8,676 research outputs found
Data analysis strategies for the detection of gravitational waves in non-Gaussian noise
In order to analyze data produced by the kilometer-scale gravitational wave
detectors that will begin operation early next century, one needs to develop
robust statistical tools capable of extracting weak signals from the detector
noise. This noise will likely have non-stationary and non-Gaussian components.
To facilitate the construction of robust detection techniques, I present a
simple two-component noise model that consists of a background of Gaussian
noise as well as stochastic noise bursts. The optimal detection statistic
obtained for such a noise model incorporates a natural veto which suppresses
spurious events that would be caused by the noise bursts. When two detectors
are present, I show that the optimal statistic for the non-Gaussian noise model
can be approximated by a simple coincidence detection strategy. For simulated
detector noise containing noise bursts, I compare the operating characteristics
of (i) a locally optimal detection statistic (which has nearly-optimal behavior
for small signal amplitudes) for the non-Gaussian noise model, (ii) a standard
coincidence-style detection strategy, and (iii) the optimal statistic for
Gaussian noise.Comment: 5 pages RevTeX, 4 figure
Deep space network support of the manned space flight network for Apollo, volume 2 Technical memorandum, 1969 - 1970
Deep Space Network support activities for Apollo 9 through 13 flights and associated equipmen
Is the squeezing of relic gravitational waves produced by inflation detectable?
Grishchuk has shown that the stochastic background of gravitational waves
produced by an inflationary phase in the early Universe has an unusual
property: it is not a stationary Gaussian random process. Due to squeezing, the
phases of the different waves are correlated in a deterministic way, arising
from the process of parametric amplification that created them. The resulting
random process is Gaussian but non-stationary. This provides a unique signature
that could in principle distinguish a background created by inflation from
stationary stochastic backgrounds created by other types of processes. We
address the question: could this signature be observed with a gravitational
wave detector? Sadly, the answer appears to be "no": an experiment which could
distinguish the non-stationary behavior would have to last approximately the
age of the Universe at the time of measurement. This rules out direct detection
by ground and space based gravitational wave detectors, but not indirect
detections via the electromagnetic Cosmic Microwave Background Radiation
(CMBR).Comment: 17 pages, 4 Postscript figures, uses revtex, psfig, to be submitted
to PRD, minor revisions - appendix B clarified, corrected typos, added
reference
Abstract Interpretation with Unfoldings
We present and evaluate a technique for computing path-sensitive interference
conditions during abstract interpretation of concurrent programs. In lieu of
fixed point computation, we use prime event structures to compactly represent
causal dependence and interference between sequences of transformers. Our main
contribution is an unfolding algorithm that uses a new notion of independence
to avoid redundant transformer application, thread-local fixed points to reduce
the size of the unfolding, and a novel cutoff criterion based on subsumption to
guarantee termination of the analysis. Our experiments show that the abstract
unfolding produces an order of magnitude fewer false alarms than a mature
abstract interpreter, while being several orders of magnitude faster than
solver-based tools that have the same precision.Comment: Extended version of the paper (with the same title and authors) to
appear at CAV 201
Traumatic brain injury: future assessment tools and treatment prospects
Traumatic brain injury (TBI) is widespread and leads to death and disability in millions of individuals around the world each year. Overall incidence and prevalence of TBI are likely to increase in absolute terms in the future. Tackling the problem of treating TBI successfully will require improvements in the understanding of normal cerebral anatomy, physiology, and function throughout the lifespan, as well as the pathological and recuperative responses that result from trauma. New treatment approaches and combinations will need to be targeted to the heterogeneous needs of TBI populations. This article explores and evaluates the research evidence in areas that will likely lead to a reduction in TBI-related morbidity and improved outcomes. These include emerging assessment instruments and techniques in areas of structural/chemical and functional neuroimaging and neuropsychology, advances in the realms of cell-based therapies and genetics, promising cognitive rehabilitation techniques including cognitive remediation and the use of electronic technologies including assistive devices and virtual reality, and the emerging field of complementary and alternative medicine
Hyperinsulinism-hyperammonaemia syndrome: novel mutations in the GLUD1 gene and genotype-phenotype correlations
Background: Activating mutations in the GLUD1 gene (which encodes for the intra-mitochondrial enzyme glutamate dehydrogenase, GDH) cause the hyperinsulinism–hyperammonaemia (HI/HA) syndrome. Patients present with HA and leucine-sensitive hypoglycaemia. GDH is regulated by another intra-mitochondrial enzyme sirtuin 4 (SIRT4). Sirt4 knockout mice demonstrate activation of GDH with increased amino acid-stimulated insulin secretion.
Objectives: To study the genotype–phenotype correlations in patients with GLUD1 mutations. To report the phenotype and functional analysis of a novel mutation (P436L) in the GLUD1 gene associated with the absence of HA.
Patients and methods: Twenty patients with HI from 16 families had mutational analysis of the GLUD1 gene in view of HA (n=19) or leucine sensitivity (n=1). Patients negative for a GLUD1 mutation had sequence analysis of the SIRT4 gene. Functional analysis of the novel P436L GLUD1 mutation was performed.
Results: Heterozygous missense mutations were detected in 15 patients with HI/HA, 2 of which are novel (N410D and D451V). In addition, a patient with a normal serum ammonia concentration (21 µmol/l) was heterozygous for a novel missense mutation P436L. Functional analysis of this mutation confirms that it is associated with a loss of GTP inhibition. Seizure disorder was common (43%) in our cohort of patients with a GLUD1 mutation. No mutations in the SIRT4 gene were identified.
Conclusion: Patients with HI due to mutations in the GLUD1 gene may have normal serum ammonia concentrations. Hence, GLUD1 mutational analysis may be indicated in patients with leucine sensitivity; even in the absence of HA. A high frequency of epilepsy (43%) was observed in our patients with GLUD1 mutations
Measuring gravitational waves from binary black hole coalescences: II. the waves' information and its extraction, with and without templates
We discuss the extraction of information from detected binary black hole
(BBH) coalescence gravitational waves, focusing on the merger phase that occurs
after the gradual inspiral and before the ringdown. Our results are: (1) If
numerical relativity simulations have not produced template merger waveforms
before BBH detections by LIGO/VIRGO, one can band-pass filter the merger waves.
For BBHs smaller than about 40 solar masses detected via their inspiral waves,
the band pass filtering signal to noise ratio indicates that the merger waves
should typically be just barely visible in the noise for initial and advanced
LIGO interferometers. (2) We derive an optimized (maximum likelihood) method
for extracting a best-fit merger waveform from the noisy detector output; one
"perpendicularly projects" this output onto a function space (specified using
wavelets) that incorporates our prior knowledge of the waveforms. An extension
of the method allows one to extract the BBH's two independent waveforms from
outputs of several interferometers. (3) If numerical relativists produce codes
for generating merger templates but running the codes is too expensive to allow
an extensive survey of the merger parameter space, then a coarse survey of this
parameter space, to determine the ranges of the several key parameters and to
explore several qualitative issues which we describe, would be useful for data
analysis purposes. (4) A complete set of templates could be used to test the
nonlinear dynamics of general relativity and to measure some of the binary
parameters. We estimate the number of bits of information obtainable from the
merger waves (about 10 to 60 for LIGO/VIRGO, up to 200 for LISA), estimate the
information loss due to template numerical errors or sparseness in the template
grid, and infer approximate requirements on template accuracy and spacing.Comment: 33 pages, Rextex 3.1 macros, no figures, submitted to Phys Rev
The horizon-entropy increase law for causal and quasi-local horizons and conformal field redefinitions
We explicitly prove the horizon-entropy increase law for both causal and
quasi-locally defined horizons in scalar-tensor and gravity theories.
Contrary to causal event horizons, future outer trapping horizons are not
conformally invariant and we provide a modification of trapping horizons to
complete the proof, using the idea of generalised entropy. This modification
means they are no longer foliated by marginally outer trapped surfaces but
fixes the location of the horizon under a conformal transformation. We also
discuss the behaviour of horizons in "veiled" general relativity and show,
using this new definition, how to locate cosmological horizons in flat
Minkowski space with varying units, which is physically identified with a
spatially flat FLRW spacetime.Comment: 23 page
Avalanche dynamics of radio pulsar glitches
We test statistically the hypothesis that radio pulsar glitches result from
an avalanche process, in which angular momentum is transferred erratically from
the flywheel-like superfluid in the star to the slowly decelerating, solid
crust via spatially connected chains of local, impulsive, threshold-activated
events, so that the system fluctuates around a self-organised critical state.
Analysis of the glitch population (currently 285 events from 101 pulsars)
demonstrates that the size distribution in individual pulsars is consistent
with being scale invariant, as expected for an avalanche process. The
waiting-time distribution is consistent with being exponential in seven out of
nine pulsars where it can be measured reliably, after adjusting for
observational limits on the minimum waiting time, as for a constant-rate
Poisson process. PSR J05376910 and PSR J08354510 are the exceptions;
their waiting-time distributions show evidence of quasiperiodicity. In each
object, stationarity requires that the rate equals , where is the angular acceleration of the
crust, is the mean glitch size, and is the
relative angular acceleration of the crust and superfluid. There is no evidence
that changes monotonically with spin-down age. The rate distribution
itself is fitted reasonably well by an exponential for . For , its exact form is unknown; the
exponential overestimates the number of glitching pulsars observed at low
, where the limited total observation time exercises a selection bias.Comment: Accepted for publication in the Astrophysical Journa
- …