**Authors**: Gabor Kupi, Pau Amaro-Seoane, Rainer Spurzem

**Date**: Mon, 6 Feb 2006

**Abstract**: Compact object clusters are likely to exist in the centre of some galaxies because of mass segregation. The high densities and velocities reached in them deserves a better understanding. The formation of binaries and their subsequent merging by gravitational radiation emission is important to the evolution of such clusters. We address the evolution of such a system in a relativistic regime. The recurrent mergers at high velocities create an object with a mass much larger than the average. For this aim we modified the direct Nbody6++ code to include post-Newtonian effects to the force during two-body encounters. We adjusted the equations of motion to include for the first time the effects of both periastron shift and energy loss by emission of gravitational waves and so to study the eventual decay and merger of radiating binaries. The method employed allows us to give here an accurate post-Newtonian description of the formation of a run-away compact object by successive mergers with surrounding particles, as well as the distribution of characteristic eccentricities in the events. This study should be envisaged as a first step towards a detailed, accurate study of possible gravitational waves sources thanks to the combination of the direct Nbody numerical tool with the implementation of post-Newtonian terms on it.

0602125
(/preprints/astro-ph)

2006-02-24, 09:42
**[edit]**

**Authors**: Dong Lai (Cornell), Roman R. Rafikov (IAS)

**Date**: Wed, 26 Jan 2005

**Abstract**: We investigate the effects of gravitational lensing in the binary pulsar system J0737-3039. Current measurement of the orbital inclination allows the millisecond pulsar (A) to pass very close (at R_{min}=4000 km) in projection to the companion pulsar (B), with R_{min} comparable to the Einstein radius (2600 km). For this separation at the conjunction, lensing causes small (about 10%) magnification of the pulsar A signal on a timescale of several seconds, and displaces the pulsar image on the sky plane by about 1200 km. More importantly, lensing introduces a correction (of several microsec) to the conventional Shapiro delay formula used in pulsar timing analysis, and gives rise to a geometric time delay together with the delays associated with the pulsar spin period. These lensing effects can influence the determination of the system parameters by both timing and scintillation studies. Given the current uncertainty in the orbital inclination, more extreme manifestations of lensing (e.g. magnification by a factor of several) are possible. We compare our predictions with the existing observations and discuss the possibility of detecting gravitational lensing signatures in the system. The anomalously high point in A's lightcurve close to superior conjunction might be caused by gravitational lensing.

0411726
(/preprints/astro-ph)

2006-02-23, 08:41
**[edit]**

**Authors**: Louis J. Rubbo, Kelly Holley-Bockelmann, Lee Samuel Finn

**Date**: Mon, 20 Feb 2006

**Abstract**: Stellar mass compact objects in short period orbits about a $10ˆ{4.5}$--$10ˆ{7.5}$ solar mass massive black hole (MBH) are thought to be a significant continuous-wave source of gravitational radiation for the ESA/NASA Laser Interferometer Space Antenna (LISA) gravitational wave detector. However, these extreme mass-ratio inspiral sources began in long-period, nearly parabolic orbits that have multiple close encounters with the MBH. The gravitational radiation emitted during these close encounters may be detectable by LISA as a gravitational wave burst if the characteristic passage timescale is less than $10ˆ5$ seconds. Scaling a static, spherical model to the size and mass of the Milky Way bulge we estimate an event rate of ~ 15 per year for such burst signals, detectable by LISA with signal-to-noise greater than five, originating in our galaxy. When extended to include Virgo cluster galaxies our estimate increases to a gravitational wave burst rate of ~ 18. We conclude that these extreme mass-ratio burst sources may be a steady and significant source of gravitational radiation in the LISA data streams.

0602445
(/preprints/astro-ph)

2006-02-23, 08:40
**[edit]**

**Authors**: Christian Röver, Renate Meyer, Nelson Christensen

**Date**: Fri, 17 Feb 2006

**Abstract**: Presented is a description of a Markov chain Monte Carlo (MCMC) parameter estimation routine for use with interferometric gravitational radiational data in searches for binary neutron star inspiral signals. Five parameters associated with the inspiral can be estimated, and summary statistics are produced. Advanced MCMC methods were implemented, including importance resampling and prior distributions based on detection probability, in order to increase the efficiency of the code. An example is presented from an application using realistic, albeit fictitious, data.

0602067
(/preprints/gr-qc)

2006-02-19, 23:39
**[edit]**

**Authors**: Guenter Sigl (APC and GReCO, IAP, Paris)

**Date**: Wed, 15 Feb 2006

**Abstract**: It has recently been suggested that collapse of neutron stars induced by a phase transition to quark matter can be a considerable source of gravitational waves with kHz frequencies. We demonstrate that if about one percent of all neutron stars undergo this process, the resulting cosmological gravitational wave background would reach about 10ˆ-10 times the critical density. The background would peak at kHz frequencies and could have an observationally significant tail down to Hz frequencies. It would be comparable or higher than other astrophysical backgrounds, for example, from ordinary core collapse supernovae, from r-mode instabilities in rapidly rotating neutron stars, or from magnetars. The scenario is consistent with cosmological backgrounds in neutrinos and photons.

0602345
(/preprints/astro-ph)

2006-02-17, 08:05
**[edit]**

**Authors**: Michele Maggiore

**Date**: Wed, 15 Feb 2006

**Abstract**: I give an overview of the motivations for gravitational-wave research, concentrating on the aspects related to ‘fundamental’ physics.

0602057
(/preprints/gr-qc)

2006-02-16, 11:57
**[edit]**

**Authors**: Marc Lachieze-Rey (APC)

**Date**: Tue, 14 Feb 2006

**Abstract**: We explore, in the general relativistic context, the properties of the recently introduced GPS coordinates, as well as those of the associated frames and coframes. We show that they are covariant, and completely independent of any observer. We show that standard spectroscopic and astrometric observations allow any observer to measure (i) the values of the GPS coordinates at his position, (ii) the components of his [four-]velocity and (iii) the components of the metric in the GPS frame. This provides to this system an unique value both for conceptual discussion (no frame dependence) and for practical use (involved quantities are directly measurable): localisation, motion monitoring, astrometry, cosmography, tests of gravitation theories. We show explicitly, in the general relativistic context, how an observer may estimate its position and motion, and reconstruct the components of the metric. This arises from two main results: the extension of the velocity fields of the probes to the whole (curved) spacetime; and the identification of the components of the observer's velocity in the GPS frame with the (inversed) observed redshifts of the probes. Specific cases (non relativistic velocities; Minkowski and Friedmann-Lema\ˆ{i}tre spacetimes; geodesic motions) are studied in details.

0602052
(/preprints/gr-qc)

2006-02-15, 12:50
**[edit]**

**Authors**: L. Villain

**Date**: Fri, 10 Feb 2006

**Abstract**: Direct observations of gravitational waves will open in the near future new windows on the Universe. Among the expected sources, instabilities of rotating compact astrophysical objects are waited to be detected with some impatience as this will sign the birth of ‘gravitational waves asteroseismology’, a crucial way to improve our knowledge of matter equation of state in conditions that cannot be reproduced in a lab. However, the theoretical work needed to really get informations from to-be-detected signals is still quite large, numerical simulations having become a necessary key ingredient. This article tries to provide a short overview of the main physical topics involved in this field (general relativity, gravitational waves, instabilities of rotating fluids, {\it etc.}), concluding with a brief description of the work that was done in Paris-Meudon Observatory by Silvano Bonazzola and collaborators.

0602234
(/preprints/astro-ph)

2006-02-14, 09:33
**[edit]**

**Authors**: J. D. Romano, G. Woan

**Date**: Wed, 8 Feb 2006

**Abstract**: Data from the Laser Interferometer Space Antenna (LISA) is expected to be dominated by frequency noise from its lasers. However the noise from any one laser appears more than once in the data and there are combinations of the data that are insensitive to this noise. These combinations, called time delay interferometry (TDI) variables, have received careful study, and point the way to how LISA data analysis may be performed. Here we approach the problem from the direction of statistical inference, and show that these variables are a direct consequence of a principal component analysis of the problem. We present a formal analysis for a simple LISA model and show that there are eigenvectors of the noise covariance matrix that do not depend on laser frequency noise. Importantly, these orthogonal basis vectors correspond to linear combinations of TDI variables. As a result we show that the likelihood function for source parameters using LISA data can be based on TDI combinations of the data without loss of information.

0602033
(/preprints/gr-qc)

2006-02-08, 22:46
**[edit]**

**Authors**: John G. Baker, Joan Centrella, Dae-Il Choi, Michael Koppitz, James van Meter

**Date**: Tue, 7 Feb 2006

**Abstract**: We study dynamics and radiation generation in the last few orbits and merger of a binary black hole system, applying recently developed techniques for simulations of moving black holes. Our analysis of the gravitational radiation waveforms and dynamical black hole trajectories produces a consistent picture for a set of simulations with black holes beginning on circular-orbit trajectories at a variety of initial separations. We find profound agreement at the level of one percent among the simulations for the last orbit, merger and ringdown. We are confident that this part of our waveform result accurately represents the predictions from Einstein's General Relativity for the final burst of gravitational radiation resulting from the merger of an astrophysical system of equal-mass non-spinning black holes. The simulations result in a final black hole with spin parameter a/m=0.69. We also find good agreement at a level of roughly 10 percent for the radiation generated in the preceding few orbits.

0602026
(/preprints/gr-qc)

2006-02-08, 14:43
**[edit]**

**Authors**: S. Pireaux (Observatoire de la Cote d'Azur, Department ARTEMIS, Grasse, France), J-P. Barriot (Observatoire Midi-Pyrenees, UMR 5562-DTP, Toulouse, France), P. Rosenblatt (Observatoire Royal de Belgique, Bruxelles, Belgique)

**Date**: Thu, 2 Feb 2006

**Abstract**: Today, the motion of spacecrafts is still described according to the classical Newtonian equations plus the so-called "relativistic corrections", computed with the required precision using the Post-(Post-)Newtonian formalism. The current approach, with the increase of tracking precision (Ka-Band Doppler, interplanetary lasers) and clock stabilities (atomic fountains) is reaching its limits in terms of complexity, and is furthermore error prone. In the appropriate framework of General Relativity, we study a method to numerically integrate the native relativistic equations of motion for a weak gravitational field, also taking into account small non-gravitational forces. The latter are treated as perturbations, in the sense that we assume that both the local structure of space-time is not modified by these forces, and that the unperturbed satellite motion follows the geodesics of the local space-time. The use of a symplectic integrator to compute the unperturbed geodesic motion insures the constancy of the norm of the proper velocity quadrivector. We further show how this general relativistic framework relates to the classical one.

0602008
(/preprints/gr-qc)

2006-02-02, 23:27
**[edit]**

© M. Vallisneri 2012 — last modified on 2010/01/29

*Tantum in modicis, quantum in maximis*