first involving a comoving world line of a
spatially-flat cosmology, the other featuring an observer placed on the axis of
symmetry of Melvin's magnetic universe.Comment: 11 pages, 1 figur
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