arXiv subjects

Stephen Fairhurst

Publications and source records attributed to Stephen Fairhurst.

At least 37 records · Page 2Linked to original sources

arXiv2020

Two-harmonic approximation for gravitational waveforms from precessing binaries

Binary-black-hole orbits precess when the black-hole spins are mis-aligned with the binary's orbital angular momentum. The apparently complicated dynamics can in most cases be described as simple precession of the orbital angular momentum about an approximately fixed total angular momentum. However, the imprint of the precession on the observed gravitational-wave signal is yet more complicated, with a non-trivial time-varying dependence on black-hole dynamics, the binary's orientation and the detector polarization. As a result, it is difficult to predict under which conditions precession effects are measurable in gravitational-wave observations, and their impact on both signal detection and source characterization. We show that the observed waveform can be simplified by decomposing it as a power series in a new precession parameter $b = \tan(β/2)$, where $β$ is the opening angle between the orbital and total angular momenta. The power series is made up of five harmonics, with frequencies that differ by the binary's precession frequency, and individually do not exhibit amplitude and phase modulations. In many cases, the waveform can be well approximated by the two leading harmonics. In this approximation we are able to obtain a simple picture of precession as caused by the beating of two waveforms of similar frequency. This enables us to identify regions of the parameter space where precession is likely to have an observable effect on the waveform, and to propose a new approach to searching for signals from precessing binaries, based upon the two-harmonic approximation.

gr-qc↗

arXiv2020

Measuring gravitational-wave higher-order modes

We investigate the observability of higher harmonics in gravitational wave signals emitted during the coalescence of binary black holes. We decompose each mode into an overall amplitude, dependent upon the masses and spins of the system, and an orientation-dependent term, dependent upon the inclination and polarization of the source. Using this decomposition, we investigate the significance of higher modes over the parameter space and show that the $\ell = 3$, $m = 3$ mode is most significant across much of the sensitive band of ground-based interferometric detectors, with the $\ell = 4$, $m = 4$ having a significant contribution at high masses. We introduce the higher mode signal-to-noise ratio (SNR), and show that a simple threshold on this SNR can be used as a criterion for observation of higher harmonics. Finally, we investigate observability in a population of binaries and observe that higher harmonics will only be observable in a few percent of binaries, typically those with unequal masses and viewed close to edge-on.

gr-qc↗

arXiv2020

When will we observe binary black holes precessing?

After eleven gravitational-wave detections from compact-binary mergers, we are yet to observe the striking general-relativistic phenomenon of orbital precession. Measurements of precession would provide valuable insights into the distribution of black-hole spins, and therefore into astrophysical binary formation mechanisms. Using our recent two-harmonic approximation of precessing-binary signals~\cite{Fairhurst:2019_2harm}, we introduce the ``precession signal-to-noise ratio'', $ρ_p$. We demonstrate that this can be used to clearly identify whether precession was measured in an observation (by comparison with both current detections and simulated signals), and can immediately quantify the measurability of precession in a given signal, which currently requires computationally expensive parameter-estimation studies. $ρ_p$ has numerous potential applications to signal searches, source-property measurements, and population studies. We give one example: assuming one possible astrophysical spin distribution, we predict that precession has a one in $\sim 25$ chance of being observed in any detection.

gr-qc↗

arXiv2018

Constraining black-hole spins with gravitational wave observations

The observation of gravitational-wave signals from merging black-hole binaries enables direct measurement of the properties of the black holes. An individual observation allows measurement of the black-hole masses, but only limited information about either the magnitude or orientation of the black hole spins is available, primarily due to the degeneracy between measurements of spin and binary mass ratio. Using the first six black-hole merger observations, we are able to constrain the distribution of black-hole spins. We perform model selection between a set of models with different spin population models combined with a power-law mass distribution to make inferences about the spin distribution. We assume a fixed power-law mass distribution on the black holes, which is supported by the data and provides a realistic distribution of binary mass-ratio. This allows us to accurately account for selection effects due to variations in the signal amplitude with spin magnitude, and provides an improved inference on the spin distribution. We conclude that the first six LIGO and Virgo observations (Abbott et al. 2016a, 2017a,b,c) disfavour highly spinning black holes against low spins by an odds-ratio of 15:1; thus providing strong constraints on spin magnitudes from gravitational-wave observations. Furthermore, we are able to rule out a population of binaries with completely aligned spins, even when the spins of the individual black holes are low, at an odds ratio of 22,000:1, significantly strengthening earlier evidence against aligned spins (Farr et al. 2017). These results provide important information that will aid in our understanding on the formation processes of black-holes.

gr-qc↗

arXiv2018

Constraining the Inclination of Binary Mergers from Gravitational-wave Observations

Much of the information we hope to extract from the gravitational-waves signatures of compact binaries is only obtainable when we can accurately constrain the inclination of the source. In this paper, we discuss in detail a degeneracy between the measurement of the binary distance and inclination which limits our ability to accurately measure the inclination using gravitational waves alone. This degeneracy is exacerbated by the expected distribution of events in the universe, which leads us to prefer face-on systems at a greater distance. We use a simplified model that only considers the binary distance and orientation, and show that this gives comparable results to the full parameter estimates obtained from the binary neutron star merger GW170817. For the advanced LIGO-Virgo network, it is only signals which are close to edge-on, with an inclination greater than $\sim 75^{\circ}$ that will be distinguishable from face-on systems. For extended networks which have good sensitivity to both gravitational wave polarizations, for face-on systems we will only be able to constrain the inclination of a signal with SNR 20 to be $45^{\circ}$ or less, and even for loud signals, with SNR of 100, the inclination of a face-on signal will only be constrained to $30^{\circ}$. For black hole mergers observed at cosmological distances, in the absence of higher modes or orbital precession, the strong degeneracy between inclination and distance dominates the uncertainty in measurement of redshift and hence the masses of the black holes.

gr-qc↗

arXiv2018

Detecting binary compact-object mergers with gravitational waves: Understanding and Improving the sensitivity of the PyCBC search

We present an improved search for binary compact-object mergers using a network of ground-based gravitational-wave detectors. We model a volumetric, isotropic source population and incorporate the resulting distribution over signal amplitude, time delay, and coalescence phase into the ranking of candidate events. We describe an improved modeling of the background distribution, and demonstrate incorporating a prior model of the binary mass distribution in the ranking of candidate events. We find a $\sim 10\%$ and $\sim 20\%$ increase in detection volume for simulated binary neutron star and neutron star--binary black hole systems, respectively, corresponding to a reduction of the false alarm rates assigned to signals by between one and two orders of magnitude.

gr-qc↗

arXiv2017

Localization of transient gravitational wave sources: beyond triangulation

Rapid, accurate localization of gravitational wave transient events has proved critical to successful electromagnetic followup. In previous papers we have shown that localization estimates can be obtained through triangulation based on timing information at the detector sites. In practice, detailed parameter estimation routines use additional information and provide better localization than is possible based on timing information alone. In this paper, we extend the timing based localization approximation to incorporate consistency of observed signals with two gravitational wave polarizations, and an astrophysically motivated distribution of sources. Both of these provide significant improvements to source localization, allowing many sources to be restricted to a single sky region, with an area 40% smaller than predicted by timing information alone. Furthermore, we show that the vast majority of sources will be reconstructed to be circularly polarized or, equivalently, indistinguishable from face-on.

gr-qc↗

arXiv2017

Localization of binary mergers with gravitational-wave detectors of second and third generation

The observation of gravitational wave signals from binary black hole mergers has established the field of gravitational wave astronomy. It is expected that future networks of gravitational wave detectors will possess great potential in probing various aspects of astronomy. An important consideration for successive improvement of current detectors or establishment on new sites is knowledge of the minimum number of detectors required to perform precision astronomy. We attempt to answer this question by assessing ability of future detector networks in detecting and localizing binary neutron stars mergers in the sky. This is an important aspect as a good localization ability is crucial for many of the scientific goals of gravitational wave astronomy, such as electromagnetic follow-up, measuring the properties of compact binaries throughout cosmic history, and cosmology. We find that although two detectors at improved sensitivity are sufficient to get a substantial increase in the number of observed signals, at least three detectors of comparable sensitivity are required to localize majority of the signals, typically to within around 10 deg$^{2}$ --- adequate for follow-up with most wide field of view optical telescopes.

gr-qc↗

arXiv2016

The PyCBC search for gravitational waves from compact binary coalescence

We describe the PyCBC search for gravitational waves from compact-object binary coalescences in advanced gravitational-wave detector data. The search was used in the first Advanced LIGO observing run and unambiguously identified two black hole binary mergers, GW150914 and GW151226. At its core, the PyCBC search performs a matched-filter search for binary merger signals using a bank of gravitational-wave template waveforms. We provide a complete description of the search pipeline including the steps used to mitigate the effects of noise transients in the data, identify candidate events and measure their statistical significance. The analysis is able to measure false-alarm rates as low as one per million years, required for confident detection of signals. Using data from initial LIGO's sixth science run, we show that the new analysis reduces the background noise in the search, giving a 30% increase in sensitive volume for binary neutron star systems over previous searches.

gr-qc↗

arXiv2015

Distinguishing compact binary population synthesis models using gravitational-wave observations of coalescing binary black holes

The coalescence of compact binaries containing neutron stars or black holes is one of the most promising signals for advanced ground-based laser interferometer gravitational-wave detectors, with the first direct detections expected over the next few years. The rate of binary coalescences and the distribution of component masses is highly uncertain, and population synthesis models predict a wide range of plausible values. Poorly constrained parameters in population synthesis models correspond to poorly understood astrophysics at various stages in the evolution of massive binary stars, the progenitors of binary neutron star and binary black hole systems. These include effects such as supernova kick velocities, parameters governing the energetics of common envelope evolution and the strength of stellar winds. Observing multiple binary black hole systems through gravitational waves will allow us to infer details of the astrophysical mechanisms that lead to their formation. Here we simulate gravitational-wave observations from a series of population synthesis models including the effects of known selection biases, measurement errors and cosmology. We compare the predictions arising from different models and show that we will be able to distinguish between them with observations (or the lack of them) from the early runs of the advanced LIGO and Virgo detectors. This will allow us to narrow down the large parameter space for binary evolution models.

astro-ph.HE↗

arXiv2014

Parameter estimation on compact binary coalescences with abruptly terminating gravitational waveforms

Gravitational-wave astronomy seeks to extract information about astrophysical systems from the gravitational-wave signals they emit. For coalescing compact-binary sources this requires accurate model templates for the inspiral and, potentially, the subsequent merger and ringdown. Models with frequency-domain waveforms that terminate abruptly in the sensitive band of the detector are often used for parameter-estimation studies. We show that the abrupt waveform termination contains significant information that affects parameter-estimation accuracy. If the sharp cutoff is not physically motivated, this extra information can lead to misleadingly good accuracy claims. We also show that using waveforms with a cutoff as templates to recover complete signals can lead to biases in parameter estimates. We evaluate when the information content in the cutoff is likely to be important in both cases. We also point out that the standard Fisher matrix formalism, frequently employed for approximately predicting parameter-estimation accuracy, cannot properly incorporate an abrupt cutoff that is present in both signals and templates; this observation explains some previously unexpected results found in the literature. These effects emphasize the importance of using complete waveforms with accurate merger and ringdown phases for parameter estimation.

gr-qc↗

arXiv2013

Reducing the effect of seismic noise in LIGO searches by targeted veto generation

The Laser Interferometer Gravitational-Wave Observatory forms part of the international effort to detect and study gravitational waves of astrophysical origin. One of the major obstacles for this project with the first generation detectors was the effect of seismic noise on instrument sensitivity - environmental disturbances causing motion of the interferometer optics, coupling as noise in the gravitational wave data output. Typically transient noise events have been identified by finding coincidence between noise in an auxiliary data signal (with negligible sensitivity to gravitational waves) and noise in the gravitational wave data, but attempts to include seismometer readings in this scheme have proven ineffective. We present a new method of generating a list of times of high seismic noise by tuning a gravitational wave burst detection pipeline to the low frequency signature of these events. This method has proven very effective at removing transients of seismic origin from the gravitational wave (GW) data with only a small loss of analysable time. We also present an outline for extending this method to other noise sources.

gr-qc↗

arXiv2013

When can gravitational-wave observations distinguish between black holes and neutron stars?

Gravitational-wave observations of compact binaries have the potential to uncover the distribution of masses and angular momenta of black holes and neutron stars in the universe. The binary components' physical parameters can be inferred from their effect on the phasing of the gravitational-wave signal, but a partial degeneracy between the components' mass ratio and their angular momenta limits our ability to measure the individual component masses. At the typical signal amplitudes expected by the Advanced Laser Interferometer Gravitational-wave Observatory (signal-to-noise ratios between 10 and 20), we show that it will in many cases be difficult to distinguish whether the components are neutron stars or black holes. We identify when the masses of the binary components could be unambiguously measured outside the range of current observations: a system with a chirp mass $\mathcal{M} \le 0.871 $ M$_\odot$ would unambiguously contain the smallest-mass neutron star observed, and a system with $\mathcal{M} \ge 2.786 \Msun$ must contain a black hole. However, additional information would be needed to distinguish between a binary containing two 1.35 M$_\odot$ neutron stars and an exotic neutron-star--black-hole binary. We also identify those configurations that could be unambiguously identified as black-hole binaries, and show how the observation of an electromagnetic counterpart to a neutron-star--black-hole binary could be used to constrain the black-hole spin.

gr-qc↗

arXiv2012

Source localization with an advanced gravitational wave detector network

We derive an expression for the accuracy with which sources can be localized using a network of gravitational wave detectors. The result is obtained via triangulation, using timing accuracies at each detector and is applicable to a network with any number of detectors. We use this result to investigate the ability of advanced gravitational wave detector networks to accurately localize signals from compact binary coalescences. We demonstrate that additional detectors can significantly improve localization results and illustrate our findings with networks comprised of the advanced LIGO, advanced Virgo and LCGT. In addition, we evaluate the benefits of relocating one of the advanced LIGO detectors to Australia.

gr-qc↗

arXiv2012

Degeneracy between mass and spin in black-hole-binary waveforms

We explore the degeneracy between mass and spin in gravitational waveforms emitted by black-hole binary coalescences. We focus on spin-aligned waveforms and obtain our results using phenomenological models that were tuned to numerical-relativity simulations. A degeneracy is known for low-mass binaries (particularly neutron-star binaries), where gravitational-wave detectors are sensitive to only the inspiral phase, and the waveform can be modelled by post-Newtonian theory. Here, we consider black-hole binaries, where detectors will also be sensitive to the merger and ringdown, and demonstrate that the degeneracy persists across a broad mass range. At low masses, the degeneracy is between mass ratio and total spin, with chirp mass accurately determined. At higher masses, the degeneracy persists but is not so clearly characterised by constant chirp mass as the merger and ringdown become more significant. We consider the importance of this degeneracy both for performing searches (including searches where only non-spinning templates are used) and in parameter extraction from observed systems. We compare observational capabilities between the early (~2015) and final (2018 onwards) versions of the Advanced LIGO detector.

gr-qc↗

arXiv2012

Improved source localization with LIGO India

A global network of advanced gravitational wave interferometric detectors is under construction. These detectors will offer an order of magnitude improvement in sensitivity over the initial detectors and will usher in the era of gravitational wave astronomy. In this paper, we evaluate the benefits of relocating one of the advanced LIGO detectors to India.

gr-qc↗

arXiv2012

The NINJA-2 catalog of hybrid post-Newtonian/numerical-relativity waveforms for non-precessing black-hole binaries

The Numerical INJection Analysis (NINJA) project is a collaborative effort between members of the numerical relativity and gravitational wave data analysis communities. The purpose of NINJA is to study the sensitivity of existing gravitational-wave search and parameter-estimation algorithms using numerically generated waveforms, and to foster closer collaboration between the numerical relativity and data analysis communities. The first NINJA project used only a small number of injections of short numerical-relativity waveforms, which limited its ability to draw quantitative conclusions. The goal of the NINJA-2 project is to overcome these limitations with long post-Newtonian - numerical relativity hybrid waveforms, large numbers of injections, and the use of real detector data. We report on the submission requirements for the NINJA-2 project and the construction of the waveform catalog. Eight numerical relativity groups have contributed 63 hybrid waveforms consisting of a numerical portion modelling the late inspiral, merger, and ringdown stitched to a post-Newtonian portion modelling the early inspiral. We summarize the techniques used by each group in constructing their submissions. We also report on the procedures used to validate these submissions, including examination in the time and frequency domains and comparisons of waveforms from different groups against each other. These procedures have so far considered only the $(\ell,m)=(2,2)$ mode. Based on these studies we judge that the hybrid waveforms are suitable for NINJA-2 studies. We note some of the plans for these investigations.

gr-qc↗

arXiv2011

A coherent triggered search for single spin compact binary coalescences in gravitational wave data

In this paper we present a method for conducting a coherent search for single spin compact binary coalescences in gravitational wave data and compare this search to the existing coincidence method for single spin searches. We propose a method to characterize the regions of the parameter space where the single spin search, both coincident and coherent, will increase detection efficiency over the existing non-precessing search. We also show example results of the coherent search on a stretch of data from LIGO's fourth science run but note that a set of signal based vetoes will be needed before this search can be run to try to make detections.

gr-qc↗