[1008.4997] The general theory of porcupines, perfect and imperfect

Authors: Latham Boyle (Perimeter Institute)

Date: 30 Aug 2010

Abstract: Porcupines are networks of gravitational wave detectors in which the detectors and the distances between them are short relative to the gravitational wavelengths of interest. Perfect porcupines are special configurations whose sensitivity to a gravitational plane wave is independent of the propagation direction or polarization of the wave. I develop the theory of porcupines, including the optimal estimator \hat{h}ˆ{ij} for the gravitational wave field; useful formulae for the spin-averaged and rotationally-averaged SNRˆ{2}; and a simple derivation of the properties of perfect porcupines. I apply these results to the interesting class of ‘simple’ porcupines, and mention some open problems.

abs pdf

Aug 31, 2010

1008.4997 (/preprints)
2010-08-31, 00:11 [edit]


[1008.5032] Relativistic spin-precession in binary pulsars

Authors: Michael Kramer (MPI fuer Radioastronomie, Germany, Jodrell Bank Centre for Astrophsyics, University of Manchester, UK)

Date: 30 Aug 2010

Abstract: After the first prediction to expect geodetic precession in binary pulsars in 1974, made immediately after the discovery of a pulsar with a companion, the effects of relativistic spin precession have now been detected in all binary systems where the magnitude of the precession rate is expected to be sufficiently high. Moreover, the first quantitative test leads to the only available constraints for spin-orbit coupling of a strongly self-gravitating body for general relativity (GR) and alternative theories of gravity. The current results are consistent with the predictions of GR, proving the effacement principle of spinning bodies. Beyond tests of theories of gravity, relativistic spin precession has also become a useful tool to perform beam tomography of the pulsar emission beam, allowing to infer the unknown beam structure, and to probe the physics of the core collapse of massive stars.

abs pdf

Aug 31, 2010

1008.5032 (/preprints)
2010-08-31, 00:11 [edit]


[1008.4622] Conservative self-force correction to the innermost stable circular orbit: comparison with multiple post-Newtonian-based methods

Authors: Marc Favata

Date: 27 Aug 2010

Abstract: [abridged] Barack & Sago have recently computed the shift of the innermost stable circular orbit (ISCO) due to the conservative self-force that arises from the finite-mass of an orbiting test-particle. This is one of the first concrete results of the self-force program, and provides an exact point of comparison with approximate post-Newtonian (PN) computations of the ISCO. Here this exact ISCO shift is compared with nearly all known PN-based methods. These include both "non-resummed" and "resummed" approaches (the latter reproduce the test-particle limit by construction). The best agreement with the exact result is found from effective-one-body (EOB) calculations that are fit to numerical relativity simulations. However, if one considers uncalibrated methods based only on the currently-known 3PN-order conservative dynamics, the best agreement is found from the gauge-invariant ISCO condition of Blanchet and Iyer (2003). This method reproduces the exact test-particle limit without any resummation. A comparison of PN methods with the equal-mass ISCO is also performed. The results of this study suggest that the EOB approach — while exactly incorporating the conservative test-particle dynamics — does not (in the absence of calibration) incorporate conservative self-force effects more accurately than standard PN methods. I also consider how the conservative self-force ISCO shift, combined with numerical relativity computations of the ISCO, can be used to constrain our knowledge of (1) the EOB effective metric, (2) phenomenological inspiral-merger-ringdown templates, and (3) 4PN and 5PN order terms in the PN orbital energy. These constraints could help in constructing better gravitational-wave templates. Lastly, I suggest a new method to calibrate unknown PN-terms in inspiral templates using "low-cost" numerical-relativity calculations.

abs pdf

Aug 29, 2010

1008.4622 (/preprints)
2010-08-29, 23:44 [edit]


[1008.4477] Croatian Black Hole School 2010 lecture notes on IMBHs in GCs

Authors: Mario Pasquato

Date: 26 Aug 2010

Abstract: Black holes are fascinating objects. As a class of solutions to the Einstein equations they have been studied a great deal, yielding a wealth of theoretical results. But do they really exist? What do astronomers really mean when they claim to have observational evidence of their existence? To answer these questions, I will focus on a particular range of black-hole masses, approximately from 100 to 10000 solar masses. Black holes of this size are named Intermediate Mass Black Holes (IMBHs) and their existence is still heavily disputed, so they will be perfect for illustrating the observational challenges faced by a black hole hunter

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Aug 26, 2010

1008.4477 (/preprints)
2010-08-26, 21:10 [edit]


[1008.4360] Intermediate-mass-ratio black hole binaries: intertwining numerical and perturbative techniques

Authors: Carlos O. Lousto, Hiroyuki Nakano, Yosef Zlochower, Manuela Campanelli

Date: 25 Aug 2010

Abstract: We describe in detail full numerical and perturbative techniques to compute the gravitational radiation from intermediate mass ratio (IMR) black-hole-binary (BHB) inspirals and mergers. We perform a series of full numerical simulations of nonspinning black holes with mass ratios q=1/10 and q=1/15 from different initial separations and for different finite difference resolutions. The highest resolution runs reach phase accuracies with errors <0.05 radians when the gravitational wave frequency is 0.2/M. In order to perform those full numerical runs, we adapted the gauge of the moving punctures approach with a variable damping term for the shift. We also derive an extrapolation (to infinite radius) formula for the waveform extracted at finite radius. For the perturbative evolutions we use the full numerical tracks, transformed into the Schwarzschild gauge, in the source terms of the Regge-Wheller-Zerilli Schwarzschild perturbations formalism. We then extend this perturbative formalism to take into account small intrinsic spins of the large black hole, and validate it by computing the quasinormal mode (QNM) frequencies, where we find good agreement for spins |a/M|<0.3. Including the final spins improves the overlap functions when comparing full numerical and perturbative waveforms, reaching 99.5% for the leading (l,m)=(2,2) and (3,3) modes, and 98.3% for the nonleading (2,1) mode in the q=1/10 case, which includes 8 orbits before merger. For the q=1/15 case, we obtain overlaps near 99.7% for all three modes. We discuss the modeling of the full inspiral and merger based on a combined matching of Post-Newtonian, Full Numerical, and Geodesic trajectories.

abs pdf

Aug 26, 2010

1008.4360 (/preprints)
2010-08-26, 21:10 [edit]


[1008.4009] Head-on infall of two compact objects: Third post-Newtonian Energy Flux

Authors: Chandra Kant Mishra, Bala R. Iyer

Date: 24 Aug 2010

Abstract: Head-on infall of two compact objects with arbitrary mass ratio is investigated using the multipolar post-Minkowskian approximation method. At the third post-Newtonian order the energy flux, in addition to the instantaneous contributions, also includes hereditary contributions consisting of the gravitational-wave tails, tails-of-tails and the tail-squared terms. The results are given both for infall from infinity and also for infall from a finite distance. These analytical expressions should be useful for the comparison with the high accuracy numerical relativity results within the limit in which post-Newtonian approximations are valid.

abs pdf

Aug 24, 2010

1008.4009 (/preprints)
2010-08-24, 20:56 [edit]


[1008.3941] Gravitational wave background from sub-luminous GRBs: prospects for second and third generation detectors

Authors: E. Howell, T. Regimbau, A. Corsi, D. Coward, R. Burman

Date: 24 Aug 2010

Abstract: We assess the detection prospects of a gravitational wave background associated with sub-luminous gamma-ray bursts (SL-GRBs). We assume that the central engines of a significant proportion of these bursts are provided by newly born magnetars and consider two plausible GW emission mechanisms. Firstly, the deformation-induced triaxial GW emission from a newly born magnetar. Secondly, the onset of a secular bar-mode instability, associated with the long lived plateau observed in the X-ray afterglows of many gamma-ray bursts (Corsi & Meszaros 2009a). With regards to detectability, we find that the onset of a secular instability is the most optimistic scenario: under the hypothesis that SL-GRBs associated with secularly unstable magnetars occur at a rate of (48; 80)Gpcˆ{-3}yrˆ{-1} or greater, cross-correlation of data from two Einstein Telescopes (ETs) could detect the GW background associated to this signal with a signal-to-noise ratio of 3 or greater after 1 year of observation. Assuming neutron star spindown results purely from triaxial GW emissions, we find that rates of around (130;350)Gpcˆ{-3}yrˆ{-1} will be required by ET to detect the resulting GW background. We show that a background signal from secular instabilities could potentially mask a primordial GW background signal in the frequency range where ET is most sen- sitive. Finally, we show how accounting for cosmic metallicity evolution can increase the predicted signal-to-noise ratio for background signals associated with SL-GRBs.

abs pdf

Aug 24, 2010

1008.3941 (/preprints)
2010-08-24, 20:56 [edit]


[0908.2356] Triangulation of gravitational wave sources with a network of detectors

Authors: Stephen Fairhurst

Date: 17 Aug 2009

Abstract: There is significant benefit to be gained by pursuing multi-messenger astronomy with gravitational wave and electromagnetic observations. In order to undertake electromagnetic follow-ups of gravitational wave signals, it will be necessary to accurately localize them in the sky. Since gravitational wave detectors are not inherently pointing instruments, localization will occur primarily through triangulation with a network of detectors. We investigate the expected timing accuracy for observed signals and the consequences for localization. In addition, we discuss the effect of systematic uncertainties in the waveform and calibration of the instruments on the localization of sources. We provide illustrative results of timing and localization accuracy as well as systematic effects for coalescing binary waveforms.

abs pdf

Aug 23, 2010

0908.2356 (/preprints)
2010-08-23, 16:14 [edit]


[1008.3324] Gravitational Radiations from a Spinning Compact Object circling a Supermassive Kerr Black Hole

Authors: Wen-Biao Han

Date: 19 Aug 2010

Abstract: The gravitational waves and energy radiations from a spinning compact object with stellar mass in a circular orbit in the equatorial plane of a supermassive Kerr black hole are investigated in this paper. The effect how the spin acts on energy and angular moment fluxes is discussed in detail. The calculation results indicate that the spin of small body should be considered in waveform-template production for the upcoming gravitational wave detections. It is clear that when the direction of spin axes is as same as the orbitally angular momentum (‘positive’ spin), spin can decrease the energy fluxes which radiate to infinity. For anti-direction spin (‘negative’), the energy fluxes to infinity be enlarged. And the relation between fluxes (both infinity and horizon) and spin looks like a quadratic function. From frequency shift due to spin, we estimate the wave-phase accumulation during inspiralling process of the particle. We find that the time of particle inspiral into the black hole is longer for ‘positive’ spin and shorter for ‘negative’ comparing with non-spinning particle. Especially, for extreme spin value, the energy radiation near the horizon of the extreme Kerr black hole is much more than the non-spinning one. And consequently, the maximum binging energy of the extreme spinning particle is much bigger than the non-spinning particle.

abs pdf

Aug 20, 2010

1008.3324 (/preprints)
2010-08-20, 08:30 [edit]


[1008.3203] A Simple Pendulum Determination of the Gravitational Constant

Authors: Harold V. Parks, James E. Faller

Date: 19 Aug 2010

Abstract: We determined the Newtonian Constant of Gravitation G by interferometrically measuring the change in spacing between two free-hanging pendulum masses caused by the gravitational field from large tungsten source masses. We find a value for G of (6.672 34 +/- 0.000 14) x 10ˆ-11 mˆ3 kgˆ-1 sˆ-2. This value is in good agreement with the 1986 Committee on Data for Science and Technology (CODATA) value of (6.672 59 +/- 0.000 85) x 10ˆ-11 mˆ3 kgˆ-1 sˆ-2 [Rev. Mod. Phys. 59, 1121 (1987)] but differs from some more recent determinations as well as the latest CODATA recommendation of (6.674 28 +/- 0.000 67) x 10ˆ-11 mˆ3 kgˆ-1 sˆ-2 [Rev. Mod. Phys. 80, 633 (2008)].

abs pdf

Aug 20, 2010

1008.3203 (/preprints)
2010-08-20, 08:30 [edit]


[1008.2961] Length requirements for numerical-relativity waveforms

Authors: Mark Hannam, Sascha Husa, Frank Ohme, P. Ajith

Date: 17 Aug 2010

Abstract: One way to produce complete inspiral-merger-ringdown gravitational waveforms from black-hole-binary systems is to connect post-Newtonian (PN) and numerical-relativity (NR) results to create ‘hybrid’ waveforms. Hybrid waveforms are central to the construction of some phenomenological models for GW search templates, and for tests of GW search pipelines. The dominant error source in hybrid waveforms arises from the PN contribution, and can be reduced by increasing the number of NR GW cycles that are included in the hybrid. Hybrid waveforms are considered sufficiently accurate for GW detection if their mismatch error is below 3% (i.e., a fitting factor about 0.97). We address the question of the length requirements of NR waveforms such that the final hybrid waveforms meet this requirement, considering nonspinning binaries with q = M_2/M_1 \in [1,4] and equal-mass binaries with \chi = S_i/M_iˆ2 \in [-0.5,0.5]. We conclude that for the cases we study simulations must contain between three (in the equal-mass nonspinning case) and ten (the \chi = 0.5 case) orbits before merger, but there is also evidence that these are the regions of parameter space for which the least number of cycles will be needed.

abs pdf

Aug 18, 2010

1008.2961 (/preprints)
2010-08-18, 18:24 [edit]


[1008.2561] Detecting massive gravitons using pulsar timing arrays

Authors: Kejia Lee, Fredrick A. Jenet, Richard H. Price, Norbert Wex, Michael Kramer

Date: 16 Aug 2010

Abstract: abbreviated:
Massive gravitons are features of some alternatives to general relativity. This has motivated experiments and observations that, so far, have been consistent with the zero mass graviton of general relativity, but further tests will be valuable. A basis for new tests may be the high sensitivity gravitational wave experiments that are now being performed, and the higher sensitivity experiments that are being planned. In these experiments it should be feasible to detect low levels of dispersion due to nonzero graviton mass. One of the most promising techniques for such a detection may be the pulsar timing program that is sensitive to nano-Hertz gravitational waves.
Here we present some details of such a detection scheme. The pulsar timing response to a gravitational wave background with the massive graviton is calculated, and the algorithm to detect the massive graviton is presented. We conclude that, with $90\%$ probability, massles gravitons can be distinguished from gravitons heavier than $3\times 10ˆ{-22}$\,eV (Compton wave length $\lambda_{\rm g}=4.1 \times 10ˆ{12}$ km), if biweekly observation of 60 pulsars are performed for 5 years with pulsar RMS timing accuracy of 100\,ns. If 60 pulsars are observed for 10 years with the same accuracy, the detectable graviton mass is reduced to $5\times 10ˆ{-23}$\,eV ($\lambda_{\rm g}=2.5 \times 10ˆ{13}$ km); for 5-year observations of 100 or 300 pulsars, the sensitivity is respectively $2.5\times 10ˆ{-22}$ ($\lambda_{\rm g}=5.0\times 10ˆ{12}$ km) and $10ˆ{-22}$ eV ($\lambda_{\rm g}=1.2\times 10ˆ{13}$ km). Finally, a 10-year observation of 300 pulsars with 100\,ns timing accuracy would probe graviton masses down to $3\times 10ˆ{-23}$\,eV ($\lambda_{\rm g}=4.1\times 10ˆ{13}$ km).

abs pdf

Aug 16, 2010

1008.2561 (/preprints)
2010-08-16, 18:43 [edit]


[1008.2032] Recoiling Massive Black Holes in Gas-Rich Galaxy Mergers

Authors: Javiera Guedes, Piero Madau, Lucio Mayer, Simone Callegari

Date: 12 Aug 2010

Abstract: The asymmetric emission of gravitational waves produced during the coalescence of a massive black hole (MBH) binary imparts a velocity "kick" to the system that can displace the hole from the center of its host. Here we study the trajectories and observability of MBHs recoiling in three (one major, two minor) gas-rich galaxy merger remnants that were previously simulated at high resolution, and in which the pairing of the MBHs had been shown to be successful. We run new simulations of MBHs recoiling in the major merger remnant with Mach numbers in the range 1<M<6 km/s, and use simulation data to construct a semi-analytical model for the orbital evolution of MBHs in gas-rich systems. We show that: 1) in major merger remnants the energy deposited by the moving hole into the rotationally supported, turbulent medium makes a negligible contribution to the thermodynamics of the gas. This contribution is more significant in minor merger remnants, potentially allowing for electromagnetic signatures in this case; 2) in major mergers, the drag from high-density gas allows even MBHs with kick velocities of 1200 km/s to remain within 1 kpc from the host's center; 3) kinematically offset nuclei can be observed for timescales of a few Myr in major merger remnants in the case of recoil velocities in the range 700-1000 km/s; 4) in minor mergers remnants the effect of gas drag is weaker, and MBHs with recoil speeds in the range 300-600 km/s will wander through the host halo and may be detectable as spatially-offset active nuclei.

abs pdf

Aug 13, 2010

1008.2032 (/preprints)
2010-08-13, 10:45 [edit]


[1008.2172] Pulsar Discovery by Global Volunteer Computing

Authors: B. Knispel, B. Allen, J. M. Cordes, J. S. Deneva, D. Anderson, C. Aulbert, N. D. R. Bhat, O. Bock, S. Bogdanov, A. Brazier, F. Camilo, D. J. Champion, S. Chatterjee, F. Crawford, P. B. Demorest, H. Fehrmann, P. C. C. Freire, M. E. Gonzalez, D. Hammer, J. W. T. Hessels, F. A. Jenet, L. Kasian, V. M. Kaspi, M. Kramer, P. Lazarus, J. van Leeuwen, D. R. Lorimer, A. G. Lyne B. Machenschalk, M. A. McLaughlin, C. Messenger, D. J. Nice, M. A. Papa, H. J. Pletsch, R. Prix, S. M. Ransom, X. Siemens, I. H. Stairs, B. W. Stappers, K. Stovall, A. Venkataraman

Date: 12 Aug 2010

Abstract: Einstein@Home aggregates the computer power of hundreds of thousands of volunteers from 192 countries to "mine" large data sets. It has now found a 40.8 Hz isolated pulsar in radio survey data from the Arecibo Observatory taken in February 2007. Additional timing observations indicate that this pulsar is likely a disrupted recycled pulsar. PSR J2007+2722's pulse profile is remarkably wide with emission over almost the entire spin period; the pulsar likely has closely aligned magnetic and spin axes. The massive computing power provided by volunteers should enable many more such discoveries.

abs pdf

Aug 13, 2010

1008.2172 (/preprints)
2010-08-13, 10:45 [edit]


[1006.0154] Numerical integration of variational equations

Authors: Ch. Skokos, E. Gerlach

Date: 1 Jun 2010

Abstract: We present and compare different numerical schemes for the integration of the variational equations of autonomous Hamiltonian systems whose kinetic energy is quadratic in the generalized momenta and whose potential is a function of the generalized positions. We apply these techniques to Hamiltonian systems of various degrees of freedom, and investigate their efficiency in accurately reproducing well-known properties of chaos indicators like the Lyapunov Characteristic Exponents (LCEs) and the Generalized Alignment Indices (GALIs). We find that the best numerical performance is exhibited by the \textit{‘tangent map (TM) method’}, a scheme based on symplectic integration techniques which proves to be optimal in speed and accuracy. According to this method, a symplectic integrator is used to approximate the solution of the Hamilton's equations of motion by the repeated action of a symplectic map $S$, while the corresponding tangent map $TS$, is used for the integration of the variational equations. A simple and systematic technique to construct $TS$ is also presented.

abs pdf

Aug 11, 2010

1006.0154 (/preprints)
2010-08-11, 21:37 [edit]


[1008.1803] Improved Time-Domain Accuracy Standards for Model Gravitational Waveforms

Authors: Lee Lindblom, John G. Baker, Benjamin J. Owen

Date: 10 Aug 2010

Abstract: Model gravitational waveforms must be accurate enough to be useful for detection of signals and measurement of their parameters, so appropriate accuracy standards are needed. Yet these standards should not be unnecessarily restrictive, making them impractical for the numerical and analytical modelers to meet. The work of Lindblom, Owen, and Brown [Phys. Rev. D 78, 124020 (2008)] is extended by deriving new waveform accuracy standards which are significantly less restrictive while still ensuring the quality needed for gravitational-wave data analysis. These new standards are formulated as bounds on certain norms of the time-domain waveform errors, which makes it possible to enforce them in situations where frequency-domain errors may be difficult or impossible to estimate reliably. These standards are less restrictive by about a factor of 20 than the previously published time-domain standards for detection, and up to a factor of 60 for measurement. These new standards should therefore be much easier to use effectively.

abs pdf

Aug 11, 2010

1008.1803 (/preprints)
2010-08-11, 21:37 [edit]


[1008.1890] Comparing the efficiency of numerical techniques for the integration of variational equations

Authors: E. Gerlach, Ch. Skokos

Date: 11 Aug 2010

Abstract: We present a comparison of different numerical techniques for the integration of variational equations. The methods presented can be applied to any autonomous Hamiltonian system whose kinetic energy is quadratic in the generalized momenta, and whose potential is a function of the generalized positions. We apply the various techniques to the well-known Hénon-Heiles system, and use the Smaller Alignment Index (SALI) method of chaos detection to evaluate the percentage of its chaotic orbits. The accuracy and the speed of the integration schemes in evaluating this percentage are used to investigate the numerical efficiency of the various techniques.

abs pdf

Aug 11, 2010

1008.1890 (/preprints)
2010-08-11, 21:37 [edit]


[1008.1782] Pulsar Timing Array Observations of Massive Black Hole Binaries

Authors: Vincent Corbin, Neil J. Cornish

Date: 10 Aug 2010

Abstract: Pulsar timing is a promising technique for detecting low frequency sources of gravitational waves. Historically the focus has been on the detection of diffuse stochastic backgrounds, such as those formed from the superposition of weak signals from a population of binary black holes. More recently, attention has turned to members of the binary population that are nearer and brighter, which stand out from the crowd and can be individually resolved. Here we show that the timing data from an array of pulsars can be used to recover the physical parameters describing an individual black hole binary to good accuracy, even for moderately strong signals. A novel aspect of our analysis is that we include the distance to each pulsar as a search parameter, which allows us to utilize the full gravitational wave signal. This doubles the signal power, improves the sky location determination by an order of magnitude, and allows us to extract the mass and the distance to the black hole binary.

abs pdf

Aug 11, 2010

1008.1782 (/preprints)
2010-08-11, 21:37 [edit]


[1008.1822] Inferring the neutron star equation of state from binary inspiral waveforms

Authors: Charalampos Markakis, Jocelyn S. Read, Masaru Shibata, Koji Uryu, Jolien D. E. Creighton, John L. Friedman

Date: 11 Aug 2010

Abstract: The properties of neutron star matter above nuclear density are not precisely known. Gravitational waves emitted from binary neutron stars during their late stages of inspiral and merger contain imprints of the neutron-star equation of state. Measuring departures from the point-particle limit of the late inspiral waveform allows one to measure properties of the equation of state via gravitational wave observations. This and a companion talk by J. S. Read reports a comparison of numerical waveforms from simulations of inspiraling neutron-star binaries, computed for equations of state with varying stiffness. We calculate the signal strength of the difference between waveforms for various commissioned and proposed interferometric gravitational wave detectors and show that observations at frequencies around 1 kHz will be able to measure a compactness parameter and constrain the possible neutron-star equations of state.

abs pdf

Aug 11, 2010

1008.1822 (/preprints)
2010-08-11, 21:37 [edit]


[1008.1577] Separating Gravitational Wave Signals from Instrument Artifacts

Authors: Tyson B. Littenberg, Neil J. Cornish

Date: 9 Aug 2010

Abstract: Central to the gravitational wave detection problem is the challenge of separating features in the data produced by astrophysical sources from features produced by the detector. Matched filtering provides an optimal solution for Gaussian noise, but in practice, transient noise excursions or ‘glitches’ complicate the analysis. Detector diagnostics and coincidence tests can be used to veto many glitches which may otherwise be misinterpreted as gravitational wave signals. The glitches that remain can lead to long tails in the matched filter search statistics and drive up the detection threshold. Here we describe a Bayesian approach that incorporates a more realistic model for the instrument noise allowing for fluctuating noise levels that vary independently across frequency bands, and deterministic ‘glitch fitting’ using wavelets as ‘glitch templates’, the number of which is determined by a trans-dimensional Markov chain Monte Carlo algorithm. We demonstrate the method's effectiveness on simulated data containing low amplitude gravitational wave signals from inspiraling binary black hole systems, and simulated non-stationary and non-Gaussian noise comprised of a Gaussian component with the standard LIGO/Virgo spectrum, and injected glitches of various amplitude, prevalence, and variety. Glitch fitting allows us to detect significantly weaker signals than standard techniques.

abs pdf

Aug 11, 2010

1008.1577 (/preprints)
2010-08-11, 08:25 [edit]


[1008.0935] Precession effect of the gravitational self-force in a Schwarzschild spacetime and the effective one-body formalism

Authors: Leor Barack, Thibault Damour, Norichika Sago

Date: 5 Aug 2010

Abstract: Using a recently presented numerical code for calculating the Lorenz-gauge gravitational self-force (GSF), we compute the $O(m)$ conservative correction to the precession rate of the small-eccentricity orbits of a particle of mass $m$ moving around a Schwarzschild black hole of mass ${\mathsf M}\gg m$. Specifically, we study the gauge-invariant function $\rho(x)$, where $\rho$ is defined as the $O(m)$ part of the dimensionless ratio $(\hat\Omega_r/\hat\Omega_{\varphi})ˆ2$ between the squares of the radial and azimuthal frequencies of the orbit, and where $x=[Gcˆ{-3}({\mathsf M}+m)\hat\Omega_{\varphi}]ˆ{2/3}$ is a gauge-invariant measure of the dimensionless gravitational potential (mass over radius) associated with the mean circular orbit. Our GSF computation of the function $\rho(x)$ in the interval $0<x\leq 1/6$ determines, for the first time, the {\em strong-field behavior} of a combination of two of the basic functions entering the Effective One Body (EOB) description of the conservative dynamics of binary systems. We show that our results agree well in the weak-field regime (small $x$) with the 3rd post-Newtonian (PN) expansion of the EOB results, and that this agreement is improved when taking into account the analytic values of some of the logarithmic-running terms occurring at higher PN orders. Furthermore, we demonstrate that GSF data give access to higher-order PN terms of $\rho(x)$ and can be used to set useful new constraints on the values of yet-undetermined EOB parameters. Most significantly, we observe that an {\em excellent global representation} of $\rho(x)$ can be obtained using a simple ‘two-point’ Pad\'{e} approximant which combines 3PN knowledge at $x=0$ with GSF information at a single strong-field point (say, $x=1/6$).

abs pdf

Aug 10, 2010

1008.0935 (/preprints)
2010-08-10, 16:18 [edit]


[1008.0397] Confronting General Relativity with Further Cosmological Data

Authors: Scott F. Daniel, Eric V. Linder

Date: 2 Aug 2010

Abstract: Deviations from general relativity in order to explain cosmic acceleration generically have both time and scale dependent signatures in cosmological data. We extend our previous work by investigating model independent gravitational deviations in bins of redshift and length scale, by incorporating further cosmological probes such as temperature-galaxy and galaxy-galaxy cross-correlations, and by examining correlations between deviations. Markov Chain Monte Carlo likelihood analysis of the model independent parameters fitting current data indicates that at low redshift general relativity deviates from the best fit at the 99\% confidence level. We trace this to two different properties of the CFHTLS weak lensing data set and demonstrate that COSMOS weak lensing data does not show such deviation. Upcoming galaxy survey data will greatly improve the ability to test time and scale dependent extensions to gravity and we calculate the constraints that the BigBOSS galaxy redshift survey could enable.

abs pdf

Aug 10, 2010

1008.0397 (/preprints)
2010-08-10, 16:15 [edit]


[1008.1061] Testing GR with Galactic-centre Stars

Authors: Raymond Angelil, Prasenjit Saha

Date: 5 Aug 2010

Abstract: The Galactic Centre S-stars orbiting the central supermassive black hole reach velocities of a few percent of the speed of light. The GR-induced perturbations to the redshift enter the dynamics via two distinct channels. The post-Newtonian regime perturbs the orbit from the Keplerian (Zucker et al., 2006, Kannan & Saha 2009), and the photons from the Minkowski (Angelil & Saha 2010). The inclusion of gravitational time dilation at order vˆ2 marks the first departure of the redshift from the line-of-sight velocities. The leading-order Schwarzschild terms curve space, and enter at order vˆ3. The classical Keplerian phenomenology dominates the total redshift. Spectral measurements of sufficient resolution will allow for the detection of these post-Newtonian effects. We estimate the spectral resolution required to detect each of these effects by fitting the redshift curve via the five keplerian elements plus black hole mass to mock data. We play with an exaggerated S2 orbit - one with a semi-major axis a fraction of that of the real S2. This amplifies the relativistic effects, and allows clear visual distinctions between the relativistic terms. We argue that spectral data of S2 with a dispersion of about 10km/s would allow for a clear detection of gravitational redshift, and about 1 km/s would suffice for leading-order space curvature detection.

abs pdf

Aug 09, 2010

1008.1061 (/preprints)
2010-08-09, 18:21 [edit]


[1008.1097] The Evolution of PSR J0737-3039B and a Model for Relativistic Spin Precession

Authors: Benetge Perera, Maura McLaughlin, Michael Kramer, Ingrid Stairs, Robert Ferdman, Paulo Freire, Andrea Possenti, Rene Breton, Richard N. Manchester, Marta Burgay, Andrew Lyne, Fernando Camilo

Date: 5 Aug 2010

Abstract: We present the evolution of the radio emission from the 2.8-s pulsar of the double pulsar system PSR J0737-3039A/B. We provide an update on the Burgay et al. (2005) analysis by describing the changes in the pulse profile and flux density over five years of observations, culminating in the B pulsar's radio disappearance in 2008 March. Over this time, the flux density decreases by 0.177 mJy/yr at the brightest orbital phases and the pulse profile evolves from a single to a double peak, with a separation rate of 2.6 deg/yr. The pulse profile changes are most likely caused by relativistic spin precession, but can not be easily explained with a circular hollow-cone beam as in the model of Clifton & Weisberg (2008). Relativistic spin precession, coupled with an elliptical beam, can model the pulse profile evolution well. This particular beam shape predicts geometrical parameters for the two bright orbital phases which are consistent and similar to those derived by Breton et al. (2008). However, the observed decrease in flux over time and B's eventual disappearance cannot be easily explained by the model and may be due to the changing influence of A on B.

abs pdf

Aug 09, 2010

1008.1097 (/preprints)
2010-08-09, 18:20 [edit]


[1008.1278] Multipole moments of bumpy black holes

Authors: Sarah J. Vigeland

Date: 6 Aug 2010

Abstract: General relativity predicts the existence of black holes, compact objects whose spacetimes depend on only their mass and spin (the famous "no hair" theorem). As various observations probe deeper into the strong fields of black hole candidates, it is becoming possible to test this prediction. Previous work suggested that such tests can be performed by measuring whether the multipolar structure of black hole candidates has the form that general relativity demands, and introduced a family of "bumpy black hole" spacetimes to be used for making these measurements. These spacetimes are black holes with the "wrong" multipoles, where the deviation from general relativity depends on the spacetime's "bumpiness." In this paper, we show how to compute the Geroch-Hansen moments of a bumpy black hole, demonstrating that there is a clean mapping between the deviations used in the bumpy black hole formalism and the Geroch-Hansen moments. We also extend our previous results to define bumpy black holes whose {\it current} moments, analogous to magnetic moments of electrodynamics, deviate from the canonical Kerr value.

abs pdf

Aug 09, 2010

1008.1278 (/preprints)
2010-08-09, 17:58 [edit]


[1008.1460] Gravitational waves from nonspinning black hole-neutron star binaries: dependence on equations of state

Authors: Koutarou Kyutoku, Masaru Shibata, Keisuke Taniguchi

Date: 9 Aug 2010

Abstract: We report results of a numerical-relativity simulation for the merger of a black hole-neutron star binary with a variety of equations of state (EOSs) modeled by piecewise polytropes. We focus in particular on the dependence of the gravitational waveform at the merger stage on the EOSs. The initial conditions are computed in the moving-puncture framework, assuming that the black hole is nonspinning and the neutron star has an irrotational velocity field. For a small mass ratio of the binaries (e.g., MBH/MNS = 2 where MBH and MNS are the masses of the black hole and neutron star, respectively), the neutron star is tidally disrupted before it is swallowed by the black hole irrespective of the EOS. Especially for less-compact neutron stars, the tidal disruption occurs at a more distant orbit. The tidal disruption is reflected in a cutoff frequency of the gravitational-wave spectrum, above which the spectrum amplitude exponentially decreases. A clear relation is found between the cutoff frequency of the gravitational-wave spectrum and the compactness of the neutron star. This relation also depends weakly on the stiffness of the EOS in the core region of the neutron star, suggesting that not only the compactness but also the EOS at high density is reflected in gravitational waveforms. The mass of the disk formed after the merger shows a similar correlation with the EOS, whereas the spin of the remnant black hole depends primarily on the mass ratio of the binary, and only weakly on the EOS. Properties of the remnant disks are also analyzed.

abs pdf

Aug 09, 2010

1008.1460 (/preprints)
2010-08-09, 17:58 [edit]


[1008.0296] Resource Letter PTG-1: Precision Tests of Gravity

Authors: Clifford M. Will

Date: 2 Aug 2010

Abstract: This resource letter provides an introduction to some of the main current topics in experimental tests of general relativity as well as to some of the historical literature. It is intended to serve as a guide to the field for upper-division undergraduate and graduate students, both theoretical and experimental, and for workers in other fields of physics who wish learn about experimental gravity. The topics covered include alternative theories of gravity, tests of the principle of equivalence, solar-system and binary-pulsar tests, searches for new physics in gravitational arenas, and tests of gravity in new regimes, involving astrophysics and gravitational radiation.

abs pdf

Aug 03, 2010

1008.0296 (/preprints)
2010-08-03, 00:16 [edit]


[1008.0320] Pulsar timing array observations of gravitational wave source timing parallax

Authors: Xihao Deng, Lee Samuel Finn

Date: 2 Aug 2010

Abstract: Pulsar timing arrays act to detect gravitational waves by observing the small, correlated effect the waves have on pulse arrival times at Earth. This effect has conventionally been evaluated assuming the gravitational wave phasefronts are planar across the array, an assumption that is valid only for sources at distances $R\gg2\pi{}Lˆ2/\lambda$, where $L$ is physical extent of the array and $\lambda$ the radiation wavelength. In the case of pulsar timing arrays (PTAs) the array size is of order the pulsar-Earth distance (kpc) and $\lambda$ is of order pc. Correspondingly, for point gravitational wave sources closer than $\sim100$~Mpc the PTA response is sensitive to the source parallax across the pulsar-Earth baseline. Here we evaluate the PTA response to gravitational wave point sources including the important wavefront curvature effects. Taking the wavefront curvature into account the relative amplitude and phase of the timing residuals associated with a collection of pulsars allows us to measure the distance to, and sky position of, the source.

abs pdf

Aug 03, 2010

1008.0320 (/preprints)
2010-08-03, 00:15 [edit]


[gr-qc/0404023] Kranc: a Mathematica application to generate numerical codes for tensorial evolution equations

Authors: Sascha Husa, Ian Hinder, Christiane Lechner

Date: 6 Apr 2004

Abstract: We present a suite of Mathematica-based computer-algebra packages, termed "Kranc", which comprise a toolbox to convert (tensorial) systems of partial differential evolution equations to parallelized C or Fortran code. Kranc can be used as a "rapid prototyping" system for physicists or mathematicians handling very complicated systems of partial differential equations, but through integration into the Cactus computational toolkit we can also produce efficient parallelized production codes. Our work is motivated by the field of numerical relativity, where Kranc is used as a research tool by the authors. In this paper we describe the design and implementation of both the Mathematica packages and the resulting code, we discuss some example applications, and provide results on the performance of an example numerical code for the Einstein equations.

abs pdf

Aug 03, 2010

0404023 (/preprints/gr-qc)
2010-08-03, 00:11 [edit]


[1007.0007] Towards relativistic orbit fitting of Galactic center stars and pulsars

Authors: Raymond Angelil, Prasenjit Saha, David Merritt

Date: 30 Jun 2010

Abstract: The S stars orbiting the Galactic center black hole reach speeds of up to a few percent the speed of light during pericenter passage. This makes, for example, S2 at pericenter much more relativistic than known binary pulsars, and opens up new possibilities for testing general relativity. This paper develops a technique for fitting nearly-Keplerian orbits with perturbations from Schwarzschild curvature, frame dragging, and spin-induced torque, to redshift measurements distributed along the orbit but concentrated around pericenter. Both orbital and light-path effects are taken into account. It turns out that absolute calibration of rest-frame frequency is not required. Hence, if pulsars on orbits similar to the S stars are discovered, the technique described here can be applied without change, allowing the much greater accuracies of pulsar timing to be taken advantage of. For example, pulse timing of 3 microsec over one hour amounts to an effective redshift precision of 30 cm/s, enough to measure frame dragging and the quadrupole moment from an S2-like orbit, provided problems like the Newtonian "foreground" due to other masses can be overcome. On the other hand, if stars with orbital periods of order a month are discovered, the same could be accomplished with stellar spectroscopy from the E-ELT at the level of 1 km/s.

abs pdf

Aug 03, 2010

1007.0007 (/preprints)
2010-08-03, 00:10 [edit]


[1007.5387] Event Horizon Deformations in Extreme Mass-Ratio Black Hole Mergers

Authors: Ryan Hamerly, Yanbei Chen

Date: 30 Jul 2010

Abstract: We study the geometry of the event horizon of a spacetime in which a small compact object plunges into a large Schwarzschild black hole. We first use the Regge-Wheeler and Zerilli formalisms to calculate the metric perturbations induced by this small compact object, then find the new event horizon by propagating null geodesics near the unperturbed horizon. A caustic is shown to exist before the merger. Focusing on the geometry near the caustic, we show that it is determined predominantly by large-l perturbations, which in turn have simple asymptotic forms near the point at which the particle plunges into the horizon. It is therefore possible to obtain an analytic characterization of the geometry that is independent of the details of the plunge. We further show that among the leading-order horizon area increase, half arises from generators that enter the horizon through the caustic, and the rest arises from area increase near the caustic, induced by the gravitational field of the compact object.

abs pdf

Aug 02, 2010

1007.5387 (/preprints)
2010-08-02, 00:51 [edit]


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