5,005 research outputs found
Coup Agency and Prospects for Democracy
This research note introduces new global data on military coups. Conventional aggregate data so far have conflated two distinct types of coups. Military interventions by leading officers are coups "from above,"characterized by political power struggles within authoritarian elite coalitions where officers move against civilian elites, executive incumbents, and their loyal security personnel. By contrast, power grabs by officers from the lower and middle ranks are coups "from below,"where military personnel outside of the political elite challenge sitting incumbents, their loyalists, and the regime itself. Disaggregating coup types offers leverage to revise important questions about the causes and consequences of military intervention in politics. This research note illustrates that coup attempts from the top of the military hierarchy are much more likely to be successful than coups from the lower and middle ranks of the military hierarchy. Moreover, coups from the top recalibrate authoritarian elite coalitions and serve to sustain autocratic rule; they rarely produce an opening for a democratic transition. Successful coups from below, by contrast, can result in the breakdown of authoritarian regimes and generate an opening for democratic transitions
Removing non-stationary, non-harmonic external interference from gravitational wave interferometer data
We describe a procedure to identify and remove a class of non-stationary and
non-harmonic interference lines from gravitational wave interferometer data.
These lines appear to be associated with the external electricity main
supply, but their amplitudes are non-stationary and they do not appear at
harmonics of the fundamental supply frequency. We find an empirical model able
to represent coherently all the non-harmonic lines we have found in the power
spectrum, in terms of an assumed reference signal of the primary supply input
signal. If this signal is not available then it can be reconstructed from the
same data by making use of the coherent line removal algorithm that we have
described elsewhere. All these lines are broadened by frequency changes of the
supply signal, and they corrupt significant frequency ranges of the power
spectrum. The physical process that generates this interference is so far
unknown, but it is highly non-linear and non-stationary. Using our model, we
cancel the interference in the time domain by an adaptive procedure that should
work regardless of the source of the primary interference. We have applied the
method to laser interferometer data from the Glasgow prototype detector, where
all the features we describe in this paper were observed. The algorithm has
been tuned in such a way that the entire series of wide lines corresponding to
the electrical interference are removed, leaving the spectrum clean enough to
detect signals previously masked by them. Single-line signals buried in the
interference can be recovered with at least 75 % of their original signal
amplitude.Comment: 14 pages, 5 figures, Revtex, psfi
Detecting gravitational waves from test-mass bodies orbiting a Kerr black hole with P-approximant templates
In this study we apply post-Newtonian (T-approximants) and resummed
post-Newtonian (P-approximants) to the case of a test-particle in equatorial
orbit around a Kerr black hole. We compare the two approximants by measuring
their effectualness (i.e. larger overlaps with the exact signal), and
faithfulness (i.e. smaller biases while measuring the parameters of the signal)
with the exact (numerical) waveforms. We find that in the case of prograde
orbits, T-approximant templates obtain an effectualness of ~0.99 for spins q <
0.75. For 0.75 < q < 0.95, the effectualness drops to about 0.82. The
P-approximants achieve effectualness of > 0.99 for all spins up to q = 0.95.
The bias in the estimation of parameters is much lower in the case of
P-approximants than T-approximants. We find that P-approximants are both
effectual and faithful and should be more effective than T-approximants as a
detection template family when q > 0. For q < 0 both T- and P-approximants
perform equally well so that either of them could be used as a detection
template family. However, for parameter estimation, the P-approximant templates
still outperforms the T-approximants.Comment: 11 Pages - 9 figures. Accepted for publication. Proceedings of GWDAW
9. Special edition of Classical and Quantum Gravit
Coherent Line Removal: Filtering out harmonically related line interference from experimental data, with application to gravitational wave detectors
We describe a new technique for removing troublesome interference from
external coherent signals present in the gravitational wave spectrum. The
method works when the interference is present in many harmonics, as long as
they remain coherent with one another. The method can remove interference even
when the frequency changes. We apply the method to the data produced by the
Glasgow laser interferometer in 1996 and the entire series of wide lines
corresponding to the electricity supply frequency and its harmonics are
removed, leaving the spectrum clean enough to detect possible signals
previously masked by them. We also study the effects of the line removal on the
statistics of the noise in the time domain. We find that this technique seems
to reduce the level of non-Gaussian noise present in the interferometer and
therefore, it can raise the sensitivity and duty cycle of the detectors.Comment: 14 pages, 8 figures, Revtex, psfig. To appear in Phys. Rev.
Data analysis of gravitational-wave signals from spinning neutron stars. II. Accuracy of estimation of parameters
We examine the accuracy of estimation of parameters of the gravitational-wave
signals from spinning neutron stars that can be achieved from observations by
Earth-based laser interferometers. We consider a model of the signal consisting
of two narrowband components and including both phase and amplitude modulation.
We calculate approximate values of the rms errors of the parameter estimators
using the Fisher information matrix. We carry out extensive Monte Carlo
simulations and obtain cumulative distribution functions of rms errors of
astrophysically interesting parameters: amplitude of the signal, wobble angle,
position of the source in the sky, frequency, and spindown coefficients. We
consider both all-sky searches and directed searches. We also examine the
possibility of determination of neutron star proper motion. We perform
simulations for all laser-interferometric detectors that are currently under
construction and for several possible lengths of the observation time and sizes
of the parameter space. We find that observations of continuous
gravitational-wave signals from neutron stars by laser-interferometric
detectors will provide a very accurate information about their astrophysical
properties. We derive several simplified models of the signal that can be used
in the theoretical investigations of the data analysis schemes independently of
the physical mechanisms generating the gravitational-wave signal.Comment: LaTeX, 34 pages, 15 figures, submitted to Phys. Rev.
Gravity from a fermionic condensate of a gauge theory
The most prominent realization of gravity as a gauge theory similar to the
gauge theories of the standard model comes from enlarging the gauge group from
the Lorentz group to the de Sitter group. To regain ordinary Einstein-Cartan
gravity the symmetry must be broken, which can be accomplished by known
quasi-dynamic mechanisms. Motivated by symmetry breaking models in particle
physics and condensed matter systems, we propose that the symmetry can
naturally be broken by a homogenous and isotropic fermionic condensate of
ordinary spinors. We demonstrate that the condensate is compatible with the
Einstein-Cartan equations and can be imposed in a fully de Sitter invariant
manner. This lends support, and provides a physically realistic mechanism for
understanding gravity as a gauge theory with a spontaneously broken local de
Sitter symmetry.Comment: 16 page
GravEn: Software for the simulation of gravitational wave detector network response
Physically motivated gravitational wave signals are needed in order to study
the behaviour and efficacy of different data analysis methods seeking their
detection. GravEn, short for Gravitational-wave Engine, is a MATLAB software
package that simulates the sampled response of a gravitational wave detector to
incident gravitational waves. Incident waves can be specified in a data file or
chosen from among a group of pre-programmed types commonly used for
establishing the detection efficiency of analysis methods used for LIGO data
analysis. Every aspect of a desired signal can be specified, such as start time
of the simulation (including inter-sample start times), wave amplitude, source
orientation to line of sight, location of the source in the sky, etc. Supported
interferometric detectors include LIGO, GEO, Virgo and TAMA.Comment: 10 Pages, 3 Figures, Presented at the 10th Gravitational Wave Data
Analysis Workshop (GWDAW-10), 14-17 December 2005 at the University of Texas,
Brownsvill
Computational Resources to Filter Gravitational Wave Data with P-approximant Templates
The prior knowledge of the gravitational waveform from compact binary systems
makes matched filtering an attractive detection strategy. This detection method
involves the filtering of the detector output with a set of theoretical
waveforms or templates. One of the most important factors in this strategy is
knowing how many templates are needed in order to reduce the loss of possible
signals. In this study we calculate the number of templates and computational
power needed for a one-step search for gravitational waves from inspiralling
binary systems. We build on previous works by firstly expanding the
post-Newtonian waveforms to 2.5-PN order and secondly, for the first time,
calculating the number of templates needed when using P-approximant waveforms.
The analysis is carried out for the four main first-generation interferometers,
LIGO, GEO600, VIRGO and TAMA. As well as template number, we also calculate the
computational cost of generating banks of templates for filtering GW data. We
carry out the calculations for two initial conditions. In the first case we
assume a minimum individual mass of and in the second, we assume
a minimum individual mass of . We find that, in general, we need
more P-approximant templates to carry out a search than if we use standard PN
templates. This increase varies according to the order of PN-approximation, but
can be as high as a factor of 3 and is explained by the smaller span of the
P-approximant templates as we go to higher masses. The promising outcome is
that for 2-PN templates the increase is small and is outweighed by the known
robustness of the 2-PN P-approximant templates.Comment: 17 pages, 8 figures, Submitted to Class.Quant.Gra
Surface Vacuum Energy in Cutoff Models: Pressure Anomaly and Distributional Gravitational Limit
Vacuum-energy calculations with ideal reflecting boundaries are plagued by
boundary divergences, which presumably correspond to real (but finite) physical
effects occurring near the boundary. Our working hypothesis is that the stress
tensor for idealized boundary conditions with some finite cutoff should be a
reasonable ad hoc model for the true situation. The theory will have a sensible
renormalized limit when the cutoff is taken away; this requires making sense of
the Einstein equation with a distributional source. Calculations with the
standard ultraviolet cutoff reveal an inconsistency between energy and pressure
similar to the one that arises in noncovariant regularizations of cosmological
vacuum energy. The problem disappears, however, if the cutoff is a spatial
point separation in a "neutral" direction parallel to the boundary. Here we
demonstrate these claims in detail, first for a single flat reflecting wall
intersected by a test boundary, then more rigorously for a region of finite
cross section surrounded by four reflecting walls. We also show how the
moment-expansion theorem can be applied to the distributional limits of the
source and the solution of the Einstein equation, resulting in a mathematically
consistent differential equation where cutoff-dependent coefficients have been
identified as renormalizations of properties of the boundary. A number of
issues surrounding the interpretation of these results are aired.Comment: 22 pages, 2 figures, 1 table; PACS 03.70.+k, 04.20.Cv, 11.10.G
Gravitational waveforms from inspiralling compact binaries to second-post-Newtonian order
The two independent ``plus" and ``cross" polarization waveforms associated
with the gravitational waves emitted by inspiralling, non-spinning, compact
binaries are presented, ready for use in the data analysis of signals received
by future laser interferometer gravitational-wave detectors such as LIGO and
VIRGO. The computation is based on a recently derived expression of the
gravitational field at the second-post-Newtonian approximation of general
relativity beyond the dominant (Newtonian) quadrupolar field. The use of these
theoretical waveforms to make measurements of astrophysical parameters and to
test the nature of relativistic gravity is discussed.Comment: 17 pages; To appear in Classical and Quantum Gravit
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