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Authors: Achamveedu Gopakumar, Mark Hannam, Sascha Husa, Bernd Brügmann

Date: 21 Dec 2007

Abstract: We compare the phase evolution of equal-mass nonspinning black-hole binaries from numerical relativity (NR) simulations with post-Newtonian (PN) results obtained from three PN approximants: the TaylorT1 and T4 approximants, for which NR-PN comparisons have already been performed in the literature, and the recently proposed approximant TaylorEt. The accumulated phase disagreement between NR and PN results over the frequency range $M\omega = 0.0455$ to $M\omega = 0.1$ is greater for TaylorEt than either T1 or T4, but has the attractive property of decreasing monotonically as the PN order is increased.

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Authors: Mark Hannam, Sascha Husa, Bernd Brügmann, Achamveedu Gopakumar

Date: 21 Dec 2007

Abstract: We compare results from numerical simulations of spinning binaries in the ‘orbital hangup’ case, where the binary completes at least nine orbits before merger, with post-Newtonian results using the approximants TaylorT1, T4 and Et. We find that, over the ten cycles before the gravitational-wave frequency reaches $M\omega = 0.1$, the accumulated phase disagreement between NR and 2.5PN results is less than three radians, and is less than 2.5 radians when using 3.5PN results. The amplitude disagreement between NR and restricted PN results increases with the black holes' spin, from about 6% in the equal-mass case to 12% when the black holes' spins are $S_i/M_iˆ2 = 0.85$. Finally, our results suggest that the merger waveform will play an important role in estimating the spin from such inspiral waveforms.

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Authors: Luciano Rezzolla, Enrico Barausse, Ernst Nils Dorband, Denis Pollney, Christian Reisswig, Jennifer Seiler, Sascha Husa

Date: 20 Dec 2007

Abstract: We provide a compact analytic formula to compute the spin of the black hole produced by the coalescence of two black holes. The expression, which uses an analytic fit of numerical-relativity data and relies on four assumptions, aims at modelling generic initial spin configurations and mass ratios. A comparison with numerical-relativity simulations already shows very accurate agreements with all of the numerical data available to date, but we also suggest a number of ways in which our predictions can be further improved.

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Authors: Stephen M. Merkowitz, Philip W. Dabney, Jeffrey C. Livas, Jan F. McGarry, Gregory A. Neumann, Thomas W. Zagwodzki

Date: 20 Dec 2007

Abstract: More precise lunar and Martian ranging will enable unprecedented tests of Einstein's theory of General Relativity and well as lunar and planetary science. NASA is currently planning several missions to return to the Moon, and it is natural to consider if precision laser ranging instruments should be included. New advanced retroreflector arrays at carefully chosen landing sites would have an immediate positive impact on lunar and gravitational studies. Laser transponders are currently being developed that may offer an advantage over passive ranging, and could be adapted for use on Mars and other distant objects. Precision ranging capability can also be combined with optical communications for an extremely versatile instrument. In this paper we discuss the science that can be gained by improved lunar and Martian ranging along with several technologies that can be used for this purpose.

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Authors: Craig J. Hogan

Date: 20 Dec 2007

Abstract: A phenomenological calculation is presented of the effect of quantum fluctuations in the spacetime metric, or holographic noise, on interferometeric measurement of the relative positions of freely falling proof masses, in theories where spacetime satisfies covariant entropy bounds and can be represented as a quantum theory on 2+1D null surfaces. The quantum behavior of the 3+1D metric, represented by a commutation relation expressing quantum complementarity between orthogonal position operators, leads to a parameter-free prediction of quantum noise in orthogonal position measurements of freely falling masses. A particular quantum weirdness of this holographic noise is that it only appears in measurements that compare transverse positions, and does not appear at all in purely radial position measurements. The effect on phase signal in an interferometer that continuously measures the difference in the length of orthogonal arms resembles that of a classical random Brownian motion of the beamsplitter with a Planck length step in orthogonal position difference every Planck time. This predicted holographic noise is comparable in magnitude with currently measured system noise, and should be detectable in the currently operating interferometer GEO600. Because of its transverse character, holographic noise is reduced relative to gravitational wave effects in some interferometer designs, such as LIGO, where beam power is much less in the beamsplitter than in the arms.

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Authors: M. Tessmer, A. Gopakumar

Date: 19 Dec 2007

Abstract: Compact binaries inspiralling along eccentric orbits are plausible gravitational wave (GW) sources for ground-based laser interferometers. We explore losses in event rates incurred when searching for GWs from compact binaries inspiralling along post-Newtonian accurate eccentric orbits with certain obvious non-optimal search templates. For the present analysis, GW signals having 2.5 post-Newtonian accurate orbital evolution are modeled following the phasing formalism, presented in [T. Damour, A. Gopakumar, and B. R. Iyer, Phys. Rev. D 70, 064028 (2004)]. The associated search templates are the usual time domain Taylor approximants for compact binaries in quasi-circular orbits, also having 2.5PN accurate non-stationary orbital phase evolution. We observe that these templates are highly inefficient in capturing our realistic GW signals having tiny residual eccentricities. We present reasons for our observations and provide certain possible remedies.

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Authors: Achamveedu Gopakumar

Date: 19 Dec 2007

Abstract: Compact binaries inspiralling along quasi-circular orbits are the most plausible gravitational wave (GW) sources for the operational, planned and proposed laser interferometers. We provide new class of restricted post-Newtonian accurate GW templates for non-spinning compact binaries inspiralling along PN accurate quasi-circular orbits. Arguments based on data analysis, theoretical and astrophysical considerations are invoked to show why these time-domain Taylor approximants should be interesting to various GW data analysis communities.

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Authors: Christopher W. Stubbs

Date: 16 Dec 2007

Abstract: Optical and infrared observations have thus far detected more celestial cataclysms than have been seen in gravity waves (GW). This argues that we should search for gravity wave signatures that correspond to flux variability seen at optical wavelengths, at precisely known positions. There is an unknown time delay between the optical and gravitational transient, but knowing the source location precisely specifies the corresponding time delays across the gravitational antenna network as a function of the GW-to-optical arrival time difference. Optical searches should detect virtually all supernovae that are plausible gravitational radiation sources. The transient optical signature expected from merging compact objects is not as well understood, but there are good reasons to expect detectable transient optical/IR emission from most of these sources as well. The next generation of deep wide-field surveys (for example PanSTARRS and LSST) will be sensitive to subtle optical variability, but we need to fill the ‘blind spots’ that exist in the Galactic plane, and for optically bright transient sources. In particular, a Galactic plane variability survey at 2 microns seems worthwhile. Science would benefit from closer coordination between the various optical survey projects and the gravity wave community.

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Authors: Barak Kol, Michael Smolkin

Date: 18 Dec 2007

Abstract: Matched Asymptotic Expansion (MAE) is a useful technique in General Relativity and other fields whenever interaction takes place between physics at two different length scales. Here MAE is argued to be equivalent quite generally to Classical Effective Field Theory (ClEFT) where one (or more) of the zones is replaced by an effective theory whose terms are organized in order of increasing irrelevancy, as demonstrated by Goldberger and Rothstein in a certain gravitational context. The ClEFT perspective has advantages as the procedure is clearer, it allows a representation via Feynman diagrams, and divergences can be regularized and renormalized in standard field theoretic methods. As a side product we obtain a wide class of classical examples of regularization and renormalization, concepts which are usually associated with Quantum Field Theories.
We demonstrate these ideas through the thermodynamics of caged black holes, both simplifying the non-rotating case, and computing the rotating case. In particular we are able to replace the computation of six two-loop diagrams by a single factorizable two-loop diagram, as well as compute certain new three-loop diagrams. The results generalize to arbitrary compactification manifolds. For caged rotating black holes we obtain the leading correction for all thermodynamic quantities. The angular momentum is found to non-renormalize at leading order.

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Authors: Stephon Alexander, Lee Samuel Finn, Nicolas Yunes

Date: 15 Dec 2007

Abstract: The Green-Schwarz anomaly-cancelling mechanism in string theories requires a Chern-Simons term in the Einstein-Hilbert action, which leads to an amplitude birefringence of spacetime for the propagation of gravitational waves. While the degree of birefringence may be intrinsically small, its effects on a gravitational wave will accumulate as the wave propagates. The proposed Laser Interferometer Space Antenna (LISA) will be sensitive enough to observe the gravitational waves from sources at cosmological distances great enough that interesting bounds on the Chern-Simons may be found. Here we evaluate the effect of a Chern-Simons induced spacetime birefringence to the propagation of gravitational waves from such systems. We find that gravitational waves from in coalescing binary black hole system are imprinted with a signature of Chern-Simons gravity. This signature appears as a time-dependent change in the apparent orientation of the binary's orbital angular momentum with respect to the observer line-of-sight, with the change magnitude reflecting the integrated history of the Chern-Simons coupling over the worldline of a radiation wavefront. While spin-orbit coupling in the binary system will also lead to an evolution of the system's orbital angular momentum, the time dependence and other details of this \emph{real} effect are different than the \emph{apparent} effect produced by Chern-Simons birefringence, allowing the two effects to be separately identified.

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Authors: Warren G. Anderson, Jolien D. E. Creighton

Date: 15 Dec 2007

Abstract: A new generation of observatories is looking for gravitational waves. These waves, emitted by highly relativistic systems, will open a new window for ob- servation of the cosmos when they are detected. Among the most promising sources of gravitational waves for these observatories are compact binaries in the final min- utes before coalescence. In this article, we review in brief interferometric searches for gravitational waves emitted by neutron star binaries, including the theory, instru- mentation and methods. No detections have been made to date. However, the best direct observational limits on coalescence rates have been set, and instrumentation and analysis methods continue to be refined toward the ultimate goal of defining the new field of gravitational wave astronomy.

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Authors: The LIGO Scientific Collaboration: B. Abbott, et al

Date: 12 Dec 2007

Abstract: We report on the first dedicated search for gravitational waves emitted during the inspiral of compact binaries with spinning component bodies. We analyze 788 hours of data collected during the third science run (S3) of the LIGO detectors. We searched for binary systems using a detection template family designed specially to capture the effects of spin-induced precession. The template bank we employed was found to yield high matches with our spin-modulated target waveform for binaries with masses in the asymmetric range 1.0 M_{\odot} < m_1 < 3.0 M_{\odot} and 12.0 M_{\odot} < m_{2} < 20.0 M_{\odot} which is where we would expect the spin of the binary's components to have significant effect. We find that our search of S3 LIGO data had good sensitivity to binaries in the Milky Way and to a small fraction of binaries in M31 and M33 with masses in the range 1.0 M_{\odot} < m_{1}, m_{2} < 20.0 M_{\odot}. No gravitational wave signals were identified during this search. Assuming a binary population with a Gaussian distribution of component body masses of a prototypical neutron star - black hole system with m_1 \simeq 1.35 M_{\odot} and m_2 \simeq 5 M_{\odot}, we calculate the 90% confidence upper limit on the rate of coalescence of these systems to be 15.9 yrˆ-1 L_10 ˆ-1, where L_10 is 10ˆ10 times the blue light luminosity of the Sun.

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Authors: Rafael A. Porto (UCSB), Ira Z. Rothstein (CMU)

Date: 12 Dec 2007

Abstract: In this comment we explain the discrepancy found between the results in arXiv:0712.1716v1 for the 3PN spin-spin potential and those previously derived in gr-qc/0604099. We point out that to compare one must include sub-leading lower order spin-orbit effects which contribute to the spin-spin potential once one transforms to the PN frame. When these effects are included the results in arXiv:0712.1716v1 do indeed reproduce those found in gr-qc/0604099.

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Authors: Bence Kocsis (Harvard), Zoltan Haiman (Columbia), Kristen Menou (Columbia)

Date: 7 Dec 2007

Abstract: Electromagnetic (EM) counterparts to supermassive black hole binary mergers observed by LISA can be localized to within the field of view of astronomical instruments ~10 degˆ2 hours to weeks prior to coalescence. The temporal coincidence of any prompt EM counterpart with a gravitationally-timed merger may offer the best chance of identifying a unique host galaxy. We discuss the challenges posed by searches for prompt EM counterparts and propose novel observational strategies to address them. In particular, we discuss the size and shape evolution of the LISA localization error ellipses on the sky, and quantify the requirements for dedicated EM surveys of the area prior to coalescence. A triggered EM counterpart search campaign will require monitoring a several-square degree area. It could aim for variability at the 24-27 mag level in optical bands, for example, which corresponds to 1-10% of the Eddington luminosity of the prime LISA sources of 10ˆ6-10ˆ7 Msun BHs at z=1-2, on time-scales of minutes to hours, the orbital time-scale of the binary in the last 2-4 weeks. A cross-correlation of the period of any variable EM signal with the quasi-periodic gravitational waveform over 10-1000 cycles may aid the detection. Alternatively, EM searches can detect a transient signal accompanying the coalescence. We highlight the measurement of differences in the arrival times of photons and gravitons from the same cosmological source as a valuable independent test of the massive character of gravity, and of possible violations of Lorentz invariance in the gravity sector.

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Authors: Tamara Bogdanovic (1,2), Britton D. Smith (1), Steinn Sigurdsson (1), Michael Eracleous (1) ((1) Pennsylvania State University,(2) University of Maryland)

Date: 2 Aug 2007

Abstract: We model the electromagnetic signatures of massive black hole binaries (MBHBs) with an associated gas component. The method comprises numerical simulations of relativistic binaries and gas coupled with calculations of the physical properties of the emitting gas. We calculate the UV/X-ray and the Halpha light curves and the Halpha emission profiles. The simulations are carried out with a modified version of the parallel tree SPH code Gadget. The heating, cooling, and radiative processes are calculated for two different physical scenarios, where the gas is approximated as a black-body or a solar metallicity gas. The calculation for the solar metallicity scenario is carried out with the photoionization code Cloudy. We focus on sub-parsec binaries which have not yet entered the gravitational radiation phase. The results from the first set of calculations, carried out for a coplanar binary and gas disk, suggest that there are pronounced outbursts in the X-ray light curve during pericentric passages. If such outbursts persist for a large fraction of the lifetime of the system, they can serve as an indicator of this type of binary. The predicted Halpha emission line profiles may be used as a criterion for selection of MBHB candidates from existing archival data. The orbital period and mass ratio of a binary may be inferred after carefully monitoring the evolution of the Halpha profiles of the candidates. The discovery of sub-parsec binaries is an important step in understanding of the merger rates of MBHBs and their evolution towards the detectable gravitational wave window.

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Authors: Jan Steinhoff, Steven Hergt, Gerhard Sch&#xe4;fer

Date: 11 Dec 2007

Abstract: Based on recents developments by the authors it is shown that the next-to-leading order spin(1)-spin(2) coupling potential recently derived by Porto and Rothstein cannot be regarded as correct if their variables, as claimed, belong to canonical ones in standard manner.

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Authors: Badri Krishnan

Date: 10 Dec 2007

Abstract: The traditional description of black holes in terms of event horizons is inadequate for many physical applications, especially when studying black holes in non-stationary spacetimes. In these cases, it is often more useful to use the quasi-local notions of trapped and marginally trapped surfaces, which lead naturally to the framework of trapping, isolated, and dynamical horizons. This framework allows us to analyze diverse facets of black holes in a unified manner and to significantly generalize several results in black hole physics. It also leads to a number of applications in mathematical general relativity, numerical relativity, astrophysics, and quantum gravity. In this review, I will discuss the basic ideas and recent developments in this framework, and summarize some of its applications with an emphasis on numerical relativity.

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Authors: Sanjeev Dhurandhar, Badri Krishnan, Himan Mukhopadhyay, John T. Whelan

Date: 10 Dec 2007

Abstract: In this paper we study the use of cross-correlations between multiple gravitational wave (GW) data streams for detecting long-lived periodic signals. Cross-correlation searches between data from multiple detectors have traditionally been used to search for stochastic GW signals, but recently they have also been used in directed searches for periodic GWs. Here we further adapt the cross-correlation statistic for periodic GW searches by taking into account both the non-stationarity and the long term-phase coherence of the signal. We study the statistical properties and sensitivity of this search, its relation to existing periodic wave searches, and describe the precise way in which the cross-correlation statistic interpolates between semi-coherent and fully-coherent methods. Depending on the maximum duration over we wish to preserve phase coherence, the cross-correlation statistic can be tuned to go from a standard cross-correlation statistic using data from distinct detectors, to the semi-coherent time-frequency methods with increasing coherent time baselines, and all the way to a full coherent search. This leads to a unified framework for studying periodic wave searches and can be used to make informed trade-offs between computational cost, sensitivity, and robustness against signal uncertainties.

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Authors: R. Umstaetter, M. Tinto

Date: 6 Dec 2007

Abstract: We estimate the probability of detecting a gravitational wave signal from coalescing compact binaries in simulated data from a ground-based interferometer detector of gravitational radiation using Bayesian model selection. The simulated waveform of the chirp signal is assumed to be a spin-less Post-Newtonian (PN) waveform of a given expansion order, while the searching template is assumed to be either of the same Post-Newtonian family as the simulated signal or one level below its Post-Newtonian expansion order. Within the Bayesian framework, and by applying a reversible jump Markov chain Monte Carlo simulation algorithm, we compare PN1.5 vs. PN2.0 and PN3.0 vs. PN3.5 wave forms by deriving the detection probabilities, the statistical uncertainties due to noise as a function of the SNR, and the posterior distributions of the parameters. Our analysis indicates that the detection probabilities are not compromised when simplified models are used for the comparison, while the accuracies in the determination of the parameters characterizing these signals can be significantly worsened, no matter what the considered Post-Newtonian order expansion comparison is.

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Authors: P. Ajith

Date: 3 Dec 2007

Abstract: Coalescing binary black-hole systems are among the most promising sources of gravitational waves for ground-based interferometers. While the \emph{inspiral} and \emph{ring-down} stages of the binary black-hole coalescence are well-modelled by analytical approximation methods in general relativity, the recent progress in numerical relativity has enabled us to compute accurate waveforms from the \emph{merger} stage also. This has an important impact on the search for gravitational waves from binary black holes. In particular, while the current gravitational-wave searches look for each stage of the coalescence separately, combining the results from analytical and numerical relativity enables us to \emph{coherently} search for all three stages using a single template family. ‘Complete’ binary black-hole waveforms can now be produced by matching post-Newtonian waveforms with those computed by numerical relativity. These waveforms can be parametrised to produce analytical waveform templates. The ‘complete’ waveforms can also be used to estimate the efficiency of different search methods aiming to detect signals from black-hole coalescences. This paper summarises some recent efforts in this direction.

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Authors: Antony C Searle, Patrick J Sutton, Massimo Tinto, Graham Woan

Date: 2 Dec 2007

Abstract: A Bayesian treatment of the problem of detecting an unmodelled gravitational wave burst with a global network of gravitational wave observatories reveals that several previously proposed statistics have implicit biases that render them sub-optimal for realistic signal populations.

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