Search For Gravitational-wave Signals Research Articles

Searching for gravitational-wave signals from precessing black hole binaries with the GstLAL pipeline

Searching for gravitational-wave signals from precessing black hole binaries with the GstLAL pipeline

Empirically estimating the distribution of the loudest candidate from a gravitational-wave search

Searches for gravitational-wave signals are often based on maximizing a detection statistic over a bank of waveform templates, covering a given parameter space with a variable level of correlation. Results are often evaluated using a noise-hypothesis test, where the background is characterized by the sampling distribution of the loudest template. In the context of continuous gravitational-wave searches, properly describing said distribution is an open problem: current approaches focus on a particular detection statistic and neglect template-bank correlations. We introduce a new approach using extreme value theory to describe the distribution of the loudest template's detection statistic in an arbitrary template bank. Our new proposal automatically generalizes to a wider class of detection statistics, including (but not limited to) line-robust statistics and transient continuous-wave signal hypotheses, and improves the estimation of the expected maximum detection statistic at a negligible computing cost. The performance of our proposal is demonstrated on simulated data as well as by applying it to different kinds of (transient) continuous-wave searches using O2 Advanced LIGO data. We release an accompanying python software package, distromax, implementing our new developments.

Ranking candidate signals with machine learning in low-latency searches for gravitational waves from compact binary mergers

In the multi-messenger astronomy era, accurate sky localization and low latency time of gravitational-wave (GW) searches are keys in triggering successful follow-up observations on the electromagnetic counterpart of GW signals. We, in this work, focus on the latency time and study the feasibility of adopting supervised machine learning (ML) method for ranking candidate GW events. We consider two popular ML methods, random forest and neural networks. We observe that the evaluation time of both methods takes tens of milliseconds for $\sim$ 45,000 evaluation samples. We compare the classification efficiency between the two ML methods and a conventional low-latency search method with respect to the true positive rate at given false positive rate. The comparison shows that about 10\% improved efficiency can be achieved at lower false positive rate $\sim 2 \times 10^{-5}$ with both ML methods. We also present that the search sensitivity can be enhanced by about 18\% at $\sim 10^{-11}$Hz false alarm rate. We conclude that adopting ML methods for ranking candidate GW events is a prospective approach to yield low latency and high efficiency in searches for GW signals from compact binary mergers.

Improving the background of gravitational-wave searches for core collapse supernovae: a machine learning approach

Based on the prior O1–O2 observing runs, about 30% of the data collected by Advanced LIGO and Virgo in the next observing runs are expected to be single-interferometer data, i.e. they will be collected at times when only one detector in the network is operating in observing mode. Searches for gravitational-wave signals from supernova events do not rely on matched filtering techniques because of the stochastic nature of the signals. If a Galactic supernova occurs during single-interferometer times, separation of its unmodelled gravitational-wave signal from noise will be even more difficult due to lack of coherence between detectors. We present a novel machine learning method to perform single-interferometer supernova searches based on the standard LIGO-Virgo coherent WaveBurst pipeline. We show that the method may be used to discriminate Galactic gravitational-wave supernova signals from noise transients, decrease the false alarm rate of the search, and improve the supernova detection reach of the detectors.

Follow-up procedure for gravitational wave searches from isolated neutron stars using the time-domain -statistic method

Among promising sources of gravitational waves are long-lived nearly periodic signals produced by rotating, asymmetric neutron stars. Depending on the astrophysical scenario, the sources of asymmetry may have thermal, viscous, elastic and/or magnetic origin. In this work we introduce a follow-up procedure for an all-sky search for gravitational wave signals from rotating neutron stars. The procedure denoted as Followup implements matched-filtering -statistic method. We describe data analysis methods and algorithms used in the procedure. We present tests of the Followup for artificial signals added to white, Gaussian noise. The tests show a good agreement with the theoretical predictions. The Followup will become part of the Time-Domain -statistic pipeline that is routinely used for all-sky searches of LIGO and Virgo detector data.

Fully-coherent all-sky search for gravitational-waves from compact binary coalescences

We introduce a fully-coherent method for searching for gravitational wave signals generated by the merger of black hole and/or neutron star binaries. This extends the coherent analysis previously developed and used for targeted gravitational wave searches to an all-sky, all-time search. We apply the search to one month of data taken during the fifth science run of the LIGO detectors. We demonstrate an increase in sensitivity of 25% over the coincidence search, which is commensurate with expectations. Finally, we discuss prospects for implementing and running a coherent search for gravitational wave signals from binary coalescence in the advanced gravitational wave detector data.

Enhanced characteristics of fused silica fibers using laser polishing

The search for gravitational wave signals from astrophysical sources has led to the current work to upgrade the two largest of the long-baseline laser interferometers, the LIGO detectors. The first fused silica mirror suspensions for the Advanced LIGO gravitational wave detectors have been installed at the LIGO Hanford and Livingston sites. These quadruple pendulums use synthetic fused silica fibers produced using a CO2 laser pulling machine to reduce thermal noise in the final suspension stage. The suspension thermal noise in Advanced LIGO is predicted to be limited by internal damping in the surface layer of the fibers, damping in the weld regions, and the strength of the fibers. We present here a new method for increasing the fracture strength of fused silica fibers by laser polishing of the stock material from which they are produced. We also show measurements of mechanical loss in laser polished fibers, showing a reduction of 30% in internal damping in the surface layer.

Searches for gravitational wave signals from rotating neutron stars

We present status of the search of the LIGO and Virgo detectors data for continuous gravitational wave signals originating from rotating neutron stars. We first review the mechanisms of gravitational wave emission from such sources then we describe the data analysis methods and finally we present recent results of the searches.

Directional Limits on Persistent Gravitational Waves Using LIGO S5 Science Data

The gravitational-wave (GW) sky may include nearby pointlike sources as well as stochastic backgrounds. We perform two directional searches for persistent GWs using data from the LIGO S5 science run: one optimized for pointlike sources and one for arbitrary extended sources. Finding no evidence to support the detection of GWs, we present 90% confidence level (C.L.) upper-limit maps of GW strain power with typical values between 2-20×10(-50) strain(2) Hz(-1) and 5-35×10(-49) strain(2) Hz(-1) sr(-1) for pointlike and extended sources, respectively. The latter result is the first of its kind. We also set 90% C.L. limits on the narrow-band root-mean-square GW strain from interesting targets including Sco X-1, SN 1987A and the Galactic center as low as ≈7×10(-25) in the most sensitive frequency range near 160 Hz.

Student-tbased filter for robust signal detection

The search for gravitational-wave signals in detector data is often hampered by the fact that many data analysis methods are based on the theory of stationary Gaussian noise, while actual measurement data frequently exhibit clear departures from these assumptions. Deriving methods from models more closely reflecting the data's properties promises to yield more sensitive procedures. The commonly used matched filter is such a detection method that may be derived via a Gaussian model. In this paper we propose a generalized matched-filtering technique based on a Student-t distribution that is able to account for heavier-tailed noise and is robust against outliers in the data. On the technical side, it generalizes the matched filter's least-squares method to an iterative, or adaptive, variation. In a simplified Monte Carlo study we show that when applied to simulated signals buried in actual interferometer noise it leads to a higher detection rate than the usual ("Gaussian") matched filter.

Assigning confidence to inspiral gravitational wave candidates with Bayesian model selection

Bayesian model selection provides a powerful and mathematically transparent framework to tackle hypothesis testing, such as detection tests of gravitational waves emitted during the coalescence of binary systems using ground-based laser interferometers. Although its implementation is computationally intensive, we have developed an efficient probabilistic algorithm based on a technique known as nested sampling that makes Bayesian model selection applicable to follow-up studies of candidate signals produced by on-going searches of inspiralling compact binaries. We discuss the performance of this approach, in terms of ‘false alarm rate’ and ‘detection probability’ of restricted second post-Newtonian inspiral waveforms from non-spinning compact objects in binary systems. The results confirm that this approach is a viable tool for detection tests in current searches for gravitational wave signals.

Applications of distance between probability distributions to gravitational wave data analysis

We present a definition of the distance between probability distributions. Our definition is based on the L1 norm on space of probability measures. We compare our distance with the well-known Kullback–Leibler divergence and with the proper distance defined using the Fisher matrix as a metric on the parameter space. We consider using our notion of distance in several problems in gravitational wave data analysis: to place templates in the parameter space in searches for gravitational-wave signals, to assess quality of search templates, and to study the signal resolution.

Optimal generalization of power filters for gravitational wave bursts from single to multiple detectors

Searches for gravitational wave signals which do not have a precise model describing the shape of their waveforms are often performed using power detectors based on a quadratic form of the data. A new, optimal method of generalizing these power detectors so that they operate coherently over a network of interferometers is presented. Such a mode of operation is useful in obtaining better detection efficiencies, and better estimates of the position of the source of the gravitational wave signal. Numerical simulations based on a realistic, computationally efficient hierarchical implementation of the method are used to characterize its efficiency, for detection and for position estimation. The method is shown to be more efficient at detecting signals than an incoherent approach based on coincidences between lists of events. It is also shown to be capable of locating the position of the source.

Gaining on Gravity Waves

In 1916 Albert Einstein published his theory of general relativity. In one of its major aspects this is a theory of the nature and operation of gravitational forces with which Einstein intended to replace the classical theory devised by Isaac Newton in the 17th century. Einstein's theory makes a number of predictions that are radically different from those of Newton. One of the most striking of these is that gravitational forces should be propagated in waves in a manner similar to the way electric and magnetic forces are. These gravitational waves should consist of cyclically fluctuating gravitational forces; they should carry energy from place to place, and they should cause minute fluctuations of the surfaces of objects they encounter. Any accelerated body could be a source of gravitational waves, but in practice physicists look to large astronomical bodies such as oblate stars or binary stars. The prediction was that gravitational waves would be extremely weak: For a cylinder a meter long the amount of surface disturbance would be a fraction of the diameter of an atomic nucleus. For 40 years no one seriously looked for gravitational waves, but in the late 1950's Dr. Joseph Weber of the University of Maryland began to develop equipment he thought would do the job. As receivers Dr. Weber uses aluminum cylinders of about a ton's weight, and he has developed piezoelectric sensors that can record fluctuations in the surfaces of these cylinders amounting to fractions of a nuclear diameter. In 1969, after about 10 years of effort, Dr. Weber announced that his equipment had recorded gravitational waves (SN: 6/21/69, p. 593). Since then he has been subjected to criticism, based mainly on his statistical analysis of the data. Throughout the last year he has maintained that further observations and more rigorous statistics support his original assertion. In spite of the critics, confidence in Dr. Weber's work has led other people to enter the search for gravitational waves. About half a dozen experiments are now in progress or in prospect in both the United States and the Soviet Union. Most of these seek to make the detectors more sensitive or to design new kinds of detectors that will record frequency ranges other than the one1,660 cycles per second (hertz)-that Dr. Weber has pioneered. Detectors for the waves can be designed either as broad-band receivers that respond to a range of frequencies or as narrow-band receivers that are excited only by a single frequency. Dr. Weber's cylinders are narrow-band receivers. To supplement the solid-bar type of detector that Dr. Weber used, Dr. David Douglass of the University of Rochester proposes development of a second class of narrow-band receivers, composed of hollow squares, hoops or U-shapes. For this second class of narrow-band receivers he predicts important practical advantages in searches for gravitational wave signals at low frequencies. Any individual detector of either of these two classes will respond to a particular resonant frequency determined by its size. For the first class of detectors the critical dimension is the length of the bar, and the resonant frequency will be inversely proportional to it. To reach low frequencies extremely long bars would be needed. For the hollow shapes the critical dimension is the length of a side or a diameter, and the resonant frequency 40~~~