75 research outputs found
On the background estimation by time slides in a network of gravitational wave detectors
Time shifting the outputs of Gravitational Wave detectors operating in
coincidence is a convenient way to estimate the background in a search for
short duration signals. However this procedure is limited as increasing
indefinitely the number of time shifts does not provide better estimates. We
show that the false alarm rate estimation error saturates with the number of
time shifts. In particular, for detectors with very different trigger rates
this error saturates at a large value. Explicit computations are done for 2
detectors, and for 3 detectors where the detection statistic relies on the
logical ``OR'' of the coincidences of the 3 couples in the network.Comment: accepted for publication in CQ
Coincidence and coherent data analysis methods for gravitational wave bursts in a network of interferometric detectors
Network data analysis methods are the only way to properly separate real
gravitational wave (GW) transient events from detector noise. They can be
divided into two generic classes: the coincidence method and the coherent
analysis. The former uses lists of selected events provided by each
interferometer belonging to the network and tries to correlate them in time to
identify a physical signal. Instead of this binary treatment of detector
outputs (signal present or absent), the latter method involves first the
merging of the interferometer data and looks for a common pattern, consistent
with an assumed GW waveform and a given source location in the sky. The
thresholds are only applied later, to validate or not the hypothesis made. As
coherent algorithms use a more complete information than coincidence methods,
they are expected to provide better detection performances, but at a higher
computational cost. An efficient filter must yield a good compromise between a
low false alarm rate (hence triggering on data at a manageable rate) and a high
detection efficiency. Therefore, the comparison of the two approaches is
achieved using so-called Receiving Operating Characteristics (ROC), giving the
relationship between the false alarm rate and the detection efficiency for a
given method. This paper investigates this question via Monte-Carlo
simulations, using the network model developed in a previous article.Comment: Spelling mistake corrected in one author's nam
Finesse and mirror speed measurement for a suspended Fabry-Perot cavity using the ringing effect
We here present an investigation of the ringing effect observed on the VIRGO
mode-cleaner prototype MC30. The results of a numerical calculation show how a
simple empirical formula can determine the cavity expansion rate from the
oscillatory behavior. We also show how the simulation output can be adjusted to
estimate the finesse value of the suspended cavity.Comment: 12 pages, 7 figure
Comparison of filters for detecting gravitational wave bursts in interferometric detectors
Filters developed in order to detect short bursts of gravitational waves in
interferometric detector outputs are compared according to three main points.
Conventional Receiver Operating Characteristics (ROC) are first built for all
the considered filters and for three typical burst signals. Optimized ROC are
shown for a simple pulse signal in order to estimate the best detection
efficiency of the filters in the ideal case, while realistic ones obtained with
filters working with several ``templates'' show how detection efficiencies can
be degraded in a practical implementation. Secondly, estimations of biases and
statistical errors on the reconstruction of the time of arrival of pulse-like
signals are then given for each filter. Such results are crucial for future
coincidence studies between Gravitational Wave detectors but also with neutrino
or optical detectors. As most of the filters require a pre-whitening of the
detector noise, the sensitivity to a non perfect noise whitening procedure is
finally analysed. For this purpose lines of various frequencies and amplitudes
are added to a Gaussian white noise and the outputs of the filters are studied
in order to monitor the excess of false alarms induced by the lines. The
comparison of the performances of the different filters finally show that they
are complementary rather than competitive.Comment: 32 pages (14 figures), accepted for publication in Phys. Rev.
Reconstruction of source location in a network of gravitational wave interferometric detectors
This paper deals with the reconstruction of the direction of a gravitational
wave source using the detection made by a network of interferometric detectors,
mainly the LIGO and Virgo detectors. We suppose that an event has been seen in
coincidence using a filter applied on the three detector data streams. Using
the arrival time (and its associated error) of the gravitational signal in each
detector, the direction of the source in the sky is computed using a chi^2
minimization technique. For reasonably large signals (SNR>4.5 in all
detectors), the mean angular error between the real location and the
reconstructed one is about 1 degree. We also investigate the effect of the
network geometry assuming the same angular response for all interferometric
detectors. It appears that the reconstruction quality is not uniform over the
sky and is degraded when the source approaches the plane defined by the three
detectors. Adding at least one other detector to the LIGO-Virgo network reduces
the blind regions and in the case of 6 detectors, a precision less than 1
degree on the source direction can be reached for 99% of the sky.Comment: Accepted in Phys. Rev.
An elliptical tiling method to generate a 2-dimensional set of templates for gravitational wave search
Searching for a signal depending on unknown parameters in a noisy background
with matched filtering techniques always requires an analysis of the data with
several templates in parallel in order to ensure a proper match between the
filter and the real waveform. The key feature of such an implementation is the
design of the filter bank which must be small to limit the computational cost
while keeping the detection efficiency as high as possible. This paper presents
a geometrical method which allows one to cover the corresponding physical
parameter space by a set of ellipses, each of them being associated to a given
template. After the description of the main characteristics of the algorithm,
the method is applied in the field of gravitational wave (GW) data analysis,
for the search of damped sine signals. Such waveforms are expected to be
produced during the de-excitation phase of black holes -- the so-called
'ringdown' signals -- and are also encountered in some numerically computed
supernova signals.Comment: Accepted in PR
Large-Scale Phylogenomic Analyses Reveal That Two Enigmatic Protist Lineages, Telonemia and Centroheliozoa, Are Related to Photosynthetic Chromalveolates
Understanding the early evolution and diversification of eukaryotes relies on a fully resolved phylogenetic tree. In recent years, most eukaryotic diversity has been assigned to six putative supergroups, but the evolutionary origin of a few major âorphanâ lineages remains elusive. Two ecologically important orphan groups are the heterotrophic Telonemia and Centroheliozoa. Telonemids have been proposed to be related to the photosynthetic cryptomonads or stramenopiles and centrohelids to haptophytes, but molecular phylogenies have failed to provide strong support for any phylogenetic hypothesis. Here, we investigate the origins of Telonema subtilis (a telonemid) and Raphidiophrys contractilis (a centrohelid) by large-scale 454 pyrosequencing of cDNA libraries and including new genomic data from two cryptomonads (Guillardia theta and Plagioselmis nannoplanctica) and a haptophyte (Imantonia rotunda). We demonstrate that 454 sequencing of cDNA libraries is a powerful and fast method of sampling a high proportion of protist genes, which can yield ample information for phylogenomic studies. Our phylogenetic analyses of 127 genes from 72 species indicate that telonemids and centrohelids are members of an emerging major group of eukaryotes also comprising cryptomonads and haptophytes. Furthermore, this group is possibly closely related to the SAR clade comprising stramenopiles (heterokonts), alveolates, and Rhizaria. Our results link two additional heterotrophic lineages to the predominantly photosynthetic chromalveolate supergroup, providing a new framework for interpreting the evolution of eukaryotic cell structures and the diversification of plastids
Detection of a close supernova gravitational wave burst in a network of interferometers, neutrino and optical detectors
Trying to detect the gravitational wave (GW) signal emitted by a type II
supernova is a main challenge for the GW community. Indeed, the corresponding
waveform is not accurately modeled as the supernova physics is very complex; in
addition, all the existing numerical simulations agree on the weakness of the
GW emission, thus restraining the number of sources potentially detectable.
Consequently, triggering the GW signal with a confidence level high enough to
conclude directly to a detection is very difficult, even with the use of a
network of interferometric detectors. On the other hand, one can hope to take
benefit from the neutrino and optical emissions associated to the supernova
explosion, in order to discover and study GW radiation in an event already
detected independently. This article aims at presenting some realistic
scenarios for the search of the supernova GW bursts, based on the present
knowledge of the emitted signals and on the results of network data analysis
simulations. Both the direct search and the confirmation of the supernova event
are considered. In addition, some physical studies following the discovery of a
supernova GW emission are also mentioned: from the absolute neutrino mass to
the supernova physics or the black hole signature, the potential spectrum of
discoveries is wide.Comment: Revised version, accepted for publication in Astroparticle Physic
Detection in coincidence of gravitational wave bursts with a network of interferometric detectors (I): Geometric acceptance and timing
Detecting gravitational wave bursts (characterised by short durations and
poorly modelled waveforms) requires to have coincidences between several
interferometric detectors in order to reject non-stationary noise events. As
the wave amplitude seen in a detector depends on its location with respect to
the source direction and as the signal to noise ratio of these bursts are
expected to be low, coincidences between antennas may not be so likely. This
paper investigates this question from a statistical point of view by using a
simple model of a network of detectors; it also estimates the timing precision
of a detection in an interferometer which is an important issue for the
reconstruction of the source location, based on time delays.Comment: low resolution figure 1 due to file size problem
Phylogenomics Reshuffles the Eukaryotic Supergroups
Background. Resolving the phylogenetic relationships between eukaryotes is an ongoing challenge of evolutionary biology. In recent years, the accumulation of molecular data led to a new evolutionary understanding, in which all eukaryotic diversity has been classified into five or six supergroups. Yet, the composition of these large assemblages and their relationships remain controversial. Methodology/Principle Findings. Here, we report the sequencing of expressed sequence tags (ESTs) for two species belonging to the supergroup Rhizaria and present the analysis of a unique dataset combining 29908 amino acid positions and an extensive taxa sampling made of 49 mainly unicellular species representative of all supergroups. Our results show a very robust relationship between Rhizaria and two main clades of the supergroup chromalveolates: stramenopiles and alveolates. We confirm the existence of consistent affinities between assemblages that were thought to belong to different supergroups of eukaryotes, thus not sharing a close evolutionary history. Conclusions. This well supported phylogeny has important consequences for our understanding of the evolutionary history of eukaryotes. In particular, it questions a single red algal origin of the chlorophyll-c containing plastids among the chromalveolates. We propose the abbreviated name âSARâ (Stramenopiles+Alveolates+Rhizaria) to accommodate this new super assemblage of eukaryotes, which comprises the largest diversity of unicellular eukaryotes
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