Progress of the low latency pipeline for the gravitational wave detector Mario Schenberg. By Da Silva Costa C., Denis Aguiar O., Fauth A. The Mario Schenberg gravitational wave antenna, located in Sao Paulo, is a spherical resonant mass detector, which is 65cm in diameter and weighs 1,150 kg. The antenna is currently being upgraded and the next run will occur later this year. Since the detector last operated in 2008, its 6 parametric transducers, measuring the sphere quadrupolar mode oscillations, have been redesigned to increase their mechanical Q. Spherical detectors present the ability to determine simultaneously both source direction and signal polarization. We have developed a low latency pipeline that takes advantage of these abilities and processes real time data in less than one-sixth the acquisition time. For each reconstructed direction, its resolution and systematic error are determined using a mapping error system. To reduce the false alarm rate, triggers are vetoed using information from the spherical modes. In the near future, we expect to add a matched filter to the pipeline. This will increase GW parameter resolution and reduce the false alarm rate. As a future part of the pipeline, a cosmic ray veto was added to the Mario Schenberg setup in December 2011. Since then, it is acquiring data and we are analyzing the local multiplicity. The energy deposition due to cosmic rays on the sphere is presently being simulated.
Inspiral Dynamics of Spin Dominant Compact Binaries. By Gopakumar A., Gupta A. We follow the secular evolution of unequal mass spinning compact binaries whose dominant spin angular momentum exceeds the orbital counterpart at \(f_0\), the initial frequency of an interferometric detector like Advanced LIGO. These binaries can experience transitional precession before their GW frequencies cross \(f_0\), provided their essentially constant dominant spin-orbit misalignments are \({>}170^{\circ }\). The usual inspiral templates turn out to be not very faithful in modeling the associated GWs, though the precessional dynamics are simple when these binaries inspiral through \(f_0\). A prescription is presented to model the GWs from the inspiral phase of such binaries in a faithful manner. Additionally, we explore the effect of gravitational radiation reaction on the orientation of the dominant spin from the initial direction of the total angular momentum \(\mathbf{j}_0\). The dominant spin orientations from \(\mathbf{j}_0\) are \({<}45^{\circ }\) at \(f_0\) for binaries with dominant spin-orbit misalignments \({<}170^{\circ }\) and rather unpredictable for higher dominant spin-orbit misalignments.
Studying the effects of tidal corrections on parameter estimation. By Wade L. Tidal deformations of neutron stars in binary systems during the infall of gravitationally radiating neutron stars before merger call for analytic corrections to the post-Newtonian gravitational-wave waveform. Tidal deformation information is important to searches for gravitational waves from these sources because it can break degeneracies in the estimation of the physical parameters of the binary, and it could also lead to insights about the neutron star equation of state. Using full Bayesian MCMC (Markov Chain Monte Carlo) simulations, we studied how tidal corrections affect and inform parameter estimation for binary neutron stars. We also investigate the systematic biases that arise in parameter estimation from using different post-Newtonian waveform families.
Measuring the neutron-star equation of state with multiple binary neutron star inspiral events. By Lackey B. Gravitational waves from compact binaries containing neutron stars can provide useful constraints on the neutron-star equation of state. In binary neutron star systems this information comes mainly from the imprint of the neutron-star tidal deformability on the waveform during inspiral. Previous work has shown that the tidal deformability is measurable with Advanced LIGO for a single event when including the tidal effect up to merger. In this work we describe a method for stacking measurements of the tidal deformability from multiple inspiral events to measure the unknown parameters of a parametrized equation of state. Specifically, we use a 4-parameter piecewise polytrope that matches theoretical equation of state models to a few percent to determine the accuracy that one can measure the pressure at twice nuclear density as well as the adiabatic index in three density regions. We also examine how the uncertainties in the equation of state parameters depend on the number of observations and on the distribution of neutron-star masses in binary neutron star systems.
Effect of the higher order modes of GWs emitted from binary black hole mergers measured by a GW burst search algorithm. By Mohapatra S., Clark J., Cadonati L. The GWs emitted from the merger of binary black holes can be expressed as the dominant quadrupole modes and sub-dominant higher order modes, in the spin-weighted spherical harmonics basis. Until now all the gravitational-wave searches that were conducted have utilized only the signal morphology of the dominant quadrupole modes for the interpretation of the search results. It is qualitatively known that these sub-dominant modes can be crucial for certain source orientations and source parameters of the binary black holes. Recently, an analytical family of binary black hole GW signals including higher order modes became available. We present a study quantifying the effect of the sub-dominant modes on the expected signal-to-noise ratio measured by a morphology-independent gravitational-wave burst search.
Inferring core-collapse supernova physics with GWs. By Logue J. Stellar collapse and the subsequent development of a core-collapse supernova explosion emit bursts of GWs that might be detected by the advanced generation of laser interferometer gravitational wave observatories such as Advanced LIGO, Advanced Virgo, and KAGRA. GW bursts from core-collapse supernovae encode information on the intricate multi-dimensional dynamics at work at the core of a dying massive star and may provide direct evidence for the yet uncertain mechanism driving supernovae in massive stars. Recent multi-dimensional simulations of core-collapse supernovae exploding via the neutrino, magneto-rotational, and acoustic explosion mechanisms have predicted GW signals which have distinct structure in both the time and frequency domains. Motivated by this, we describe a promising method for determining the most likely explosion mechanism underlying a hypothetical GW signal, based on Principal Component Analysis and Bayesian model selection. Using simulated Advanced LIGO noise and assuming a single detector and linear waveform polarization for simplicity, we demonstrate that our method can distinguish magneto-rotational explosions throughout the Milky Way (10 kpc) and explosions driven by the neutrino and acoustic mechanisms to 2 kpc. Furthermore, we show that we can differentiate between models for rotating accretion-induced collapse of massive white dwarfs and models of rotating iron core collapse with high reliability out to several kpc.
A gravitational-wave search algorithm for non-precessing spinning binary black holes. By Privitera S., Mohapatra S., Hanna C., Fotpolous N., Ajith P., Weinstein A., Whelan J. Previous searches for gravitational waves in LIGO-Virgo data from the inspiral, merger and ringdown of binary black holes have used a matched-filter approach with non-spinning templates. However, astrophysical black holes in binaries are expected to have significant spin; neglecting such effects in the templates may reduce the detection efficiency. Gravitational-wave signal models from the coalescences of non-precessing spinning binary black holes have recently become available, but it is not known whether using these templates in a search can improve detection efficiency of generic-spinning binary black holes signals in realistic, non-Gaussian noise from the Initial LIGO detectors. We present [43] a search method for gravitational-waves that employs non-precessing spinning templates and compare the performance of this method to one in which the templates neglect spin effects.
Science reach of stochastic gravitational wave background searches with second-generation detectors. By Mandic V. The stochastic GW background is produced as an incoherent superposition of gravitational waves from many cosmological and astrophysical sources, and could therefore carry unique information about cosmological and astrophysical processes that gave rise to it. I present a new formalism designed to extract this information from the stochastic background measurements. The formalism offers the prospect of estimating the energy budget of the stochastic gravitational wave background, and provides a natural framework for inclusion of other measurements to further constrain model parameters. I discuss applications of this formalism to some specific situations, for example, to measure possible parity violation in the early universe, or to probe the stochastic background models based on coalescences of compact binaries.
Inferences from the post-merger gravitational wave signal in binary neutron star coalescence. By Clark J. The inspiral phase of binary neutron star coalescence is widely considered to be one of the strongest sources in the next generation of ground based gravitational wave detectors, yielding detection rates in the range of 0.4–400 events per year of operation. Amongst the most exciting prospects for gravitational wave astronomy is the measurement of neutron star mass and radius, which would lead to constraints on the neutron star equation of state. Recent numerical simulations suggest that, rather than prompt collapse to a black hole, the favored outcome of the merger is the formation of a relatively long-lived hyper-massive neutron star whose gravitational wave signal constitutes a significantly weaker but complementary gravitational wave source to the inspiral phase. We present results of deploying a Bayesian nested sampling algorithm in the context of a gravitational wave inspiral-triggered search for, and characterization of, this post-merger signal.
Impact of higher harmonics in searching for gravitational waves from binary black hole coalescence in advanced LIGO. By Capuano C. Current searches for gravitational waves from compact binary coalescence use dominant- mode only waveforms as templates in a matched filter. It has been shown that neglecting additional modes causes mismatch between these templates and expected signals. We investigate the effect of this mismatch on signal-based vetoes and the detection efficiency of advanced LIGO detectors for non-spinning stellar-mass binary black holes. We also consider what improvement could be expected if a search were developed that utilized these additional modes.
Gravitational-wave parameter estimation with compressed likelihood evaluations. By Canizares P., Field S., Gair J., Tiglio M. One of the main bottlenecks in GW astronomy is the high cost of performing parameter estimation and GW searches on the fly. We propose a novel technique based on Reduced Order Quadratures (ROQs), an application and data-specific quadrature rule, to perform fast and accurate likelihood evaluations. These are the dominant cost in Markov chain Monte Carlo (MCMC) algorithms, which are widely employed in parameter estimation studies, and so ROQs offer a new way to accelerate GW parameter estimation. We illustrate our approach using a four dimensional GW burst model embedded in noise. We build an ROQ for this model, and perform four dimensional MCMC searches with both the standard and ROQs quadrature rules, showing that, for this model, the ROQ approach is around 25 times faster than the standard approach with essentially no loss of accuracy. The speed-up from using ROQs is expected to increase for more complex GW signal models and therefore has significant potential to accelerate parameter estimation of GW sources such as compact binary coalescences.
Methods for the first all-sky search for continuous gravitational waves from spinning neutron stars in binary systems. By Goetz E. An all-sky search for continuous gravitational waves from unknown neutron stars in binary systems is daunting in its computational challenge, because one much search over additional binary orbital parameters. A new search algorithm, called TwoSpect, has been developed and implemented; it exploits the periodic orbital modulation of the source waves by searching for patterns in doubly Fourier transformed data. This technique enables a more computationally efficient search compared to other StackSlide-like, all-sky search algorithms. We present the analysis methods and current status of the first ongoing search for sources in binary systems in the LIGO Science Run 6 and Virgo Science Runs 2 and 3 data sets.
Resolving noise structures in Virgo and LIGO data: an application of non-linear system identification. By Piergiovanni F., Guidi G., Carini A. A gravitational wave detector features many kinds of non-linear physical processes. Disturbances present in auxiliary channels, including narrow spectral features, may be converted linearly and non-linearly into noise, polluting the gravitational channel in a wide frequency range. Uncovering such relationships between auxiliary channels and the gravitational wave channel can be very useful for characterizing the detector and possibly also for improving the confidence of gravitational wave searches. We present a system identification tool developed to characterize linear and non-linear noise in the gravitational wave detector output. The signal is expanded in a Volterra series of the auxiliary channels, and the model parameters are determined by means of the Sorted Fast Orthogonal Search technique. Cross-correlation is used to perform dephasing identification, to minimize the number of time lags in Volterra expansions. The magnitude of the specific contribution of any channel or combination of channels is estimated; this allows for the recognition of the channels that are most involved in the noise structures, giving hints about noise sources. We applied the tool to interferometric detector data and showed that it is effective in performing blind identification among several hundreds of auxiliary channels in linear and bilinear combination.
Gravitational waves from SCO X-1 : prospects for detection and a comparison of methods. By Crowder G., Dergachev V., Galloway D., Goetz E., Meadors G., Messenger C., Premachandra S., Riles K., Sammut L., Thrane E., Whelan J. The low-mass X-ray binary Scorpius X-1 is potentially our most luminous source of continuous gravitational radiation. Unlike for the recycled pulsars already targeted by LIGO-Virgo, this radiation would be powered by the accretion of matter from its binary companion rather than its rotational energy. With the advanced detector era fast approaching, work is underway to develop an array of robust tools for maximizing the science and detection potential of Sco X-1. It will be possible with advanced detector data to attain sensitivities below the current theoretical torque-balance limits, implying that signal detection is a possibility. We describe the plans and progress of a project designed to compare and contrast the numerous independent search algorithms currently employable. We describe a mock-data-challenge by which the search pipelines will test their relative proficiencies in parameter estimation, search volume dependence, computational efficiency, robustness, and most importantly, search sensitivity.
Parameter-space metric for All-sky coherent searches for gravitational-wave pulsars. By Wette K., Prix R. All-sky, broadband searches for gravitational-wave pulsars are computationally limited. It is therefore important to make efficient use of available computational resources, for example by minimizing the number of templates needed to cover the parameter space of sky position and frequency evolution. For searches over the sky, the required template resolution is different for each sky position. This makes it difficult to achieve an efficient covering. Previous work on this problem has found choices of sky and frequency coordinates, with respect to which the parameter space metric (which determines the template resolution) is constant. These approaches, however, are limited to coherent integration times of a few days, which in turn limits the sensitivity achievable by e.g. a hierarchical search pipeline. We present recent work on new sky and frequency coordinates, with a flat parameter space metric, that do not suffer from this limitation. By allowing integration times of, e.g., longer than a week, improvements in search sensitivity may be possible using the new coordinates.
Effect of sine-gaussian glitches on searches for binary coalescence. By Dal Canton T., Bhagwat S., Dhurandhar S., Lundgren A. We investigate the effect of an important class of glitches occurring in the detector data on matched filter searches of gravitational waves from coalescing compact binaries in the advanced detector era. The glitches, which can be modeled as sine-Gaussians, can produce triggers with significant time delays and thus have important bearing on veto procedures. We provide approximated analytical estimates of the trigger SNR and time as a function of the parameters describing the sine-Gaussian (center time, center frequency and Q-factor) and the inspiral waveform (chirp mass). We validate our analytical predictions through simple numerical simulations, performed by filtering noiseless sine-Gaussians with the inspiral matched filter and recovering the time and value of the maximum of the resulting SNR time series. Although we identify regions of the parameter space in which each approximation no longer reproduces the numerical results, the approximations complement each other and together effectively cover the whole parameter space.
New Einstein@Home directed search for young compact objects. By Prix R. Einstein@Home is beginning a new semi-coherent search on S6 data, targeting the sky-positions of about 20 young non-pulsing neutron stars (NS) or NS candidates in supernova remnants (including Cas-A and VelaJr). These objects may be detectable if they spin within the (best) LIGO band, and are slowing down due to gravitational waves, starting from a near-breakup rotation at birth. Non-detection would therefore allow us to set constraining upper limits. The sensitivity of this search is expected to improve by about a factor of 2 on previous upper limits set on these objects. Here we present some of the considerations that went into designing this search, addressing the following questions: What astrophysical priors can we use? What parameter space can we cover? How can we distribute computing power to obtain the best detection probability? How can we balance these aims with the practical constraints of an Einstein@Home search?
An F-statistic based multi-detector veto for detector artifacts in continuous gravitational wave searches. By Keitel D., Prix R., Papa M.A., Leaci P., Siddiqi M. The emission of continuous gravitational waves (CWs) is expected from spinning neutron stars with non-axisymmetric deformations. Detecting these very weak signals requires very sensitive instruments and data analysis techniques. The standard multi-detector F-statistic often used for CW data analysis is optimal in Gaussian noise, but susceptible to false alarms from noise artifacts in the form of strong monochromatic lines. In the past, ad-hoc post-processing vetoes have been used to remove these artifacts. Here we provide a systematic framework to derive a generalized form of such line vetoes (LVs). With an extended noise model including a hypothesis for single-detector lines, we can use a Bayesian odds ratio to derive a generalized detection statistic: the LV-statistic. Compared to the F-statistic, it requires very little extra computational effort. We test this LV-statistic on both simulated and real detector data from the first year of the fifth LIGO science run. We show that the LV-statistic retains most of the detection power of the F-statistic in Gaussian noise, while being much more robust in the presence of line artifacts. Furthermore, we briefly describe the advantages, in the context of Einstein@Home, of applying the LV-statistic directly on the host machines, as done in a recent search that analyzed data from the sixth LIGO science run, and whose post-processing is currently underway.
Application of the 5-vector analysis method to narrow-band searches of continuous gravitational wave signals. By Serafinelli R., Astone P., Colla A., D’Antonio S., Frasca S., Palomba C. Targeted searches of continuous waves from spinning neutron stars normally start with the assumption that the frequency of the gravitational wave signal is at a given known ratio with respect to the rotational frequency of the source, e.g. twice for an asymmetric neutron star rotating around a principal axis of inertia. In fact, this assumption may well be invalid if, for instance, the gravitational wave signal is due to a solid core rotating at a slightly different rate with respect to the star crust. Then, it is important to perform narrow-band searches in which a small frequency band (and frequency derivative range) around the electromagnetic-inferred values is explored. In order to implement this, we have adapted a computationally efficient analysis method based on 5-vectors, which was originally developed for targeted searches. In this work, the basic principles of this new procedure, together with results of tests done using Virgo VSR1 data, are discussed. We have also estimated the sensitivity of the method, on the base of which we expect a narrow-band search of 0.02 Hz around Crab pulsar central frequency, done over Virgo VSR4 data, could improve results of other similar searches and overcome the spin-down limit by about a factor of 2.
Avoiding selection bias in gravitational wave astronomy. By Veitch J. , Messenger C. Ground-based GW searches typically use a detection threshold to reduce the number of background triggers, but imposing such a threshold will also discard some real signals of low amplitude. This process can produce a selection bias in results drawn from the population of triggers unless the discarded data is properly accounted for. We will describe how selection bias can be naturally avoided by considering both the triggers and our ignorance of the sub- threshold triggers which are discarded. This approach produces unbiased estimates of population parameters even in the presence of false alarms and incomplete data.
Low latency search for gravitational waves from compact binary coalescence. By Wen L. For advanced gravitational-wave detectors, signals from coalescing binaries of neutron stars and stellar-mass black holes could be detected before or near their merger. I will discuss the astrophysical motivation for fast low-latency detection of such signals and present a recently developed time-domain search technique aiming at extremely low latency search for gravitational waves from binary coalescence. I show the result of the sensitivity our pipeline tested on simulated data as well as existing detector data and recent Engineering Run data that simulate online data from advanced gravitational-wave detectors. I’ll also present our on-going effort to improve the computational efficiency of the pipeline by using the cost-effective Graphics Processing Unit as well as a new template interpolation strategy. Implications for future joint gravitational wave and electromagnetic observations will be discussed.
Running the frequency Hough all-sky continuous wave analysis on the grid: a job submission and control framework. By Colla A., Astone P., D’Antonio S., Palomba C., Frasca S. In the all-sky search for continuous gravitational wave signals from unknown sources, one must apply a hierarchical approach in searching the source parameter space (source coordinates, frequency and frequency derivatives). The very high computational requirements of this search are addressed by most analysis pipelines, such as the one based on the Frequency Hough algorithm, by splitting the analysis in a series of parallel and independent tasks and running them in a distributed computing environment. This translates into tens of thousands of jobs which have to be configured, submitted and monitored. We will describe the software framework we have developed to automatize the submission, monitoring, failure recovery and output retrieval of the analysis jobs within the Grid environment. We are currently applying it to the Frequency Hough all-sky search using data from the Virgo second and fourth science runs, and we will report on how the framework helps to increase the overall efficiency of the analysis.
Testing the validity of the single-spin approximation in IMR waveforms. By Pürrer M., Hannam M., Ajith P., Husa S. An effective single spin parameter allows us to capture the dominant spin effects in coalescences of non-precessing compact binaries, while reducing the number of physical parameters in a waveform model. To leading order an optimal effective spin parameter is available in the PN regime and an inspiral model based on it has been shown to be effectual (and faithful when the masses or spins are equal). Recent phenomenological inspiral-merger-ringdown (IMR) models for black-hole binaries have used a similar parameter. We quantify how well the single spin approximation works for a set of BBH configurations with mass-ratio 4 and effective spin 0.45 in terms of parameter biases and uncertainties.
Parameter estimation improvements using a new hybrid waveform. By Aoudia S., Babak S., Hinder I., Ohme F., Petiteau A., Sesana A., Wardell B. Detection of gravitational waves by a space-based gravitational wave observatory such as eLISA-NGO requires not only an optimization over all the instrumental equipments but also an optimization of our knowledge about waveforms, especially to improve our ability to extract information on the physical parameters of the source. Our work aims to illustrate, indeed, the importance and power of including the full waveform modeling. Especially, it reinforces a recent study which shows that by comprising all the stages of the binary merger, it is possible to reach a high level of precision measurements. For this study we build a new hybrid model, resulting from a very accurate matching between PN and NR waveforms. The matching is done for several mass ratios (q = 1, 2, 3 and 4) by fixing spins and orbital angular momentum of the system. By the use of this new hybrid model, errors on parameters based on Fisher matrix was computed for several thousand sources by randomly choosing 7 free parameters among the 15 characterizing a coalescent binary. For the same sources the same work was repeated twice, using a pure PN model (i.e. including only the inspiral phase): (1) with 7 free parameters, and (2) with 15 free parameters. The comparison of all these results enables us to compute a new law which may be used to shift or improve the precision on the measurements coming from any more realistic catalogue where the Fisher matrices were only computed using a pure PN model.
The noise characterization framework for Advanced Virgo detector. By Cuoco E., Hemming G., Berni F., Cortese S., Colla A., Drago M., Re V., Piergiovanni F., Guidi G., Vajente G. We are approaching the advanced detector era and we have to be ready with useful tools to help the commissioning phase. We gained experience from the Virgo detector, understanding the importance of having tools to be used either for a fast noise characterization or for prompt reaction to mitigate the noise disturbances. We are working on the upgrade of the Noise Monitor Application Programming Interface (NMAPI), which gathers both in-time and on-line noise analysis pipelines. Our goal is to set up a framework in which spectral characterization is integrated with linear and non-linear noise coupling and with slow and fast non-stationarity tracking. Moreover we plan to give the users, either data analysis or commissioning people, a simple web interface to retrieve information from different pipelines. In this work, we describe the framework into which these tools are integrated, and examples of the frameworks implementation and applications.
Calculating the significance of candidate binary coalescence signals. By Dent T. In order to report detections of transient gravitational wave signals it is necessary to obtain high statistical confidence, i.e. low false alarm probability. Doing this in real data containing a population of loud, unmodeled non-Gaussian transients (‘glitches’) presents several technical challenges. The established method of background estimation is to apply unphysical time-shifts to detectors at different sites. However, this can be computationally expensive, requiring long stretches of data to establish high confidence, and prone to large statistical fluctuations. I will present the results of investigations into computationally efficient and reliable methods of finding the statistical significance of candidate events, including comparisons with existing methods, with a view to application on advanced detector data.
Impact of noise cancellation on the search for gravitational wave transient signals. By Re V., Drago M., Klimenko S., Mazzolo G., Necula V., Prodi G., Salemi F., Tiwari V., Vedovato G., Yakushin I. One of the prominent problems in the search for GW is the presence of non gaussian excess noise which may hide a GW signal due to increased false alarm rate. Regression analysis of auxiliary environmental and instrumental channels can provide a partial noise cancellation by measuring the linear (or non linear) coupling of the auxiliary channels to the GW channel, thus subtracting such predicted contributions from the noise. In this work we explore the impact that this method has on the search for transient GW signals. We present preliminary results of the application of noise cancellation on a set of real data from the first joint LIGO-Virgo run. The efficiency of the method and the impact on the search sensitivity is tested by means of software injections simulating transient signals in the data. Investigations are being pursued in particular within the lower frequency band (\({<}100\) Hz ) which will be of particular astrophysical interest in future observations by the advanced detectors, being crucial for the achievable range of BH–BH coalescences.
Searching for a stochastic GW background from populations of neutron stars in data from the LIGO & Virgo detectors. By Bose S. We describe a search in LIGO and Virgo data for a stochastic gravitational-wave background from populations of rotating non-axisymmetric neutron stars in our galaxy and in the Virgo cluster. Employing multi-baseline radiometry, bounds on the GW strain power from these populations can be obtained. These bounds can in turn constrain neutron star equations of state. The current status of the search will be presented. We also assess the expected performance of this search using forthcoming second-generation detectors, including the improvement from locating one of the Advanced LIGO detectors in India.
Generic black-hole-binary waveform models: issues and progress. By Hannam M., Schmidt P., Ajith P., Bohe A., Husa S., Ohme F., Püerrer M. Current phenomenological waveform models for the inspiral, merger and ringdown of non-precessing black-hole binaries make use of a single spin parameter, which is an appropriately weighted sum of the two black-hole spins. The errors incurred in making this approximation in GW searches and parameter estimation are in most cases smaller than the errors due to waveform degeneracies between the binary’s mass ratio and the black-hole spins. In generic (precessing) binaries, additional approximate degeneracies, plus the recent insight that inspiral and precession effects can be effectively decoupled, open the possibility of constructing simple generic waveform models. We present progress in constructing such models.
Approximation methods for bayesian detection statistics in a targeted search for continuous gravitational waves. By Whelan J. , Prix R. Prix and Krishnan [44] showed that the standard maximum-likelihood statistic used in continuous gravitational-wave searches, known as the F-statistic, could also be interpreted as a Bayes factor using an unphysical prior distribution on the amplitude parameter space. They defined an alternative statistic using physical priors on the amplitude parameters, particularly the geometrical parameters of neutron star inclination and polarization angles, known as the B-statistic, and showed it to be more powerful in the case where the unknown amplitude parameters are drawn from the physical prior distribution. Marginalizing over the amplitude parameters requires a multi-dimensional integral which must, in general, be done numerically. We describe [45] approximation methods which allow analytic evaluation of this integral, allowing the more powerful search to be done without a large increase in computational resources.
A model-based cross-correlation search for gravitational waves from Scorpius X-1. By Whelan J., Sundaresan S., Peiris P. The low-mass X-ray binary (LMXB) Scorpius X-1 (Sco X-1) is a promising source of gravitational waves in the advanced detector era. A variety of methods have been used or proposed to perform the directed search for gravitational waves from a binary source in a known sky location with unknown frequency and residual uncertainty in binary orbital parameters. These include a fully coherent search over a short observation time, a search for an unmodeled narrowband stochastic signal, and a search for a pattern of sidebands arising from the Doppler modulation of the signal by the binary orbit. A modification of the cross-correlation method used in the stochastic-background search has been proposed, which takes into account the signal model of a rotating neutron star to allow cross-correlation of data from different times. By varying the maximum allowed time lag between cross-correlated segments, one can tune this semi-coherent search and strike a balance between sensitivity and computing cost. We describe the details and prospects for application of this method to searches for Sco X-1 and other LMXBs. We also present some recent enhancements to the Cross-Correlation search method.
Construction and validation of multi-mode hybrids obtained from gluing post-newtonian and numerical relativity waveforms. By Calderon Bustillo J. The construction and accuracy of hybrid post-Newtonian/numerical relativity waveforms of the dominant \(l=|m|=2\) spherical harmonic modes has been studied in the past with considerable detail. In this work we generalize to non-dominant modes and study impact and the errors of the procedure from a data analysis point of view.
Sensitivity of coincident and coherent CBC searches at finite computing cost. By Dal Canton T., Keppel D. Searches for gravitational radiation from coalescing compact binaries use single-detector matched filters followed by a coincidence stage. An alternative method is the so-called coherent matched filter, where data from all interferometers are combined coherently into a single detection statistic, automatically taking into account the different responses and time delays of the instruments. This method is expected to be more sensitive but also computationally more expensive and has never been used in a blind all-sky CBC search. Important open questions for the advanced detector era are i) at what computing cost the coherent method becomes more sensitive and ii) whether an interesting sensitivity increase can be achieved at a lower cost by combining the two methods into a “hierarchical” search. We address these issues by estimating the sensitivity and computing cost of the coincident, coherent and hierarchical methods under the main assumption of stationary Gaussian noise. We compare the sensitivities at fixed computing cost for different configurations of advanced detectors, including the projected evolution of advanced sensitivity curves.
Stochastic background of gravitational waves generated by compact binary systems. By Evangelista E. Soon after the publication of the General Theory of Relativity in its definitive form in 1916, it was noticed that some of its solutions depicted gravitational waves, that is, perturbations in the spacetime which propagate at speed of light and that could in principle be detected. According to such solutions, any mass distributions that underwent some kind of time variation would become a source of gravitational radiation, since at a given moment the spherical symmetry is broken. Thus, from the astrophysical viewpoint, virtually all the processes involving mass deformations and movements, such as stars and black holes, could be considered as potential sources of gravitational waves. Particularly, we are dealing with cosmological compact binary systems in circular and eccentric orbits and the spectra generated by the population of those sources. The main purpose of our work is the formulation of a new method of calculating the spectra generated by such a population during the periodic and quasi-periodic regimes. We used an analogy to a problem of Statistical Mechanics in order to establish the fundamentals of such a method, besides taking into account the time variation of the orbital parameters such as eccentricities and frequencies.