372 research outputs found
The search for continuous gravitational waves: metric of the multi-detector F-statistic
We develop a general formalism for the parameter-space metric of the
multi-detector F-statistic, which is a matched-filtering detection statistic
for continuous gravitational waves. We find that there exists a whole family of
F-statistic metrics, parametrized by the (unknown) amplitude parameters of the
gravitational wave. The multi-detector metric is shown to be expressible in
terms of noise-weighted averages of single-detector contributions, which
implies that the number of templates required to cover the parameter space does
not scale with the number of detectors. Contrary to using a longer observation
time, combining detectors of similar sensitivity is therefore the
computationally cheapest way to improve the sensitivity of coherent
wide-parameter searches for continuous gravitational waves.
We explicitly compute the F-statistic metric family for signals from isolated
spinning neutron stars, and we numerically evaluate the quality of different
metric approximations in a Monte-Carlo study. The metric predictions are tested
against the measured mismatches and we identify regimes in which the local
metric is no longer a good description of the parameter-space structure.Comment: 20 pages, 15 figures, revtex4; v2: some edits of style and notation,
fixed minor typo
Global parameter-space correlations of coherent searches for continuous gravitational waves
The space of phase-parameters (sky-position, frequency, spindowns) of a
coherent matched-filtering search for continuous gravitational waves from
isolated neutron stars shows strong global correlations (``circles in the
sky''). In the local limit this can be analysed in terms of a parameter-space
metric, but the global properties are less well studied. In this work we report
on our recent progress in understanding these global correlations analytically
for short to intermediate (less than a month, say) observation times and
neglecting spindowns. The location of these correlation-circles in
parameter-space is found to be determined mostly by the orbital velocity of the
earth, while the spin-motion of the detector and the antenna-patterns only
contribute significantly to the amplitude of the detection statistic along
these circles.Comment: 10 pages, 6 figures; contribution to GWDAW9, submitted to CQ
Improved all-sky search method for continuous gravitational waves from unknown neutron stars in binary systems
Continuous gravitational waves from spinning deformed neutron stars have not been detected yet, and are one of the most promising signals for future detection. All-sky searches for continuous gravitational waves from unknown neutron stars in binary systems are the most computationally challenging search type. Consequently, very few search algorithms and implementations exist for these sources, and only a handful of such searches have been performed so far. In this paper, we present a new all-sky binary search method, BinarySkyHou, which extends and improves upon the earlier BinarySkyHough method, and which was the basis for a recent search (Covas et al. [1]). We compare the sensitivity and computational cost to previous methods, showing that it is both more sensitive and computationally efficient, which allows for broader and more sensitive searches. <br
Improved short-segment detection statistic for continuous gravitational waves
Continuous gravitational waves represent one of the long-sought types of signals that have yet to be detected. Due to their small amplitude, long observational datasets (months-years) have to be analyzed together, thereby vastly increasing the computational cost of these searches. All-sky searches face the most severe computational obstacles, especially searches for sources in unknown binary systems, which need to break the data into very short segments in order to be computationally feasible. In this paper, we present a new detection statistic that improves sensitivity by up to 19% compared to the standard -statistic for segments shorter than a few hours
Targeted search for continuous gravitational waves: Bayesian versus maximum-likelihood statistics
We investigate the Bayesian framework for detection of continuous
gravitational waves (GWs) in the context of targeted searches, where the phase
evolution of the GW signal is assumed to be known, while the four amplitude
parameters are unknown. We show that the orthodox maximum-likelihood statistic
(known as F-statistic) can be rediscovered as a Bayes factor with an unphysical
prior in amplitude parameter space. We introduce an alternative detection
statistic ("B-statistic") using the Bayes factor with a more natural amplitude
prior, namely an isotropic probability distribution for the orientation of GW
sources. Monte-Carlo simulations of targeted searches show that the resulting
Bayesian B-statistic is more powerful in the Neyman-Pearson sense (i.e. has a
higher expected detection probability at equal false-alarm probability) than
the frequentist F-statistic.Comment: 12 pages, presented at GWDAW13, to appear in CQ
Constraints on r-modes and mountains on millisecond neutron stars in binary systems
Continuous gravitational waves are nearly monochromatic signals emitted by
asymmetries in rotating neutron stars. These signals have not yet been
detected. Deep all-sky searches for continuous gravitational waves from
isolated neutron stars require significant computational expense. Deep searches
for neutron stars in binary systems are even more expensive, but potentially
these targets are more promising emitters, especially in the hundreds-Hz
region, where ground-based gravitational wave detectors are most sensitive. We
present here an all-sky search for continuous signals with frequency between
300 and 500 Hz, from neutron stars in binary systems with orbital period
between 15 and 60 days, and projected semi-major axis between 10 and 40
light-seconds. This is the only binary search on Advanced-LIGO data that probes
this frequency range. Compared to previous results, our search is over an order
of magnitude more sensitive. We do not detect any signals, but our results
exclude plausible and unexplored neutron star configurations, for example,
neutron stars with relative deformations greater than 3e-6 within 1 kpc from
Earth and r-mode emission at the level of alpha ~ few 1e-4 within the same
distance.Comment: Accepted for publication in The Astrophysical Journal Letter
Building a stochastic template bank for detecting massive black hole binaries
Coalescence of two massive black holes is the strongest and most promising
source for LISA. In fact, gravitational signal from the end of inspiral and
merger will be detectable throughout the Universe. In this article we describe
the first step in the two-step hierarchical search for gravitational wave
signal from the inspiraling massive BH binaries. It is based on the routinely
used in the ground base gravitational wave astronomy method of filtering the
data through the bank of templates. However we use a novel Monte-Carlo based
(stochastic) method to lay a grid in the parameter space, and we use the
likelihood maximized analytically over some parameters, known as F-statistic,
as a detection statistic. We build a coarse template bank to detect
gravitational wave signals and to make preliminary parameter estimation. The
best candidates will be followed up using Metropolis-Hasting stochastic search
to refine the parameter estimation. We demonstrate the performance of the
method by applying it to the Mock LISA data challenge 1B (training data set).Comment: revtex4, 8 figure
Non-equilibrium beta processes in superfluid neutron star cores
The influence of nucleons superfluidity on the beta relaxation time of
degenerate neutron star cores, composed of neutrons, protons and electrons, is
investigated. We numerically calculate the implied reduction factors for both
direct and modified Urca reactions, with isotropic pairing of protons or
anisotropic pairing of neutrons. We find that due to the non-zero value of the
temperature and/or to the vanishing of anisotropic gaps in some directions of
the phase-space, superfluidity does not always completely inhibit beta
relaxation, allowing for some reactions if the superfluid gap amplitude is not
too large in respect to both the typical thermal energy and the chemical
potential mismatch. We even observe that if the ratio between the critical
temperature and the actual temperature is very small, a suprathermal regime is
reached for which superfluidity is almost irrelevant. On the contrary, if the
gap is large enough, the composition of the nuclear matter can stay frozen for
very long durations, unless the departure from beta equilibrium is at least as
important as the gap amplitude. These results are crucial for precise
estimation of the superfluidity effect on the cooling/slowing-down of pulsars
and we provide online subroutines to be implemented in codes for simulating
such evolutions.Comment: 11 pages, 6 Figs., published, minor changes, subroutines can be found
on line at http://luth2.obspm.fr/~etu/villain/Micro/Resolution.htm
Gravitational wave background from rotating neutron stars
The background of gravitational waves produced by the ensemble of rotating
neutron stars (which includes pulsars, magnetars and gravitars) is
investigated. A formula for \Omega(f) (commonly used to quantify the
background) is derived, properly taking into account the time evolution of the
systems since their formation until the present day. Moreover, the formula
allows one to distinguish the different parts of the background: the
unresolvable (which forms a stochastic background) and the resolvable. Several
estimations of the background are obtained, for different assumptions on the
parameters that characterize neutron stars and their population. In particular,
different initial spin period distributions lead to very different results. For
one of the models, with slow initial spins, the detection of the background can
be rejected. However, other models do predict the detection of the background
by the future ground-based gravitational wave detector ET. A robust upper limit
for the background of rotating neutron stars is obtained; it does not exceed
the detection threshold of two cross-correlated Advanced LIGO interferometers.
If gravitars exist and constitute more than a few percent of the neutron star
population, then they produce an unresolvable background that could be detected
by ET. Under the most reasonable assumptions on the parameters characterizing a
neutron star, the background is too faint. Previous papers have suggested
neutron star models in which large magnetic fields (like the ones that
characterize magnetars) induce big deformations in the star, which produce a
stronger emission of gravitational radiation. Considering the most optimistic
(in terms of the detection of gravitational waves) of these models, an upper
limit for the background produced by magnetars is obtained; it could be
detected by ET, but not by BBO or DECIGO.Comment: 25 pages, 15 figure
Vortex in a weakly relativistic Bose gas at zero temperature and relativistic fluid approximation
The Bogoliubov procedure in quantum field theory is used to describe a
relativistic almost ideal Bose gas at zero temperature. Special attention is
given to the study of a vortex. The radius of the vortex in the field
description is compared to that obtained in the relativistic fluid
approximation. The Kelvin waves are studied and, for long wavelengths, the
dispersion relation is obtained by an asymptotic matching method and compared
with the non relativistic result.Comment: 20 page
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