21 research outputs found
Sliding coherence window technique for hierarchical detection of continuous gravitational waves
A novel hierarchical search technique is presented for all-sky surveys for
continuous gravitational-wave sources, such as rapidly spinning nonaxisymmetric
neutron stars. Analyzing yearlong detector data sets over realistic ranges of
parameter space using fully coherent matched-filtering is computationally
prohibitive. Thus more efficient, so-called hierarchical techniques are
essential. Traditionally, the standard hierarchical approach consists of
dividing the data into nonoverlapping segments of which each is coherently
analyzed and subsequently the matched-filter outputs from all segments are
combined incoherently. The present work proposes to break the data into
subsegments shorter than the desired maximum coherence time span (size of the
coherence window). Then matched-filter outputs from the different subsegments
are efficiently combined by sliding the coherence window in time: Subsegments
whose timestamps are closer than coherence window size are combined coherently,
otherwise incoherently. Compared to the standard scheme at the same coherence
time baseline, data sets longer by about 50-100% would have to be analyzed to
achieve the same search sensitivity as with the sliding coherence window
approach. Numerical simulations attest to the analytically estimated
improvement.Comment: 11 pages, 4 figure
Parameter-space correlations of the optimal statistic for continuous gravitational-wave detection
The phase parameters of matched-filtering searches for continuous
gravitational-wave signals are sky position, frequency and frequency
time-derivatives. The space of these parameters features strong global
correlations in the optimal detection statistic. For observation times smaller
than one year, the orbital motion of the Earth leads to a family of
global-correlation equations which describes the "global maximum structure" of
the detection statistic. The solution to each of these equations is a different
hypersurface in parameter space. The expected detection statistic is maximal at
the intersection of these hypersurfaces. The global maximum structure of the
detection statistic from stationary instrumental-noise artifacts is also
described by the global-correlation equations. This permits the construction of
a veto method which excludes false candidate events.Comment: 17 pages, 10 figure
Efficient generation and optimization of stochastic template banks by a neighboring cell algorithm
Placing signal templates (grid points) as efficiently as possible to cover a
multi-dimensional parameter space is crucial in computing-intensive
matched-filtering searches for gravitational waves, but also in similar
searches in other fields of astronomy. To generate efficient coverings of
arbitrary parameter spaces, stochastic template banks have been advocated,
where templates are placed at random while rejecting those too close to others.
However, in this simple scheme, for each new random point its distance to every
template in the existing bank is computed. This rapidly increasing number of
distance computations can render the acceptance of new templates
computationally prohibitive, particularly for wide parameter spaces or in large
dimensions. This work presents a neighboring cell algorithm that can
dramatically improve the efficiency of constructing a stochastic template bank.
By dividing the parameter space into sub-volumes (cells), for an arbitrary
point an efficient hashing technique is exploited to obtain the index of its
enclosing cell along with the parameters of its neighboring templates. Hence
only distances to these neighboring templates in the bank are computed,
massively lowering the overall computing cost, as demonstrated in simple
examples. Furthermore, we propose a novel method based on this technique to
increase the fraction of covered parameter space solely by directed template
shifts, without adding any templates. As is demonstrated in examples, this
method can be highly effective..Comment: PRD accepte
Gamma-ray Timing of Redback PSR J2339-0533: Hints for Gravitational Quadrupole Moment Changes
We present the results of precision gamma-ray timing measurements of the
binary millisecond pulsar PSR J23390533, an irradiating system of "redback"
type, using data from the Fermi Large Area Telescope. We describe an optimized
analysis method to determine a long-term phase-coherent timing solution
spanning more than six years, including a measured eccentricity of the binary
orbit and constraints on the proper motion of the system. A major result of
this timing analysis is the discovery of an extreme variation of the nominal
4.6-hour orbital period over time, showing alternating epochs of
decrease and increase. We inferred a cyclic modulation of with an
approximate cycle duration of 4.2 years and a modulation amplitude of . Considering different possible
physical causes, the observed orbital-period modulation most likely results
from a variable gravitational quadrupole moment of the companion star due to
cyclic magnetic activity in its convective zone.Comment: 9 pages, 2 figure
Optimized Blind Gamma-ray Pulsar Searches at Fixed Computing Budget
The sensitivity of blind gamma-ray pulsar searches in multiple years worth of
photon data, as from the Fermi LAT, is primarily limited by the finite
computational resources available. Addressing this "needle in a haystack"
problem, we here present methods for optimizing blind searches to achieve the
highest sensitivity at fixed computing cost. For both coherent and semicoherent
methods, we consider their statistical properties and study their search
sensitivity under computational constraints. The results validate a multistage
strategy, where the first stage scans the entire parameter space using an
efficient semicoherent method and promising candidates are then refined through
a fully coherent analysis. We also find that for the first stage of a blind
search incoherent harmonic summing of powers is not worthwhile at fixed
computing cost for typical gamma-ray pulsars. Further enhancing sensitivity, we
present efficiency-improved interpolation techniques for the semicoherent
search stage. Via realistic simulations we demonstrate that overall these
optimizations can significantly lower the minimum detectable pulsed fraction by
almost 50% at the same computational expense.Comment: 22 pages, 13 figures; includes ApJ proof correction
Parameter-space metric of semicoherent searches for continuous gravitational waves
Continuous gravitational-wave (CW) signals such as emitted by spinning
neutron stars are an important target class for current detectors. However, the
enormous computational demand prohibits fully coherent broadband all-sky
searches for prior unknown CW sources over wide ranges of parameter space and
for yearlong observation times. More efficient hierarchical "semicoherent"
search strategies divide the data into segments much shorter than one year,
which are analyzed coherently; then detection statistics from different
segments are combined incoherently. To optimally perform the incoherent
combination, understanding of the underlying parameter-space structure is
requisite. This problem is addressed here by using new coordinates on the
parameter space, which yield the first analytical parameter-space metric for
the incoherent combination step. This semicoherent metric applies to broadband
all-sky surveys (also embedding directed searches at fixed sky position) for
isolated CW sources. Furthermore, the additional metric resolution attained
through the combination of segments is studied. From the search parameters (sky
position, frequency, and frequency derivatives), solely the metric resolution
in the frequency derivatives is found to significantly increase with the number
of segments.Comment: 14 pages, 5 figures (matching Phys.Rev.D version
Exploiting Large-Scale Correlations to Detect Continuous Gravitational Waves
Fully coherent searches (over realistic ranges of parameter space and
year-long observation times) for unknown sources of continuous gravitational
waves are computationally prohibitive. Less expensive hierarchical searches
divide the data into shorter segments which are analyzed coherently, then
detection statistics from different segments are combined incoherently. The
novel method presented here solves the long-standing problem of how best to do
the incoherent combination. The optimal solution exploits large-scale
parameter-space correlations in the coherent detection statistic. Application
to simulated data shows dramatic sensitivity improvements compared with
previously available (ad hoc) methods, increasing the spatial volume probed by
more than 2 orders of magnitude at lower computational cost.Comment: 4 pages, 2 figure
Quantum state preparation and macroscopic entanglement in gravitational-wave detectors
Long-baseline laser-interferometer gravitational-wave detectors are operating
at a factor of 10 (in amplitude) above the standard quantum limit (SQL) within
a broad frequency band. Such a low classical noise budget has already allowed
the creation of a controlled 2.7 kg macroscopic oscillator with an effective
eigenfrequency of 150 Hz and an occupation number of 200. This result, along
with the prospect for further improvements, heralds the new possibility of
experimentally probing macroscopic quantum mechanics (MQM) - quantum mechanical
behavior of objects in the realm of everyday experience - using
gravitational-wave detectors. In this paper, we provide the mathematical
foundation for the first step of a MQM experiment: the preparation of a
macroscopic test mass into a nearly minimum-Heisenberg-limited Gaussian quantum
state, which is possible if the interferometer's classical noise beats the SQL
in a broad frequency band. Our formalism, based on Wiener filtering, allows a
straightforward conversion from the classical noise budget of a laser
interferometer, in terms of noise spectra, into the strategy for quantum state
preparation, and the quality of the prepared state. Using this formalism, we
consider how Gaussian entanglement can be built among two macroscopic test
masses, and the performance of the planned Advanced LIGO interferometers in
quantum-state preparation
Searching for a Stochastic Background of Gravitational Waves with LIGO
The Laser Interferometer Gravitational-wave Observatory (LIGO) has performed
the fourth science run, S4, with significantly improved interferometer
sensitivities with respect to previous runs. Using data acquired during this
science run, we place a limit on the amplitude of a stochastic background of
gravitational waves. For a frequency independent spectrum, the new limit is
. This is currently the most sensitive
result in the frequency range 51-150 Hz, with a factor of 13 improvement over
the previous LIGO result. We discuss complementarity of the new result with
other constraints on a stochastic background of gravitational waves, and we
investigate implications of the new result for different models of this
background.Comment: 37 pages, 16 figure