1,057 research outputs found
On the falloff of radiated energy in black hole spacetimes
The goal of much research in relativity is to understand gravitational waves
generated by a strong-field dynamical spacetime. Quantities of particular
interest for many calculations are the Weyl scalar , which is simply
related to the flux of gravitational waves far from the source, and the flux of
energy carried to distant observers, . Conservation laws guarantee
that, in asympotically flat spacetimes, and as . Most calculations extract these quantities at
some finite extraction radius. An understanding of finite radius corrections to
and allows us to more accurately infer their asymptotic
values from a computation. In this paper, we show that, if the final state of
the system is a black hole, then the leading correction to is , and that to the energy flux is --- not and as one might naively guess. Our argument only
relies on the behavior of the curvature scalars for black hole spacetimes.
Using black hole perturbation theory, we calculate the corrections to the
leading falloff, showing that it is quite easy to correct for finite extraction
radius effects.Comment: 5 pages, no figures, accepted to Phys. Rev. D. This version corrects
several typos and minor errors in the earlier submissio
A Fuzzy Logic Based Algorithm for Finding Astronomical Objects in Wide-Angle Frames
Accurate automatic identification of astronomical objects in an imperfect
world of non-linear wide-angle optics, imperfect optics, inaccurately pointed
telescopes, and defect-ridden cameras is not always a trivial first step. In
the past few years, this problem has been exacerbated by the rise of digital
imaging, providing vast digital streams of astronomical images and data. In the
modern age of increasing bandwidth, human identifications are many times
impracticably slow. In order to perform an automatic computer-based analysis of
astronomical frames, a quick and accurate identification of astronomical
objects is required. Such identification must follow a rigorous transformation
from topocentric celestial coordinates into image coordinates on a CCD frame.
This paper presents a fuzzy logic based algorithm that estimates needed
coordinate transformations in a practical setting. Using a training set of
reference stars, the algorithm statically builds a fuzzy logic model. At
runtime, the algorithm uses this model to associate stellar objects visible in
the frames to known-catalogued objects, and generates files that contain
photometry information of objects visible in the frame. Use of this algorithm
facilitates real-time monitoring of stars and bright transients, allowing
identifications and alerts to be issued more reliably. The algorithm is being
implemented by the Night Sky Live all-sky monitoring global network and has
shown itself significantly more reliable than the previously used non-fuzzy
logic algorithm.Comment: Accepted for publication in PAS
Uniaxial magnetocrystalline anisotropy in
is a paramagnetic metal and since its low temperature
resistivity is described by with , it
is also considered a non-Fermi liquid (NFL) metal. We have performed extensive
magnetoresistance and Hall effect measurements of untwinned epitaxial films of
. These measurements reveal that exhibits
uniaxial magnetocrystalline anisotropy. In addition, the low-temperature NFL
behavior is most effectively suppressed when a magnetic field is applied along
the easy axis, suggesting that critical spin fluctuations, possibly due to
proximity of a quantum critical phase transition, are related to the NFL
behavior.Comment: 7 figure
Computational Efficiency of Frequency-- and Time--Domain Calculations of Extreme Mass--Ratio Binaries: Equatorial Orbits
Gravitational waveforms and fluxes from extreme mass--ratio inspirals can be
computed using time--domain methods with accuracy that is fast approaching that
of frequency--domain methods. We study in detail the computational efficiency
of these methods for equatorial orbits of fast spinning Kerr black holes, and
find the number of modes needed in either method --as functions of the orbital
parameters-- in order to achieve a desired accuracy level. We then estimate the
total computation time and argue that for high eccentricity orbits the
time--domain approach is more efficient computationally. We suggest that in
practice low-- modes are computed using the frequency--domain approach, and
high-- modes are computed using the time--domain approach, where is the
azimuthal mode number.Comment: 19 figures, 6 table
Late-time Kerr tails: generic and non-generic initial data sets, "up" modes, and superposition
Three interrelated questions concerning Kerr spacetime late-time scalar-field
tails are considered numerically, specifically the evolutions of generic and
non-generic initial data sets, the excitation of "up" modes, and the resolution
of an apparent paradox related to the superposition principle. We propose to
generalize the Barack-Ori formula for the decay rate of any tail multipole
given a generic initial data set, to the contribution of any initial multipole
mode. Our proposal leads to a much simpler expression for the late-time power
law index. Specifically, we propose that the late-time decay rate of the
spherical harmonic multipole moment because of an initial
multipole is independent of the azimuthal number , and is
given by , where for and
for . We also show explicitly that the angular symmetry group of
a multipole does not determine its late-time decay rate.Comment: 12 pages, 13 figures, 4 tables. Substantially revised manuscrip
- …