190 research outputs found
Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion
Pulsar timing arrays (PTAs) will be sensitive to a finite number of
gravitational wave (GW) "point" sources (e.g. supermassive black hole
binaries). N quiet pulsars with accurately known distances d_{pulsar} can
characterize up to 2N/7 distant chirping sources per frequency bin \Delta
f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta
\gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are
poorly known, a PTA with F frequency bins can still characterize up to
(2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing
residuals in the vicinity of a GW source still allows the source to be
localized quasi-topologically within roughly the smallest quadrilateral of
quiet pulsars that encircles it on the sky, down to a limiting resolution
\delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be
unconfused, even at the lowest frequencies, with matched filtering always
appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio
Summed Parallel Infinite Impulse Response (SPIIR) Filters For Low-Latency Gravitational Wave Detection
With the upgrade of current gravitational wave detectors, the first detection
of gravitational wave signals is expected to occur in the next decade.
Low-latency gravitational wave triggers will be necessary to make fast
follow-up electromagnetic observations of events related to their source, e.g.,
prompt optical emission associated with short gamma-ray bursts. In this paper
we present a new time-domain low-latency algorithm for identifying the presence
of gravitational waves produced by compact binary coalescence events in noisy
detector data. Our method calculates the signal to noise ratio from the
summation of a bank of parallel infinite impulse response (IIR) filters. We
show that our summed parallel infinite impulse response (SPIIR) method can
retrieve the signal to noise ratio to greater than 99% of that produced from
the optimal matched filter. We emphasise the benefits of the SPIIR method for
advanced detectors, which will require larger template banks.Comment: 9 pages, 6 figures, for PR
Learning about compact binary merger: the interplay between numerical relativity and gravitational-wave astronomy
Activities in data analysis and numerical simulation of gravitational waves
have to date largely proceeded independently. In this work we study how
waveforms obtained from numerical simulations could be effectively used within
the data analysis effort to search for gravitational waves from black hole
binaries. We propose measures to quantify the accuracy of numerical waveforms
for the purpose of data analysis and study how sensitive the analysis is to
errors in the waveforms. We estimate that ~100 templates (and ~10 simulations
with different mass ratios) are needed to detect waves from non-spinning binary
black holes with total masses in the range 100 Msun < M < 400 Msun using
initial LIGO. Of course, many more simulation runs will be needed to confirm
that the correct physics is captured in the numerical evolutions. From this
perspective, we also discuss sources of systematic errors in numerical waveform
extraction and provide order of magnitude estimates for the computational cost
of simulations that could be used to estimate the cost of parameter space
surveys. Finally, we discuss what information from near-future numerical
simulations of compact binary systems would be most useful for enhancing the
detectability of such events with contemporary gravitational wave detectors and
emphasize the role of numerical simulations for the interpretation of eventual
gravitational-wave observations.Comment: 19 pages, 12 figure
When the Earth trembles in the americas: the experience of haiti and chile 2010.
The response of the nephrological community to the Haiti and Chile earthquakes which occurred in the first months of 2010 is described. In Haiti, renal support was organized by the Renal Disaster Relief Task Force (RDRTF) of the International Society of Nephrology (ISN) in close collaboration with Médecins Sans Frontières (MSF), and covered both patients with acute kidney injury (AKI) and patients with chronic kidney disease (CKD). The majority of AKI patients (19/27) suffered from crush syndrome and recovered their kidney function. The remaining 8 patients with AKI showed acute-to-chronic renal failure with very low recovery rates. The intervention of the RDRTF-ISN involved 25 volunteers of 9 nationalities, lasted exactly 2 months, and was characterized by major organizational difficulties and problems to create awareness among other rescue teams regarding the availability of dialysis possibilities. Part of the Haitian patients with AKI reached the Dominican Republic (DR) and received their therapy there. The nephrological community in the DR was able to cope with this extra patient load. In both Haiti and the DR, dialysis treatment was able to be prevented in at least 40 patients by screening and adequate fluid administration. Since laboratory facilities were destroyed in Port-au-Prince and were thus lacking during the first weeks of the intervention, the use from the very beginning on of a point-of-care device (i-STAT®) was very efficient for the detection of aberrant kidney function and electrolyte parameters. In Chile, nephrological problems were essentially related to difficulties delivering dialysis treatment to CKD patients, due to the damage to several units. This necessitated the reallocation of patients and the adaptation of their schedules. The problems could be handled by the local nephrologists. These observations illustrate that local and international preparedness might be life-saving if renal problems occur in earthquake circumstances
Angular Resolution of the LISA Gravitational Wave Detector
We calculate the angular resolution of the planned LISA detector, a
space-based laser interferometer for measuring low-frequency gravitational
waves from galactic and extragalactic sources. LISA is not a pointed
instrument; it is an all-sky monitor with a quadrupolar beam pattern. LISA will
measure simultaneously both polarization components of incoming gravitational
waves, so the data will consist of two time series. All physical properties of
the source, including its position, must be extracted from these time series.
LISA's angular resolution is therefore not a fixed quantity, but rather depends
on the type of signal and on how much other information must be extracted.
Information about the source position will be encoded in the measured signal in
three ways: 1) through the relative amplitudes and phases of the two
polarization components, 2) through the periodic Doppler shift imposed on the
signal by the detector's motion around the Sun, and 3) through the further
modulation of the signal caused by the detector's time-varying orientation. We
derive the basic formulae required to calculate the LISA's angular resolution
for a given source. We then evaluate for
two sources of particular interest: monchromatic sources and mergers of
supermassive black holes. For these two types of sources, we calculate (in the
high signal-to-noise approximation) the full variance-covariance matrix, which
gives the accuracy to which all source parameters can be measured. Since our
results on LISA's angular resolution depend mainly on gross features of the
detector geometry, orbit, and noise curve, we expect these results to be fairly
insensitive to modest changes in detector design that may occur between now and
launch. We also expect that our calculations could be easily modified to apply
to a modified design.Comment: 15 pages, 5 figures, RevTex 3.0 fil
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
Use and Abuse of the Fisher Information Matrix in the Assessment of Gravitational-Wave Parameter-Estimation Prospects
The Fisher-matrix formalism is used routinely in the literature on
gravitational-wave detection to characterize the parameter-estimation
performance of gravitational-wave measurements, given parametrized models of
the waveforms, and assuming detector noise of known colored Gaussian
distribution. Unfortunately, the Fisher matrix can be a poor predictor of the
amount of information obtained from typical observations, especially for
waveforms with several parameters and relatively low expected signal-to-noise
ratios (SNR), or for waveforms depending weakly on one or more parameters, when
their priors are not taken into proper consideration. In this paper I discuss
these pitfalls; show how they occur, even for relatively strong signals, with a
commonly used template family for binary-inspiral waveforms; and describe
practical recipes to recognize them and cope with them.
Specifically, I answer the following questions: (i) What is the significance
of (quasi-)singular Fisher matrices, and how must we deal with them? (ii) When
is it necessary to take into account prior probability distributions for the
source parameters? (iii) When is the signal-to-noise ratio high enough to
believe the Fisher-matrix result? In addition, I provide general expressions
for the higher-order, beyond--Fisher-matrix terms in the 1/SNR expansions for
the expected parameter accuracies.Comment: 24 pages, 3 figures, previously known as "A User Manual for the
Fisher Information Matrix"; final, corrected PRD versio
Random template placement and prior information
In signal detection problems, one is usually faced with the task of searching
a parameter space for peaks in the likelihood function which indicate the
presence of a signal. Random searches have proven to be very efficient as well
as easy to implement, compared e.g. to searches along regular grids in
parameter space. Knowledge of the parameterised shape of the signal searched
for adds structure to the parameter space, i.e., there are usually regions
requiring to be densely searched while in other regions a coarser search is
sufficient. On the other hand, prior information identifies the regions in
which a search will actually be promising or may likely be in vain. Defining
specific figures of merit allows one to combine both template metric and prior
distribution and devise optimal sampling schemes over the parameter space. We
show an example related to the gravitational wave signal from a binary inspiral
event. Here the template metric and prior information are particularly
contradictory, since signals from low-mass systems tolerate the least mismatch
in parameter space while high-mass systems are far more likely, as they imply a
greater signal-to-noise ratio (SNR) and hence are detectable to greater
distances. The derived sampling strategy is implemented in a Markov chain Monte
Carlo (MCMC) algorithm where it improves convergence.Comment: Proceedings of the 8th Edoardo Amaldi Conference on Gravitational
Waves. 7 pages, 4 figure
Custo-efetividade dos stents recobertos por rapamicina em procedimentos percutâneos coronarianos no Brasil
Performance of Newtonian filters in detecting gravitational waves from coalescing binaries
Coalescing binary systems are one of the most promising sources of
gravitational waves. The technique of matched filtering used in the detection
of gravitational waves from coalescing binaries relies on the construction of
accurate templates. Until recently filters modelled on the quadrupole or the
Newtonian approximation were deemed sufficient. Recently it was shown that
post-Newtonian effects contribute to a secular growth in the phase difference
between the actual signal and its corresponding Newtonian template. In this
paper we investigate the possibility of compensating for the phase difference
caused by the post-Newtonian terms by allowing for a shift in the Newtonian
filter parameters. We find that Newtonian filters perform adequately for the
purpose of detecting the presence of the signal for both the initial and the
advanced LIGO detectors.Comment: Revtex 9 pages + 6 figures ( Can be obtained by "anonymous" ftp from
144.16.31.1 in dir /pub/rbs. Submitted to Physical Review D. IUCAA 1
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