by Nissanke, Samaya and Vallisneri, Michele and Nelemans, Gijs and Prince, Thomas A.
17 pages, 3 figures, 5 tables, to be submitted to the Astrophysical Journal

Compact Galactic binaries where at least one member is a white dwarf (WD) or neutron star constitute the majority of individually detectable sources for future low-frequency space-based gravitational-wave (GW) observatories; in addition, they form an unresolved continuum, the dominant Galactic foreground at frequencies below a few mHz. A handful of ultra-compact binaries, observed at optical, ultraviolet and X-ray wavelengths, are known verification sources for space-based GW interferometers. Due to the paucity of electromagnetic observations, the majority of studies of Galactic-binary populations so far have been based on population-synthesis simulations. However, recent surveys have reported several new detections of compact binaries including double WDs, providing new constraints for population estimates. In this article, we evaluate the impact of revised local densities of interacting WD binaries on future low-frequency GW observations. Specifically: we consider five scenarios that explain these densities with different assumptions on the formation of interacting systems; we simulate corresponding populations of detached and interacting WD binaries; we estimate the number of individually detectable GW sources and the magnitude of the confusion-noise foreground, in the case of two GW interferometers with armlengths of 1 and 5 Mkm. We confirm earlier estimates of thousands of detached-binary detections, but project only a few ten to a few hundred detections of interacting systems. We also confirm estimates for the confusion-noise foreground (except in one scenario that explains smaller local densities of interacting systems with fewer progenitor detached systems). Last, we provide a general scaling argument that shows that the magnitude of the GW foreground can be derived robustly from the merger rate of Galactic WD binaries, and depends only weakly on the structure of the Galaxy.

by Corda, Christian
9 pages, 2 figures, published in Modern Physics Letters A. arXiv admin note: substantial text overlap with arXiv:0901.1193

The cosmological bound of the stochastic background of gravitational waves is analyzed with the aid of the WMAP data, differently from lots of works in literature, where the old COBE data were used. From our analysis, it will result that the WMAP bounds on the energy spectrum and on the characteristic amplitude of the stochastic background of gravitational waves are greater than the COBE ones, but they are also far below frequencies of the earth-based antennas band. At the end of this letter a lower bound for the integration time of a potential detection with advanced LIGO is released and compared with the previous one arising from the old COBE data. Even if the new lower bound is minor than the previous one, it results very long, thus for a possible detection we hope in the LISA interferometer and in a further growth in the sensitivity of advanced projects.

by Regimbau, Tania and Giampanis, Stefanos and Siemens, Xavier and Mandic, Vuk
21 pages, 11 figures, submitted to PRD

In the era of the next generation of gravitational wave experiments a stochastic background from cusps of cosmic (super)strings is expected to be probed and, if not detected, to be significantly constrained. A popcorn-like background can be, for part of the parameter space, as pronounced as the (Gaussian) continuous contribution from unresolved sources that overlap in frequency and time. We study both contributions from unresolved cosmic string cusps over a range of frequencies relevant to ground based interferometers, such as LIGO/Virgo second generation (AdLV) and Einstein Telescope (ET) third generation detectors, the space antenna LISA and Pulsar Timing Arrays (PTA). We compute the sensitivity (at $latex 2 \sigma$ level) in the parameter space for AdLV, ET, LISA and PTA. We conclude that the popcorn regime is complementary to the continuous background. Its detection could therefore enhance confidence in a stochastic background detection and possibly help determine fundamental string parameters such as the string tension and the reconnection probability.

by Marx, Jay and Danzmann, Karsten and Hough, James and Kuroda, Kazuaki and McClelland, David and Mours, Benoit and Phinney, Sterl and Rowan, Sheila and Sathyaprakash, B. and Vetrano, Flavio and Vitale, Stefano and Whitcomb, Stan and Will, Clifford
116 pages. Original document in higher resolution can be found at https://gwic.ligo.org/roadmap/

Gravitational wave science is on the verge of direct observation of the waves predicted by Einstein’s General Theory of Relativity and opening the exciting new field of gravitational wave astronomy. In the coming decades, ultra-sensitive arrays of ground-based instruments and complementary spaced-based instruments will observe the gravitational wave sky, inevitably discovering entirely unexpected phenomena while providing new insight into many of the most profound astrophysical phenomena known. in July 2007 the Gravitational Wave International Committee (GWIC) initiated the development of a strategic roadmap for the field of gravitational wave science with a 30-year horizon. The goal of this roadmap is to serve the international gravitational wave community and its stakeholders as a tool for the development of capabilities and facilities needed to address the exciting scientific opportunities on the intermediate and long-term horizons.

by Benacquista, Matthew J. and Downing, Jonathan M. B.
88 pages, 13 figures. Submitted update of Living Reviews article

Galactic globular clusters are old, dense star systems typically containing 10\super{4}–10\super{7} stars. As an old population of stars, globular clusters contain many collapsed and degenerate objects. As a dense population of stars, globular clusters are the scene of many interesting close dynamical interactions between stars. These dynamical interactions can alter the evolution of individual stars and can produce tight binary systems containing one or two compact objects. In this review, we discuss theoretical models of globular cluster evolution and binary evolution, techniques for simulating this evolution that leads to relativistic binaries, and current and possible future observational evidence for this population. Our discussion of globular cluster evolution will focus on the processes that boost the production of hard binary systems and the subsequent interaction of these binaries that can alter the properties of both bodies and can lead to exotic objects. Direct {\it N}-body integrations and Fokker–Planck simulations of the evolution of globular clusters that incorporate tidal interactions and lead to predictions of relativistic binary populations are also discussed. We discuss the current observational evidence for cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries as well as possible future detection of relativistic binaries with gravitational radiation.

by Benacquista, Matthew
5 pages, submitted to ApJL

The recently discovered J0651+2844 is a detached, eclipsing white dwarf binary with an orbital period of 765 s. We investigate the prospects for the detection of gravitational radiation from this system and estimate the effect of the tidal deformation of the low-mass component on the period evolution of the system. Because of the high inclination of the system, the amplitude of the gravitational waves at Earth will be as much as a factor of two lower than that from an optimally oriented system. The dominant contribution of tidal corrections to the period evolution comes from the increase in rotational energy of the components as they spin up to remain tied to the orbital period. This contribution results in an advance of the timing of the eclipses by an additional 0.3 s after one year.

by Rosado, Pablo A.

Basic aspects of the background of gravitational waves and its mathematical characterization are reviewed. The spectral energy density parameter $latex \Omega(f)$, commonly used as a quantifier of the background, is derived for an ensemble of many identical sources emitting at different times and locations. For such an ensemble, $latex \Omega(f)$ is generalized to account for the duration of the signals and of the observation, so that one can distinguish the resolvable and unresolvable parts of the background. The unresolvable part, often called confusion noise or stochastic background, is made by signals that cannot be either individually identified or subtracted out of the data. To account for the resolvability of the background, the overlap function is introduced. This function is a generalization of the duty cycle, which has been commonly used in the literature, in some cases leading to incorrect results. The spectra produced by binary systems (stellar binaries and massive black hole binaries) are presented over the frequencies of all existing and planned detectors. A semi-analytical formula for $latex \Omega(f)$ is derived in the case of stellar binaries (containing white dwarfs, neutron stars or stellar-mass black holes). Besides a realistic expectation of the level of background, upper and lower limits are given, to account for the uncertainties in some astrophysical parameters such as binary coalescence rates. One interesting result concerns all current and planned ground-based detectors (including the Einstein Telescope). In their frequency range, the background of binaries is resolvable and only sporadically present. In other words, there is no stochastic background of binaries for ground-based detectors.

by Valsecchi, Francesca and Farr, Will M. and Willems, Bart and Deloye, Christopher . J. and Kalogera, Vassiliki
21 pages, 7 figures, submitted to APJ

Galactic interacting double white dwarfs (DWD) are guaranteed gravitational wave (GW) sources for the GW detector LISA, with more than 10^4 binaries expected to be detected over the mission’s lifetime. While the majority of DWDs are expected to be circular, dynamical interactions in globular clusters can lead to a sub-population of eccentric DWDs detectable by LISA. Here we investigate the potential for constraining the white dwarf (WD) properties through apsidal precession in these binaries. We analyze the tidal, rotational, and general relativistic contributions to apsidal precession by using detailed He WD models, where the evolution of the star’s interior is followed throughout the cooling phase. In agreement with previous studies of zero-temperature WDs, we find that apsidal precession in eccentric DWDs can lead to a detectable shift in the emitted GW signal when binaries with cool (old) components are considered. This shift increases significantly for hot (young) WDs. We find that apsidal motion in hot (cool) DWDs is dominated by tides at orbital frequencies above ~10^(-4) Hz [10^(-3) Hz]. The analysis of apsidal precession in these sources while ignoring the tidal component would lead to an extreme bias in the mass determination, and could lead us to misidentify WDs as neutron stars or black holes. We use the detailed WD models to show that for older, cold WDs, there is a unique relationship that ties the radius and apsidal precession constant to the WD masses, therefore allowing tides to be used as a tool to constrain the source masses.

by Marsh, T. R.
11 pages, 2 figures, in press Classical and Quantum Gravity. Summary of talk given at 8th LISA Symposium, Stanford, July 2010

Close pairs of white dwarfs are potential progenitors of Type~Ia supernovae and they are common, with of order 100 — 300 million in the Galaxy. As such they will be significant, probably dominant, sources of the gravitational waves detectable by LISA. In the context of LISA’s goals for fundamental physics, double white dwarfs are a source of noise, but from an astrophysical perspective, they are of considerable interest in their own right. In this paper I discuss our current knowledge of double white dwarfs and their close relatives (and possible descendants) the AM~CVn stars. LISA will add to our knowledge of these systems by providing the following unique constraints: (i) an almost direct measurement of the Galactic merger rate of DWDs from the detection of short period systems and their period evolution, (ii) an accurate and precise normalisation of binary evolution models at the shortest periods, (iii) a determination of the evolutionary pathways to the formation of AM~CVn stars, (iv) measurements of the influence of tidal coupling in white dwarfs and its significance for stabilising mass transfer, and (v) discovery of numerous examples of eclipsing white dwarfs with the potential for optical follow-up to test models of white dwarfs.

by Schneider, Raffaella and Marassi, Stefania and Ferrari, Valeria
10 pages, 9 figures, proceedings of the GWDAW 10 Conference, submitted to Class. & Quantum Grav

Astrophysical sources emit gravitational waves in a large variety of processes occurred since the beginning of star and galaxy formation. These waves permeate our high redshift Universe, and form a background which is the result of the superposition of different components, each associated to a specific astrophysical process. Each component has different spectral properties and features that it is important to investigate in view of a possible, future detection. In this contribution, we will review recent theoretical predictions for backgrounds produced by extragalactic sources and discuss their detectability with current and future gravitational wave observatories.

by Adams, Matthew R. and Cornish, Neil J.
10 Pages, 10 Figures

The detection of a stochastic background of gravitational waves could significantly impact our understanding of the physical processes that shaped the early Universe. The challenge lies in separating the cosmological signal from other stochastic processes such as instrument noise and astrophysical foregrounds. One approach is to build two or more detectors and cross correlate their output, thereby enhancing the common gravitational wave signal relative to the uncorrelated instrument noise. When only one detector is available, as will likely be the case with the Laser Interferometer Space Antenna (LISA), alternative analysis techniques must be developed. Here we show that models of the noise and signal transfer functions can be used to tease apart the gravitational and instrument noise contributions. We discuss the role of gravitational wave insensitive “null channels” formed from particular combinations of the time delay interferometry, and derive a new combination that maintains this insensitivity for unequal arm length detectors. We show that, in the absence of astrophysical foregrounds, LISA could detect signals with energy densities as low as $latex \Omega_{\rm gw} = 6 \times 10^{-13}$ with just one month of data. We describe an end-to-end Bayesian analysis pipeline that is able to search for, characterize and assign confidence levels for the detection of a stochastic gravitational wave background, and demonstrate the effectiveness of this approach using simulated data from the third round of Mock LISA Data Challenges.

by Błaut, Arkadiusz and Babak, Stanislav and Królak, Andrzej
36 pages, 13 figures

We present data analysis methods used in detection and the estimation of parameters of gravitational wave signals from the white dwarf binaries in the Mock LISA Data Challenge. Our main focus is on the analysis of Challenge 3.1, where the gravitational wave signals from more than 50 mln. Galactic binaries were added to the simulated Gaussian instrumental noise. Majority of the signals at low frequencies are not resolved individually. The confusion between the signals is strongly reduced at frequencies above 5 mHz. Our basic data analysis procedure is the maximum likelihood detection method. We filter the data through the template bank at the first step of the search, then we refine parameters using the Nelder-Mead algorithm, we remove the strongest signal found and we repeat the procedure. We detect reliably and estimate parameters accurately of more than ten thousand signals from white dwarf binaries.

by Stroeer, Alexander and Nelemans, Gijs
5 pages, 3 figures, accepted for publication in MNRAS Letters

We study the effect of short term variations of the evolution of AM CVn systems on their gravitational wave emissions and in particular LISA observations. We model the systems according to their equilibrium mass-transfer evolution as driven by gravitational wave emission and tidal interaction, and determine their reaction to a sudden perturbation of the system. This is inspired by the suggestion to explain the orbital period evolution of the ultra-compact binary systems V407 Vul and RX-J0806+1527 by non-equilibrium mass transfer. The characteristics of the emitted gravitational wave signal are deduced from a Taylor expansion of a Newtonian quadrupolar emission model, and the changes in signal structure as visible to the LISA mission are determined. We show that short term variations can significantly change the higher order terms in the expansion, and thus lead to spurious (non) detection of frequency derivatives. This may hamper the estimation of the parameters of the system, in particular their masses and distances. However, we find that overall detection is still secured as signals still can be described by general templates. We conclude that a better modelling of the effects of short term variations is needed to prepare the community for astrophysical evaluations of real gravitational wave data of AM CVn systems.

by Caprini, Chiara and Durrer, Ruth and Servant, Geraldine
44 pages, 17 figures

We analytically derive the spectrum of gravitational waves due to magneto-hydrodynamical turbulence generated by bubble collisions in a first-order phase transition. In contrast to previous studies, we take into account the fact that turbulence and magnetic fields act as sources of gravitational waves for many Hubble times after the phase transition is completed. This modifies the gravitational wave spectrum at large scales. We also model the initial stirring phase preceding the Kolmogorov cascade, while earlier works assume that the Kolmogorov spectrum is set in instantaneously. The continuity in time of the source is relevant for a correct determination of the peak position of the gravitational wave spectrum. We discuss how the results depend on assumptions about the unequal-time correlation of the source and motivate a realistic choice for it. Our treatment gives a similar peak frequency to previous analyses but the amplitude of the signal is reduced due to the use of a more realistic power spectrum for the MHD turbulence. For a strongly first-order electroweak phase transition, the signal is observable by LISA.

by Whelan, John T. and Prix, Reinhard and Khurana, Deepak
26 pages, 11 figures

We describe the application of an F-statistic search for continuous gravitational waves to the search for galactic white-dwarf binaries in the Second Mock LISA Data Challenge. The search method employs a hierarchical template-grid based exploration of the parameter space, using a coincidence step to distinguish between primary (”true”) and secondary maxima, followed by a final (multi-TDI) “zoom” stage to provide an accurate parameter estimation of the final candidates. Suitably tuned, the pipeline is able to extract 1989 true signals with only 5 false alarms. The use of the rigid adiabatic approximation allows recovery of signal parameters comparable to statistical expectations, although there is still some systematic excess above expected statistical errors due to Gaussian noise. An experimental iterative pipeline with seven rounds of subtraction and re-analysis allows us to increase the number of signals recovered, up to a total of 3419 with 29 false alarms.

by Seto, Naoki
18 pages, to appear in PRD

We study an observational method to analyze non-Gaussianity of a gravitational wave (GW) background made by superposition of weak burst signals. The proposed method is based on fourth-order correlations of data from four detectors, and might be useful to discriminate the origin of a GW background. With a formulation newly developed to discuss geometrical aspects of the correlations, it is found that the method provides us with linear combinations of two interesting parameters, I_2 and V_2 defined by the Stokes parameters of individual GW burst signals. We also evaluate sensitivities of specific detector networks to these parameters.

by Stroeer, Alexander and Veitch, John
18 pages, 8 figures, 3 tables, submitted to PRD

The Laser Interferometer Space Antenna (LISA) defines new demands on data analysis efforts in its all-sky gravitational wave survey, recording simultaneously thousands of galactic compact object binary foreground sources and tens to hundreds of background sources like binary black hole mergers and extreme mass ratio inspirals. We approach this problem with an adaptive and fully automatic Reversible Jump Markov Chain Monte Carlo sampler, able to sample from the joint posterior density function (as established by Bayes theorem) for a given mixture of signals “out of the box”, handling the total number of signals as an additional unknown parameter beside the unknown parameters of each individual source and the noise floor. We show in examples from the LISA Mock Data Challenge implementing the full response of LISA in its TDI description that this sampler is able to extract monochromatic Double White Dwarf signals out of colored instrumental noise and additional foreground and background noise successfully in a global fitting approach. We introduce 2 examples with fixed number of signals (MCMC sampling), and 1 example with unknown number of signals (RJ-MCMC), the latter further promoting the idea behind an experimental adaptation of the model indicator proposal densities in the main sampling stage. We note that the experienced runtimes and degeneracies in parameter extraction limit the shown examples to the extraction of a low but realistic number of signals.

by Nelemans, G.
Accepted for publication in Classical and Quantum Gravity (special issue for proceedings of 7th International LISA Symposium)

I present an overview of the Galactic binaries that form the foreground for the ESA/NASA Laser Interferometer Space Antenna (LISA). The currently known population is discussed, as well as current and near-future large-scale surveys that will find new systems. The astrophysics that can be done when the LISA data becomes available is presented, with particular attention to verification binaries, the overall Galactic populations, neutron star and black hole binaries and sources in globular clusters. I discuss the synergy with electro-magnetic observations and correct an error in the estimate of the number of LISA systems that can be found in the optical compared to Nelemans (2006a) and conclude that at least several hundreds of systems should be detectable.