2,051 research outputs found
Gravitational wave forms for a three-body system in Lagrange's orbit: parameter determinations and a binary source test
Continuing work initiated in an earlier publication [Torigoe et al. Phys.
Rev. Lett. {\bf 102}, 251101 (2009)], gravitational wave forms for a three-body
system in Lagrange's orbit are considered especially in an analytic method.
First, we derive an expression of the three-body wave forms at the mass
quadrupole, octupole and current quadrupole orders. By using the expressions,
we solve a gravitational-wave {\it inverse} problem of determining the source
parameters to this particular configuration (three masses, a distance of the
source to an observer, and the orbital inclination angle to the line of sight)
through observations of the gravitational wave forms alone. For this purpose,
the chirp mass to a three-body system in the particular configuration is
expressed in terms of only the mass ratios by deleting initial angle positions.
We discuss also whether and how a binary source can be distinguished from a
three-body system in Lagrange's orbit or others.Comment: 21 pages, 3 figures, 1 table; text improved, typos corrected;
accepted for publication in PR
Choreographic solution to the general relativistic three-body problem
We revisit the three-body problem in the framework of general relativity. The
Newtonian N-body problem admits choreographic solutions, where a solution is
called choreographic if every massive particles move periodically in a single
closed orbit. One is a stable figure-eight orbit for a three-body system, which
was found first by Moore (1993) and re-discovered with its existence proof by
Chenciner and Montgomery (2000). In general relativity, however, the periastron
shift prohibits a binary system from orbiting in a single closed curve.
Therefore, it is unclear whether general relativistic effects admit a
choreographic solution such as the figure eight. We carefully examine general
relativistic corrections to initial conditions so that an orbit for a
three-body system can be closed and a figure eight. This solution is still
choreographic. This illustration suggests that the general relativistic N-body
problem also may admit a certain class of choreographic solutions.Comment: 10 pages, 4 figures, text improved, accepted for publication in PR
Action minimizing orbits in the n-body problem with simple choreography constraint
In 1999 Chenciner and Montgomery found a remarkably simple choreographic
motion for the planar 3-body problem (see \cite{CM}). In this solution 3 equal
masses travel on a eight shaped planar curve; this orbit is obtained minimizing
the action integral on the set of simple planar choreographies with some
special symmetry constraints. In this work our aim is to study the problem of
masses moving in \RR^d under an attractive force generated by a potential
of the kind , , with the only constraint to be a simple
choreography: if are the orbits then we impose the
existence of x \in H^1_{2 \pi}(\RR,\RR^d) such that q_i(t)=x(t+(i-1) \tau),
i=1,...,n, t \in \RR, where . In this setting, we first
prove that for every d,n \in \NN and , the lagrangian action
attains its absolute minimum on the planar circle. Next we deal with the
problem in a rotating frame and we show a reacher phenomenology: indeed while
for some values of the angular velocity minimizers are still circles, for
others the minima of the action are not anymore rigid motions.Comment: 24 pages; 4 figures; submitted to Nonlinearit
Depth of interaction and bias voltage depenence of the spectral response in a pixellated CdTe detector operating in time-over-threshold mode subjected to monochromatic X-rays
High stopping power is one of the most important figures of merit for X-ray detectors. CdTe is a promising material but suffers from: material defects, non-ideal charge transport and long range X-ray fluorescence. Those factors reduce the image quality and deteriorate spectral information. In this project we used a monochromatic pencil beam collimated through a 20μm pinhole to measure the detector spectral response in dependance on the depth of interaction. The sensor was a 1mm thick CdTe detector with a pixel pitch of 110μm, bump bonded to a Timepix readout chip operating in Time-Over-Threshold mode. The measurements were carried out at the Extreme Conditions beamline I15 of the Diamond Light Source. The beam was entering the sensor at an angle of \texttildelow20 degrees to the surface and then passed through \texttildelow25 pixels before leaving through the bottom of the sensor. The photon energy was tuned to 77keV giving a variation in the beam intensity of about three orders of magnitude along the beam path. Spectra in Time-over-Threshold (ToT) mode were recorded showing each individual interaction. The bias voltage was varied between -30V and -300V to investigate how the electric field affected the spectral information. For this setup it is worth noticing the large impact of fluorescence. At -300V the photo peak and escape peak are of similar height. For high bias voltages the spectra remains clear throughout the whole depth but for lower voltages as -50V, only the bottom part of the sensor carries spectral information. This is an effect of the low hole mobility and the longer range the electrons have to travel in a low field
Uniqueness of collinear solutions for the relativistic three-body problem
Continuing work initiated in an earlier publication [Yamada, Asada, Phys.
Rev. D 82, 104019 (2010)], we investigate collinear solutions to the general
relativistic three-body problem. We prove the uniqueness of the configuration
for given system parameters (the masses and the end-to-end length). First, we
show that the equation determining the distance ratio among the three masses,
which has been obtained as a seventh-order polynomial in the previous paper,
has at most three positive roots, which apparently provide three cases of the
distance ratio. It is found, however, that, even for such cases, there exists
one physically reasonable root and only one, because the remaining two positive
roots do not satisfy the slow motion assumption in the post-Newtonian
approximation and are thus discarded. This means that, especially for the
restricted three-body problem, exactly three positions of a third body are true
even at the post-Newtonian order. They are relativistic counterparts of the
Newtonian Lagrange points L1, L2 and L3. We show also that, for the same masses
and full length, the angular velocity of the post-Newtonian collinear
configuration is smaller than that for the Newtonian case. Provided that the
masses and angular rate are fixed, the relativistic end-to-end length is
shorter than the Newtonian one.Comment: 18 pages, 1 figure; typos corrected, text improved; accepted by PR
Collinear solution to the general relativistic three-body problem
The three-body problem is reexamined in the framework of general relativity.
The Newtonian three-body problem admits Euler's collinear solution, where three
bodies move around the common center of mass with the same orbital period and
always line up. The solution is unstable. Hence it is unlikely that such a
simple configuration would exist owing to general relativistic forces dependent
not only on the masses but also on the velocity of each body. However, we show
that the collinear solution remains true with a correction to the spatial
separation between masses. Relativistic corrections to the Sun-Jupiter Lagrange
points L1, L2 and L3 are also evaluated.Comment: 12 pages, 2 figures, accepted for publication in PR
The effect of haptic guidance, aging, and initial skill level on motor learning of a steering task
In a previous study, we found that haptic guidance from a robotic steering wheel can improve short-term learning of steering of a simulated vehicle, in contrast to several studies of other tasks that had found that the guidance either impairs or does not aid motor learning. In this study, we examined whether haptic guidance-as-needed can improve long-term retention (across 1 week) of the steering task, with age and initial skill level as independent variables. Training with guidance-as-needed allowed all participants to learn to steer without experiencing large errors. For young participants (age 18–30), training with guidance-as-needed produced better long-term retention of driving skill than did training without guidance. For older participants (age 65–92), training with guidance-as-needed improved long-term retention in tracking error, but not significantly. However, for a subset of less skilled, older subjects, training with guidance-as-needed significantly improved long-term retention. The benefits of guidance-based training were most evident as an improved ability to straighten the vehicle direction when coming out of turns. In general, older participants not only systematically performed worse at the task than younger subjects (errors ∼3 times greater), but also apparently learned more slowly, forgetting a greater percentage of the learned task during the 1 week layoffs between the experimental sessions. This study demonstrates that training with haptic guidance can benefit long-term retention of a driving skill for young and for some old drivers. Training with haptic guidance is more useful for people with less initial skill
DeltaPhish: Detecting Phishing Webpages in Compromised Websites
The large-scale deployment of modern phishing attacks relies on the automatic
exploitation of vulnerable websites in the wild, to maximize profit while
hindering attack traceability, detection and blacklisting. To the best of our
knowledge, this is the first work that specifically leverages this adversarial
behavior for detection purposes. We show that phishing webpages can be
accurately detected by highlighting HTML code and visual differences with
respect to other (legitimate) pages hosted within a compromised website. Our
system, named DeltaPhish, can be installed as part of a web application
firewall, to detect the presence of anomalous content on a website after
compromise, and eventually prevent access to it. DeltaPhish is also robust
against adversarial attempts in which the HTML code of the phishing page is
carefully manipulated to evade detection. We empirically evaluate it on more
than 5,500 webpages collected in the wild from compromised websites, showing
that it is capable of detecting more than 99% of phishing webpages, while only
misclassifying less than 1% of legitimate pages. We further show that the
detection rate remains higher than 70% even under very sophisticated attacks
carefully designed to evade our system.Comment: Preprint version of the work accepted at ESORICS 201
Rac GTPase isoform-specific regulation of NADPH oxidase and chemotaxis in murine neutrophils in vivo: Role of the C-terminal polybasic domain
The Rho family GTPase Rac acts as a molecular switch for signal transduction to regulate various cellular functions. Mice deficient in the hematopoietic-specific Rac2 isoform exhibit agonist-specific defects in neutrophil chemotaxis and superoxide production, despite expression of the highly homologous Rac1 isoform. To examine whether functional defects in rac2–/– neutrophils reflect effects of an overall decrease in total cellular Rac or an isoform-specific role for Rac2, retroviral vectors were used to express exogenous Rac1 or Rac2 at levels similar to endogenous. In rac2–/– neutrophils differentiated from transduced myeloid progenitors in vitro, increasing cellular Rac levels by expression of either exogenous Rac1 or Rac2 increased formylmethionylleucylphenylalanine- or phorbol ester-stimulated NADPH oxidase activity. Of note, placement of an epitope tag on the N terminus of Rac1 or Rac2 blunted reconstitution of responses in rac2–/– neutrophils. In rac2–/– neutrophils isolated from mice transplanted with Rac-transduced bone marrow cells, superoxide production and chemotaxis were fully reconstituted by expression of exogenous Rac2, but not Rac1. A chimeric Rac1 protein in which the Rac1 C-terminal polybasic domain, which contains six lysines or arginines, was replaced with that of the human Rac2 polybasic domain containing only three basic residues, also reconstituted superoxide production and chemotaxis, whereas expression of a Rac2 derivative in which the polybasic domain was replaced with that of Rac1 did not and resulted in disoriented cell motility. Thus, the composition of the polybasic domain is sufficient for determining Rac isoform specificity in the production of superoxide and chemotaxis in murine neutrophils in vivo
A genoserotyping system for a fast and objective identification of Salmonella serotypes commonly isolated from poultry and pork food sectors in Belgium
Humans are mostly contaminated by Salmonella through the consumption of pork- and poultry-derived food products. Therefore, a strict monitoring of Salmonella serotypes in food-producing animals is needed to limit the transmission of the pathogen to humans. Additionally, Salmonella can lead to economic loss in the food sector. Previously, a genoserotyping method using the MOL-PCR and Luminex technology was developed for the identification of the 6 Salmonella serotypes, and their variants, subjected to an official control in the Belgian food sector. In this study, 3 additional assays using the same technology were developed for the rapid and cost-effective detection of 13 dangerous highly invasive serotypes or other serotypes frequently isolated from the Belgian poultry and pork sector, i.e. Agona, Anatum, Brandenburg, Choleraesuis, Derby, Enteritidis vaccine strains, Gallinarum var. Gallinarum/Pullorum, Livingstone, Mbandaka, Minnesota, Ohio, Rissen and Senftenberg. Moreover, the previously developed first MOL-PCR assay was improved for S. Paratyphi B and serogroup O:3 detection. Finally, a Decision Support System hosted by a web application was created for an automatic and objective interpretation of the Luminex raw data. The 3 new assays and the modifications of the first assay were validated with a 100% accuracy, using 553 Salmonella and non-Salmonella strains in total
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