B. S. Sathyaprakash

Massive black-hole binary inspirals: results from the LISA parameter estimation taskforce

The LISA Parameter Estimation Taskforce was formed in September 2007 to provide the LISA Project with vetted codes, source distribution models and results related to parameter estimation. The Taskforces goal is to be able to quickly calculate the impact of any mission design changes on LISAs science capabilities, based on reasonable estimates of the distribution of astrophysical sources in the universe. This paper describes our Taskforces work on massive black-hole binaries (MBHBs). Given present uncertainties in the formation history of MBHBs, we adopt four different population models, based on (i) whether the initial black-hole seeds are small or large and (ii) whether accretion is efficient or inefficient at spinning up the holes. We compare four largely independent codes for calculating LISAs parameter-estimation capabilities. All codes are based on the Fisher-matrix approximation, but in the past they used somewhat different signal models, source parametrizations and noise curves. We show that once these differences are removed, the four codes give results in extremely close agreement with each other. Using a code that includes both spin precession and higher harmonics in the gravitational-wave signal, we carry out Monte Carlo simulations and determine the number of events that can be detected and accurately localized in our four population models.

Cosmography with the Einstein Telescope

The Einstein Telescope, a third-generation gravitational-wave detector under a design study, could detect millions of binary neutron star inspirals each year. A small fraction of these events might be observed as gamma-ray bursts, helping to measure both the luminosity distance DL to and redshift of the source. By fitting these measured values of DL and to a cosmological model, it would be possible to infer the dark energy equation of state to within 1.5% without the need to correct for errors in DL caused by weak lensing. This compares favourably with 0.3–10% accuracy that can be achieved with the Laser Interferometer Space Antenna (where weak lensing will need to be dealt with) as well as with dedicated dark energy missions that have been proposed, where 3.5–11% uncertainty is expected.

Comparison of search templates for gravitational waves from binary inspiral: 3.5PN update

Phasing formulas in a recent paper of ours’ [Phys. Rev. D 63, 044023 (2001)] are updated taking into account the recent 3.5PN results. Some misprints in our recent paper are also corrected.

The Mock LISA Data Challenges: from Challenge 1B to Challenge 3

The Mock LISA Data Challenges are a programme to demonstrate and encourage the development of LISA data-analysis capabilities, tools and techniques. At the time of this workshop, three rounds of challenges had been completed, and the next was about to start. In this paper we provide a critical analysis of the entries to the latest completed round, Challenge 1B. The entries confirm the consolidation of a range of data-analysis techniques for galactic and massive-black-hole binaries, and they include the first convincing examples of detection and parameter estimation of extreme-mass-ratio inspiral sources. In this paper we also introduce the next round, Challenge 3. Its data sets feature more realistic waveform models (e.g., galactic binaries may now chirp, and massive-black-hole binaries may precess due to spin interactions), as well as new source classes (bursts from cosmic strings, isotropic stochastic backgrounds) and more complicated nonsymmetric instrument noise.

Testing the no-hair theorem with black hole ringdowns using TIGER

The Einstein Telescope, a proposed third-generation gravitational-wave observatory, would enable tests of the no-hair theorem by looking at the characteristic frequencies and damping times of black hole ringdown signals. In previous work it was shown that with a single 500–1000u2009u2009M ⊙ black hole at a distance ≲6 Gpc (or redshift z≲1 ), deviations of a few percent in the frequencies and damping times of dominant and subdominant modes would be within the range of detectability. Given that such sources may be relatively rare, it is of interest to see how well the no-hair theorem can be tested with events at much larger distances and with smaller signal-to-noise ratios, thus accessing a far bigger volume of space and a larger number of sources. We employ a model-selection scheme called TIGER (Test Infrastructure for GEneral Relativity), which was originally developed to test general relativity with weak binary coalescence signals that will be seen in second-generation detectors, such as Advanced LIGO and Advanced Virgo. TIGER is well suited for the regime of low signal-to-noise ratios, and information from a population of sources can be combined so as to arrive at a stronger test. By performing a range of simulations using the expected noise power spectral density of the Einstein Telescope, we show that with TIGER, similar deviations from the no-hair theorem (such as those considered in previous works) will be detectable with great confidence using O(10) sources distributed uniformly in a comoving volume out to 50u2009u2009Gpcu2009(z≲5) .

Neutron-star Radius from a Population of Binary Neutron Star Mergers

We show how gravitational-wave observations with advanced detectors of tens to several tens of neutron-star binaries can measure the neutron-star radius with an accuracy of several to a few percent, for mass and spatial distributions that are realistic, and with none of the sources located within 100xa0Mpc. We achieve such an accuracy by combining measurements of the total mass from the inspiral phase with those of the compactness from the postmerger oscillation frequencies. For estimating the measurement errors of these frequencies, we utilize analytical fits to postmerger numerical relativity waveforms in the time domain, obtained here for the first time, for four nuclear-physics equations of state and a couple of values for the mass. We further exploit quasiuniversal relations to derive errors in compactness from those frequencies. Measuring the average radius to well within 10% is possible for a sample of 100 binaries distributed uniformly in volume between 100 and 300xa0Mpc, so long as the equation of state is not too soft or the binaries are not too heavy. We also give error estimates for the Einstein Telescope.

The Missing Link: Bayesian Detection and Measurement of Intermediate-Mass Black-Hole Binaries

We perform Bayesian analysis of gravitational-wave signals from nonspinning, intermediate-mass black-hole binaries (IMBHBs) with observed total mass, Mobs, from 50M⊙ to 500M⊙ and mass ratio 1–4 using advanced LIGO and Virgo detectors. We employ inspiral-merger-ringdown waveform models based on the effective-one-body formalism and include subleading modes of radiation beyond the leading (2,2) mode. The presence of subleading modes increases signal power for inclined binaries and allows for improved accuracy and precision in measurements of the masses as well as breaking of degeneracies in distance, orientation and polarization. For low total masses, Mobs≲50M⊙, for which the inspiral signal dominates, the observed chirp mass Mobs=Mobsη3/5 (η being the symmetric mass ratio) is better measured. In contrast, as increasing power comes from merger and ringdown, we find that the total mass Mobs has better relative precision than Mobs. Indeed, at high Mobs (≥300M⊙), the signal resembles a burst and the measurement thus extracts the dominant frequency of the signal that depends on Mobs. Depending on the binary’s inclination, at signal-to-noise ratio (SNR) of 12, uncertainties in Mobs can be as large as ∼20–25% while uncertainties in Mobs are ∼50–60% in binaries with unequal masses (those numbers become ∼17% vs. ∼22% in more symmetric mass-ratio binaries). Although large, those uncertainties in Mobs will establish the existence of IMBHs. We find that effective-one-body waveforms with subleading modes are essential to confirm a signal’s presence in the data, with calculated Bayesian evidences yielding a false alarm probability below 10−5 for SNR≳9 in Gaussian noise. Our results show that gravitational-wave observations can offer a unique tool to observe and understand the formation, evolution and demographics of IMBHs, which are difficult to observe in the electromagnetic window.

Is black-hole ringdown a memory of its progenitor?

We perform an extensive numerical study of coalescing black-hole binaries to understand the gravitational-wave spectrum of quasinormal modes excited in the merged black hole. Remarkably, we find that the masses and spins of the progenitor are clearly encoded in the mode spectrum of the ringdown signal. Some of the mode amplitudes carry the signature of the binarys mass ratio, while others depend critically on the spins. Simulations of precessing binaries suggest that our results carry over to generic systems. Using Bayesian inference, we demonstrate that it is possible to accurately measure the mass ratio and a proper combination of spins even when the binary is itself invisible to a detector. Using a mapping of the binary masses and spins to the final black-hole spin allows us to further extract the spin components of the progenitor. Our results could have tremendous implications for gravitational astronomy by facilitating novel tests of general relativity using merging black holes.

The Status of GEO600

The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode.

Digging deeper: Observing primordial gravitational waves below the binary black hole produced stochastic background

The merger rate of black hole binaries inferred from the detections in the first Advanced LIGO science run implies that a stochastic background produced by a cosmological population of mergers will likely mask the primordial gravitational wave background. Here we demonstrate that the next generation of ground-based detectors, such as the Einstein Telescope and Cosmic Explorer, will be able to observe binary black hole mergers throughout the Universe with sufficient efficiency that the confusion background can potentially be subtracted to observe the primordial background at the level of Ω_{GW}≃10^{-13} after 5xa0years of observation.