2,032 research outputs found
Measurement of Parity Violation in the Early Universe using Gravitational-wave Detectors
A stochastic gravitational-wave background (SGWB) is expected to arise from
the superposition of many independent and unresolved gravitational-wave
signals, of either cosmological or astrophysical origin. Some cosmological
models (characterized, for instance, by a pseudo-scalar inflaton, or by some
modification of gravity) break parity, leading to a polarized SGWB. We present
a new technique to measure this parity violation, which we then apply to the
recent results from LIGO to produce the first upper limit on parity violation
in the SGWB, assuming a generic power-law SGWB spectrum across the LIGO
sensitive frequency region. We also estimate sensitivity to parity violation of
the future generations of gravitational-wave detectors, both for a power-law
spectrum and for a model of axion inflation. This technique offers a new way of
differentiating between the cosmological and astrophysical sources of the
isotropic SGWB, as astrophysical sources are not expected to produce a
polarized SGWB.Comment: 5 pages, 2 figures, 1 tabl
Registration of retinal images from Public Health by minimising an error between vessels using an affine model with radial distortions
In order to estimate a registration model of eye fundus images made of an
affinity and two radial distortions, we introduce an estimation criterion based
on an error between the vessels. In [1], we estimated this model by minimising
the error between characteristics points. In this paper, the detected vessels
are selected using the circle and ellipse equations of the overlap area
boundaries deduced from our model. Our method successfully registers 96 % of
the 271 pairs in a Public Health dataset acquired mostly with different
cameras. This is better than our previous method [1] and better than three
other state-of-the-art methods. On a publicly available dataset, ours still
better register the images than the reference method
A Solution to the Galactic Foreground Problem for LISA
Low frequency gravitational wave detectors, such as the Laser Interferometer
Space Antenna (LISA), will have to contend with large foregrounds produced by
millions of compact galactic binaries in our galaxy. While these galactic
signals are interesting in their own right, the unresolved component can
obscure other sources. The science yield for the LISA mission can be improved
if the brighter and more isolated foreground sources can be identified and
regressed from the data. Since the signals overlap with one another we are
faced with a ``cocktail party'' problem of picking out individual conversations
in a crowded room. Here we present and implement an end-to-end solution to the
galactic foreground problem that is able to resolve tens of thousands of
sources from across the LISA band. Our algorithm employs a variant of the
Markov Chain Monte Carlo (MCMC) method, which we call the Blocked Annealed
Metropolis-Hastings (BAM) algorithm. Following a description of the algorithm
and its implementation, we give several examples ranging from searches for a
single source to searches for hundreds of overlapping sources. Our examples
include data sets from the first round of Mock LISA Data Challenges.Comment: 19 pages, 27 figure
The role of fingerprints in the coding of tactile information probed with a biomimetic sensor
In humans, the tactile perception of fine textures (spatial scale <200
micrometers) is mediated by skin vibrations generated as the finger scans the
surface. To establish the relationship between texture characteristics and
subcutaneous vibrations, a biomimetic tactile sensor has been designed whose
dimensions match those of the fingertip. When the sensor surface is patterned
with parallel ridges mimicking the fingerprints, the spectrum of vibrations
elicited by randomly textured substrates is dominated by one frequency set by
the ratio of the scanning speed to the interridge distance. For human touch,
this frequency falls within the optimal range of sensitivity of Pacinian
afferents, which mediate the coding of fine textures. Thus, fingerprints may
perform spectral selection and amplification of tactile information that
facilitate its processing by specific mechanoreceptors.Comment: 25 pages, 11 figures, article + supporting materia
Observed Limits on Charge Exchange Contributions to the Diffuse X-ray Background
We present a high resolution spectrum of the diffuse X-ray background from
0.1 to 1 keV for a ~1 region of the sky centered at l=90, b=+60 using a
36-pixel array of microcalorimeters flown on a sounding rocket. With an energy
resolution of 11 eV FWHM below 1 keV, the spectrum's observed line ratios help
separate charge exchange contributions originating within the heliosphere from
thermal emission of hot gas in the interstellar medium. The X-ray sensitivity
below 1 keV was reduced by about a factor of four from contamination that
occurred early in the flight, limiting the significance of the results. The
observed centroid of helium-like O VII is 568+2-3 eV at 90% confidence. Since
the centroid expected for thermal emission is 568.4 eV while for charge
exchange is 564.2 eV, thermal emission appears to dominate for this line
complex, consistent with much of the high-latitude O VII emission originating
in 2-3 x 10^6 K gas in the Galactic halo. On the other hand, the observed ratio
of C VI Ly gamma to Ly alpha is 0.3+-0.2. The expected ratios are 0.04 for
thermal emission and 0.24 for charge exchange, indicating that charge exchange
must contribute strongly to this line and therefore potentially to the rest of
the ROSAT R12 band usually associated with 10^6 K emission from the Local Hot
Bubble. The limited statistics of this experiment and systematic uncertainties
due to the contamination require only >32% thermal emission for O VII and >20%
from charge exchange for C VI at the 90% confidence level. An experimental gold
coating on the silicon substrate of the array greatly reduced extraneous
signals induced on nearby pixels from cosmic rays passing through the
substrate, reducing the triggered event rate by a factor of 15 from a previous
flight of the instrument.Comment: 14 pages, 7 figures, to be published in Ap
Gravitational waves from Sco X-1: A comparison of search methods and prospects for detection with advanced detectors
The low-mass X-ray binary Scorpius X-1 (Sco X-1) is potentially the most
luminous source of continuous gravitational-wave radiation for interferometers
such as LIGO and Virgo. For low-mass X-ray binaries this radiation would be
sustained by active accretion of matter from its binary companion. With the
Advanced Detector Era fast approaching, work is underway to develop an array of
robust tools for maximizing the science and detection potential of Sco X-1. We
describe the plans and progress of a project designed to compare the numerous
independent search algorithms currently available. We employ a mock-data
challenge in which the search pipelines are tested for their relative
proficiencies in parameter estimation, computational efficiency, robust- ness,
and most importantly, search sensitivity. The mock-data challenge data contains
an ensemble of 50 Scorpius X-1 (Sco X-1) type signals, simulated within a
frequency band of 50-1500 Hz. Simulated detector noise was generated assuming
the expected best strain sensitivity of Advanced LIGO and Advanced VIRGO ( Hz). A distribution of signal amplitudes was then
chosen so as to allow a useful comparison of search methodologies. A factor of
2 in strain separates the quietest detected signal, at
strain, from the torque-balance limit at a spin frequency of 300 Hz, although
this limit could range from (25 Hz) to (750 Hz) depending on the unknown frequency of Sco X-1. With future
improvements to the search algorithms and using advanced detector data, our
expectations for probing below the theoretical torque-balance strain limit are
optimistic.Comment: 33 pages, 11 figure
Gas damping force noise on a macroscopic test body in an infinite gas reservoir
We present a simple analysis of the force noise associated with the
mechanical damping of the motion of a test body surrounded by a large volume of
rarefied gas. The calculation is performed considering the momentum imparted by
inelastic collisions against the sides of a cubic test mass, and for other
geometries for which the force noise could be an experimental limitation. In
addition to arriving at an accurated estimate, by two alternative methods, we
discuss the limits of the applicability of this analysis to realistic
experimental configurations in which a test body is surrounded by residual gas
inside an enclosure that is only slightly larger than the test body itself.Comment: 8 pages. updated with correct translational damping coefficient for
cylinder on axis. added cylinder orthogonal to symmetry axis, force and
torque. slightly edited throughou
LISA Data Analysis using MCMC methods
The Laser Interferometer Space Antenna (LISA) is expected to simultaneously
detect many thousands of low frequency gravitational wave signals. This
presents a data analysis challenge that is very different to the one
encountered in ground based gravitational wave astronomy. LISA data analysis
requires the identification of individual signals from a data stream containing
an unknown number of overlapping signals. Because of the signal overlaps, a
global fit to all the signals has to be performed in order to avoid biasing the
solution. However, performing such a global fit requires the exploration of an
enormous parameter space with a dimension upwards of 50,000. Markov Chain Monte
Carlo (MCMC) methods offer a very promising solution to the LISA data analysis
problem. MCMC algorithms are able to efficiently explore large parameter
spaces, simultaneously providing parameter estimates, error analyses and even
model selection. Here we present the first application of MCMC methods to
simulated LISA data and demonstrate the great potential of the MCMC approach.
Our implementation uses a generalized F-statistic to evaluate the likelihoods,
and simulated annealing to speed convergence of the Markov chains. As a final
step we super-cool the chains to extract maximum likelihood estimates, and
estimates of the Bayes factors for competing models. We find that the MCMC
approach is able to correctly identify the number of signals present, extract
the source parameters, and return error estimates consistent with Fisher
information matrix predictions.Comment: 14 pages, 7 figure
Space Use and Relative Habitat Selection for Immature Green Turtles Within a Caribbean Marine Protected Area
Background
A better understanding of sea turtle spatial ecology is critical for the continued conservation of imperiled sea turtles and their habitats. For resource managers to develop the most effective conservation strategies, it is especially important to examine how turtles use and select for habitats within their developmental foraging grounds. Here, we examine the space use and relative habitat selection of immature green turtles (Chelonia mydas) using acoustic telemetry within the marine protected area, Buck Island Reef National Monument (BIRNM), St. Croix, United States Virgin Islands. Results
Space use by turtles was concentrated on the southern side of Buck Island, but also extended to the northeast and northwest areas of the island, as indicated by minimum convex polygons (MCPs) and 99%, 95%, and 50% kernel density estimations (KDEs). On average space use for all categories was \u3c 3 km2 with mean KDE area overlap ranging from 41.9 to 67.7%. Cumulative monthly MCPs and their proportions to full MCPs began to stabilize 3 to 6 detection months after release, respectively. Resource selection functions (RSFs) were implemented using a generalized linear mixed effects model with turtle ID as the random effect. After model selection, the accuracy of the top model was 77.3% and showed relative habitat selection values were highest at shallow depths, for areas in close proximity to seagrass, and in reef zones for both day and night, and within lagoon zones at night. The top model was also extended to predict across BIRNM at both day and night. Conclusion
More traditional acoustic telemetry analyses in combination with RSFs provide novel insights into animal space use and relative resource selection. Here, we demonstrated immature green turtles within the BIRNM have small, specific home ranges and core use areas with temporally varying relative selection strengths across habitat types. We conclude the BIRNM marine protected area is providing sufficient protection for immature green turtles, however, habitat protection could be focused in both areas of high space use and in locations where high relative selection values were determined. Ultimately, the methodologies and results presented here may help to design strategies to expand habitat protection for immature green turtles across their greater distribution
Report on the first round of the Mock LISA Data Challenges
The Mock LISA Data Challenges (MLDCs) have the dual purpose of fostering the
development of LISA data analysis tools and capabilities, and demonstrating the
technical readiness already achieved by the gravitational-wave community in
distilling a rich science payoff from the LISA data output. The first round of
MLDCs has just been completed: nine data sets containing simulated
gravitational wave signals produced either by galactic binaries or massive
black hole binaries embedded in simulated LISA instrumental noise were released
in June 2006 with deadline for submission of results at the beginning of
December 2006. Ten groups have participated in this first round of challenges.
Here we describe the challenges, summarise the results, and provide a first
critical assessment of the entries.Comment: Proceedings report from GWDAW 11. Added author, added reference,
clarified some text, removed typos. Results unchanged; Removed author, minor
edits, reflects submitted versio
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