Overview of Astrophysical Gravitational Wave Sources
I will review current expectations about various gravitational wave sources over the whole potential observing band from 0.1 milliHertz to 10 kiloHertz. Of particular interest is the reliability of our predictions of waveforms and therefore the quality of our search templates. Also important are estimates of the size of the parameter spaces that must be searched, and the reliability with which sources can be extracted from confusion-limited backgrounds. Most important of all is the information about astronomy and fundamental physics that we are likely to get from observations of different types of sources.
Gravitational Wave Detectors and Detection in the Year 2012
In the year 2012 we can anticipate LISA entering science operations, the third generation of ground-based interferometric detectors, and broadband, millikelvin or better acoustic detectors with spherical geometries. In this talk we'll discuss the kind of technologies will be involved in these detectors and what we can expect in terms of their sensitivity to different gravitational wave sources.
The Road to Prediction: Toward Gravitational Wave Signatures from Core Collapse Supernovae
Core collapse supernovae are among the sources expected to produce gravitational radiation at amplitudes and frequencies within the range detectable by current laser interferometric gravitational wave observatories. Gravitational waves from core collapse supernovae will arise from a number of phenomena, including nonspherical stellar core collapse, instabilities in the proto-neutron star, convection below the supernova shock wave, and a potential instability of the shock wave itself. To compute the gravitational waveforms from these phenomena and others in this context will ultimately require three-dimensional radiation magnetohydrodynamics with realistic nuclear and weak interaction physics. Progress toward this goal is being made. I will discuss these efforts, as well as present results from a few completed simulations isolating one or a subset of the physics components in a supernova model to illustrate the sensitivities of the stellar core dynamics to changes in model components, the interdependencies between these components, and the need to pursue complete, fully coupled models before gravitational wave predictions can be made with confidence.
Gravitational Waves From Stellar Collapse (First Stars and Recent 3D results)
The collapse of massive stars remain one of the most promising sources for gravitational waves. Studies of star formation in the first generation of stars imply that most of these stars are more massive than stars formed today. This trend, combined with the fact that the low metal abundances in the first generation of stars lead to much less mass loss through stellar winds, causes these stars to be very massive at collapse. Such collapses will produce black holes and strong gravitational wave signals - large enough that even at the high redshifts of their formation, these objects could be observable. I will review these systems and their gravitational wave signals. I will compare these results to the latest 3-dimensional collapse simulations of 15 solar mass stars.
Collapse of Rotating Supermassive Stars to Supermassive Black Holes: Relativistic Simulations
There is compelling evidence that supermassive black holes (SMBHs) exist. Yet the origin of these objects, or their seeds, is still unknown. We are performing general relativistic simulations of gravitational collapse to black holes in various scenarios to help reveal how, when and where SMBH seeds form in the universe. SMBHs with ~ 10^9 solar masses must have formed by z > 6, or within 10^9 yrs after the Big Bang, to power quasars. It may be difficult for gas accretion to build up such a SMBH by this time unless the initial seed black hole already has a substantial mass. One plausible progenitor of a massive seed black hole is a supermassive star (SMS). We have followed the collapse of a SMS to a SMBH by means of hydrodynamic simulations in post-Newtonian gravitation (in 3D) and in full general relativity (in axisymmetry). The initial SMS of arbitrary mass M in these simulations rotates uniformly at the mass--shedding limit and is marginally unstable to radial collapse. The final black hole mass and spin are determined to be M_h/M ~ 0.9 and J_h/M_h^2 ~ 0.75. The remaining mass goes into a disk of mass M_disk/M ~ 0.1. This disk arises even though the total spin of the progenitor star, J/M^2 = 0.97, is safely below the Kerr limit. The collapse itself will generate a mild burst of gravitational radiation; nonaxisymmetric bars or one-armed spirals, which may arise during the collapse or in the ambient disk, are potential sources of quasi-periodic waves. We will show a computer-generated video to highlight the final phases of the collapse.
Constraining Binary Evolution With Gravitational Wave Measurements ofChirp Masses
We estimate the observed distribution of chirp masses of compact object binaries for the gravitational wave detectors. The stellar binary evolution is modeled using the StarTrack population synthesis code. The distribution of the predicted "observed" chirp masses vary with variation of different parameters describing stellar binary evolution. We estimate the sensitivity of the observed distribution to variation of these parameters and show which of the parameters can be constrained after observing 20, 100, and 500 compact object mergers. As a general feature of all our models we find that the population of observed binaries is dominated by the double black hole mergers.
Possible Class of Nearby Gamma-Ray Burst / Gravitational Wave Sources
Jay P. Norris
A possible subclass of gamma-ray bursts -- those with few, wide pulses, spectral lags of order one to several seconds, and soft spectra -- has been identified. Their Log[N]-Log[Fp] distribution approximates a -3/2 power-law, suggesting homogeneity and relatively nearby sources. These relatively dim bursts account for ~ 50% of BATSE bursts near that instrument's trigger threshold, suggesting that this subluminous class constitutes a more common variety than the more familiar burst sources which lie at truly cosmological distances. Theoretical scenarios predicted such a class, motivated by their exemplar GRB 980425 (SN 1998bw) at a distance of ~ 38 Mpc; the observations were explained by invoking off-axis viewing of the GRB jet and/or bulk Lorentz factors of order a few. These long-lag bursts show a tendency to concentrate near the Supergalactic Plane with a quadrupole moment of ~ -0.10 ħ 0.04, similar to the moment for SNe type Ib/c found within the same volume. The frequency of the observed subluminous bursts is ~ one quarter that of the SNe Ib/c. Evidence for a sequential relationship between SNe Ib/c and GRBs will be discussed for two cases; simultaneity of the SN and GRB events may be important for detection of the expected gravitational wave signal.
Gravitational Waves from Gamma-Ray Bursts
I review the main stellar progenitor models of gamma-ray bursts, and discuss the various phases before and during the burst which can give rise to gravitational wave emission. The characteristic strains are estimated as a function of the various uncertain astrophysical parameters, and compared to the LIGO II sensitivity. Estimates are given for the probability of detection within a year in LIGO II as a function of these parameters. The character of the gravitational wave polarization is discussed in GRB, and its relation to the properties of the gamma-ray and/or X-ray emission.
The Astrophysics of Captures
The capture of stellar mass compact objects by supermassive black holes is one of the primary classes of sources for LISA observations. I review the physics of capture and the contributing stellar population. Briefly discuss plausible rates and contributing populations, and then review some of the uncertainties and open issues in estimating the true astrophysical rate and distribution of merging objects.
Gravitational Waves From Captures
Gravitational waves from capture binaries --- stellar mass compact objects captured by galactic mass black holes --- are an interesting source we hope to observe using LISA. These sources are extremely clean and thereby amenable to precise modeling. Capture binaries will be "laboratories" for studying the extreme strong field gravity of massive black holes, probing the characteristics of black holes with high precision. In this talk, I will describe the state of our current efforts to understand how to model these systems and how to extract their signals from the LISA datastream.
Middleweight Black Holes
The possible existence of "middle weight" black holes with a mass between 20-5000 Msun is of great interest. In the last 20 years a class of "ultra-luminous" point-like x-ray sources in nearby galaxies has been found. Their are >4x1039 ergs/sec and require objects of masses greater than 20Msun, if they radiating at the Eddington limit. Since the most massive black hole that can produced by the collapse of a single star is ~20Msun, the origin of these objects requires special circumstances. It is also possible that the true luminosity of these objects is considerably less than the observed luminosity if the radiation is intrinsically beamed. I will review the recent x-ray, optical and radio data for these objects, stressing the x-ray timing and spectral data to determine the true nature of these objects. I will also review the number of these objects and their relation to their host galaxies.
Optical Constraints on the Existence of Intermediate-Mass Black Holes in the Universe
Astronomers have long believed that black holes naturally form in the Universe in two mass ranges. Stellar-mass black holes form when a heavy star collapses under its own weight in a supernova explosion, and have been identified in X-ray binaries. Super-massive black holes probably form as a byproduct of galaxy formation, and are found in the centers of galaxies where they sometimes generate prodigious activity. However, black holes could have formed in the Universe in different mass ranges. Intermediate-mass black holes (IMBHs) of 10^2-10^5 solar masses are an especially interesting possibility. IMBHs might plausible have formed through a variety of processes. I will review the observational constraints on the existence of IMBHs, focusing in particular on the optical wavelength regime. IMBHs have been hypothesized to be the missing baryonic dark matter in the Universe ½ Ċ 10-1.7. This is constrained by a variety of dynamical and gravitational lensing issues, but cannot be ruled out. The cosmic mass density of IMBHs could certainly exceed that of supermassive BHs ½ Ċ 10-5.7. Unambiguous detections of individual IMBHs currently do not exist. However, I will review possible observational hints for their existence from studies of microlensing events, centers of nearby galaxies and globular clusters. IMBHs have potential importance for several fields of astrophysics and are likely to grow as a focus of research attention.
Black holes with hundreds to thousands of solar masses are more massive than can be formed from a single star in the current universe, yet the best candidates for these objects are not located in gas-rich environments where gradual accretion could build up the mass. Three formation scenarios have been suggested in the literature: that intermediate-mass black holes are the remnants of the first, metal-poor, stars; that they result from direct stellar collisions in young clusters; or that they are produced by gradual interactions and mergers of compact objects in dense stellar clusters. I will discuss each of these in turn and speculate on future observations that may help sharpen our understanding of the formation of intermediate-mass black holes.
Massive Black Hole Growth and Formation: A Theoretical Overview
Prospects for Detecting Gravitational Radiation from Supermassive Black Holes
An analysis of 25 galaxies suggests that approximately all galaxies host massive (10^6 to 10^9 Msolar) black holes. If this is the case, these objects must play a major role in the formation and evolution of galaxies. This population of massive black holes in the present universe is consistent with the masses and numbers of bh predicted by the luminosity function of quasars in the z >~ 2 universe. These objects merge. Although the merger rates are uncertain, the gravitational radiation expected from lower mass objects (black holes in the mass range 10^5 - 10^7 Msolar) is likely to be easily observable with LISA, a proposed NASA/ESA mission.
The spectrum of the hard X--ray background records the history of accretion processes integrated over the cosmic time. Several observational and theoretical evidences indicate that a large fraction of the energy density is obscured by large columns of gas and dust. X--ray surveys are the most efficient way to trace accretion onto supermassive black holes, since obscured, accreting sources are more difficult to select at all other wavelengths. I will review the current status of hard X--ray surveys after the recent observations carried out with Chandra and XMM satellites along with the results of extensive follow--up multiwavelength observations. In particular I will present recent results concerning the physical and evolutive properties of the supermassive black holes hosted by X--ray selected Active Galactic Nuclei at high redshifts.
Observational Evidence for Massive Black Hole Binaries
Mergers of massive binary black holes produce strong gravitational wave signals which will be detectable for the first time with the future space-borne mission LISA. Hierarchical merger models of galaxy formation predict that such binary black holes should be common in galaxies. Observations of such systems therefore allow to test and refine the theoretical models, and allow to estimate the rate with which the future mission LISA can detect these events. Here, we present a review of the observational evidence for the presence of supermassive binary black holes in galaxies.
Hierarchical Build-up of Massive Black Holes
We investigate a hierarchical structure formation scenario describing the evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first POPIII stars. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. Binaries decay efficiently both as a result of stellar slingshots and, at very high redshifts, due to triple BH interactions. A simple model in which the damage done to a stellar cusps by decaying BH pairs is cumulative is able to reproduce the observed scaling relation between galaxy luminosity and core size. An accretion model connecting quasar activity with major mergers and the observed BH mass-velocity dispersion correlation reproduces remarkably well the observed luminosity function of optically-selected quasars in the redshift range 1<z<5.
Gravitational Radiation From Super-Massive Black-Hole Binaries
Recent observational evidence ties the mass of supermassive black-holes to the circular velocities of their host dark-matter halos, and suggests that this relation is independent of redshift. I will show that a simple model including the combination of the black-hole - circular velocity relation with the dark matter halo mass function and merger rate reproduces the details of the optical and X-ray quasar luminosity functions at high redshift within a standard Lambda-CDM cosmology. A similar approach can be used to estimate the expected rate of gravitational wave emission from merging super-massive black-holes. These calculations suggest an event rate for the LISA satellite that may be as high as one hundred per year unless most super-massive black-hole binaries have hardening timescales longer than a Hubble time. I will discuss the effect of reionization on the redshift distribution of gravitational wave sources and show that most sources for LISA may come from z>6. Finally I will show that the nHz frequency back-ground is dominated by black-hole binaries at z<2, and that pulsar timing experiments are already nearing the level where limits may be placed on the fraction of black-hole binaries that achieve coalescence.
Mergers of Massive Black Hole Binaries
We discuss the formation and the evolution of massive black hole binaries in the nuclei of galaxies that have experienced mergers. We review aspects of stellar dynamics in the binary black hole nuclei including the gravitational slingshot, the Brownian motion, the loss-cone refilling. We attempt to resolve the "final parsec problem", i.e., how black hole binaries manage to shrink to separations at which emission of gravity waves becomes efficient. We also discuss the suitability of N-body experiments for probing these mechanisms.
Binary/Multiple Black Holes in Hierarchically Merging Galaxies
In hierarchically merging galaxies the formation of binary and binary and multiple black holes is expected to be frequent. I discuss issues concerning their formation and further dynamical evolution with emphasis on estimates of the number of massive black hole mergers that the the planned space-based gravitational wave interferometer LISA may see.
Calculating Gravitational Wave Signatures from Black Hole Binary Mergers
Joan M. Centrella
The coalescence of black hole binaries by the emission of gravitational radiation begins with an adiabatic inspiral; this is followed by a dynamical merger phase and then a ÒringdownÓ to a final Kerr black hole. While the gravitational waves emitted in the inspiral and ringdown phases can be calculated analytically, the merger phase requires the solution of the full Einstein equations using numerical simulations. This talk will review the current status of and future prospects for numerical relativity computations of binary black hole mergers. The wealth of astrophysical information about the sources that can be gained from observations of merger waveforms will be highlighted.
Galactic Binaries as Sources of Gravitational Waves
I will give an overview of the types of binaries in the Galaxy that are (low-frequency) gravitational wave sources. Theoretical predictions from population synthesis models are presented and compared to observations which are used to test and improve the models. The expected sources detectable by LISA will be presented with emphasis on the overlap between possible optical, X-ray and GWR detections.
Do Old Neutron Stars Shiver to Keep Warm?
Rossby waves (r-modes) in the cores of rapidly rotating neutron stars may grow because of the emission of gravitational radiation, preventing accretion from spinning up the star further. Whether and how r-modes affect the spin evolution of low-mass X-ray binaries depends strongly on the nature of the r-mode instability and how the r-modes saturate. I will describe the r-modes and explain why they may become unstable in LMXBs. Then using recent results which describe how the r-modes saturate, I will outline the spin-evolution of LMXBs, calculate how many LXMBs could be detected by LIGO, and speculate how these gravitational-wave sources might appear in photons.
Gravitational wave detectors such as LIGO and LISA are very similar to quadrupole radio wave antennae. The key difference is that radio wave detectors can be turned into radio telescopes using a focusing dish, while gravitational wave detectors can not. Turning LIGO or LISA into a gravitational wave telescope is a data analysis problem, not an engineering problem. I will describe the current state of the art in LISA data analysis, and what it means for space based gravitational wave astronomy.
Searching for Gravitational Waves with Ground-based Interferometers
The advent of kilometer-scale interferometers, such as those in LIGO, makes detection of gravitational waves from astrophysical sources a realistic goal in the near-term. Data collected simultaneously from the LIGO and GEO interferometers has started to flow during dedicated science runs and is being analyzed by working groups in the LIGO Scientific Collaboration. I will present an overview of data analysis techniques that are being used with an emphasis on science that can be extracted as the instruments approach design sensitivity.
Analysis of Data From the LIGO's First Science Run
I will present results from the first science run ('S1') of the LIGO interferometers. The S1 data have attained unprecedented goals for gravitational-wave interferometers in that they had the highest strain sensitivity over the largest bandwidth with the greatest number of hours of coincident operation. I will report on astrophysical searches carried out primarily by the working groups looking for gravitational radiation from four types of sources: modeled chirps from binary neutron star inspirals, unmodeled bursts in the absence of triggers from other observations, continuous waves from known pulsars, and a stochastic background of astrophysical or cosmological origin. A brief description of the present status of the performance of the LIGO detectors will also be included as a preview of what can be expected in the near future.
Posters Session 4
Conformally Flat Hydrodynamics Calculations With a Spectral-Methods Field Solver
We present the first results from our new relativistic hydrodynamics code. Using the equations of the conformally flat (CF) formalism, we solve for the fields using the LORENE code, developed by the Meudon group. This code, previously used for high-accuracy studies of equilibria, has been combined with a smoothed particle hydrodynamics (SPH) evolution treatment, to allow for a combination of high accuracy as well as computational speed. We have tested this code extensively, finding that it performs well for calculations of a collapsing relativistic dust cloud as well as single-star configurations, for which the exact matter and field solution can be computed. We have also tested the code by computing the orbits of binary neutron stars, with the effects of radiation reaction not included, finding good long-term stability. We have used the code, with an approximate treatment of radiation reaction, to study the case of merging neutron star binaries, finding that we can successfully compute the fields even as the stars become tidally disrupted during the mergers. Our results about gravitational wave emission, as well as preliminary studies on the applicability of the code to collapsing stellar models will be shown
How Black Holes Get Their Kicks! Gravitational Radiation Recoil in the Extreme-Mass-Ratio Limit
Black hole-black hole binaries are likely products of galaxy mergers and the dynamics of globular clusters. If tight enough, the orbits of these binaries will decay by emission of gravitational radiation. The emitted radiation carries a linear momentum flux which can impart a "kick" to the system's center of mass. Previous work has speculated that this kick may be large enough for the system to escape its host galaxy or cluster. We revisit this problem using the tools of black hole perturbation theory applied to extreme mass ratio binaries. This allows us to, in principle, obtain a very accurate calculation of the recoil in the limit in which one black hole is much more massive than the other, and to explore the recoil's dependence on parameters such as black hole spin and relative inclination of the spin and orbit. We argue that these results can be extrapolated into the comparable mass ratio regime with accuracy of about 20 percent. Although still in progress, our results so far indicate that astrophysically interesting recoils (tens to hundreds of km/sec) are certainly possible.
Captures of Stars by a Massive Black Hole: Investigations in Numerical Stellar Dynamics
Among the astrophysical systems targeted by LISA, stars on relativistic orbits around massive black holes (MBHs) are particularly promising sources. From the gravitational signal emitted by the stars as they spiral in, precise information about the mass and spin of MBHs can be obtained. However, the prediction for the number and characteristics of such sources suffer from many uncertainties. Stellar dynamical Monte Carlo simulations of the evolution of galactic nucleus models allow more realistic estimates of these quantities and an exploration of their dependencies on the model's parameters. Furthermore, they strongly suggest that the closest such extreme mass-ratio binary to be detected by LISA could be a MS star orbiting the MBH at the center of our Milky Way.
The Statistical Analysis of Binary Pulsar Coalescence Rates
Chunglee Kim, Vassiliki Kalogera
Duncan R. Lorimer
Inspiraling binary pulsar systems are good candidates for being detected by gravitational-wave detectors such as LIGO and LISA. In this poster, we will present a newly developed statistical analysis method to estimate the coalescence rate, R, of double neutron star (NS-NS) systems and the detection rate of these systems by LIGO. The method involves the simulation of selection effects inherent in all relevant radio pulsar surveys and a Bayesian analysis to calculate the probability distribution of R. One of the advantages of this method is that it can be applied to any type of pulsar population. Results for the case of neutron star-white dwarf (NS-WD) binaries and the estimated detection rate of such systems by LISA will also be presented and compared with those of the NS-NS systems. Finally, we will discuss the statistical significance of each estimated value as well as the correlation between the model parameters and R for both cases.
Polarized Gravitational Waves from Gamma-Ray Bursts
Shiho Kobayashi, and Peter Meszaros
Significant gravitational wave emission is expected from gamma-ray bursts arising from compact stellar mergers, and possibly also from bursts associated with fast-rotating massive stellar core collapses. These models have in common a high angular rotation rate, and observations provide evidence for jet collimation of the photon emission, with properties depending on the polar angle. Here we consider the gravitational wave emission and its polarization as a function of angle which is expected from such sources. We discuss possible correlations between the burst photon luminosity, or the delay between gravitational wave bursts and X-ray flashes, and the polarization degree of the gravitational waves.
New Estimation of Black Holes Merging Rate in Binary Systems
V.M. Lipunov and E.I. Panchenko
We revise observational status of Black Holes in our Galaxy and give the new low limit to binary black holes merging rate in different scenario of the evolution of binary stars. Our new estimation is about one order greater than previous (First LIGO events: binary black holes mergings (V.M. Lipunov et al., New Astronomy, 1997, v.2, pp. 43-52).
Compact Object Capture Rates
We compute rates of capture of compact objects by massive black holes, based on new insights from loss cone theory.
Perturbative Approach to an Orbital Evolution Around a Supermassive Black Hole
A charge-free, point particle of infinitesimal mass orbiting a Kerr black hole is known to move along a geodesic. When the particle has a finite mass or charge, it emits radiation which carries away orbital energy and angular momentum, and the orbit deviates from a geodesic. In this paper we assume that the deviation is small and show that the half-advanced minus half-retarded field surprisingly provides the correct radiation reaction force, in a time-averaged sense, and determines the orbit of the particle.
Posters Session 6
Gravitational Wave Bursts from Cosmic String Loops
Abstract: We directly observe the energy loss mechanism for a certain class of cosmic string loops. Through simulations, we show that the power radiated from loops is dominated by sharp, beamed, periodic gravitational wave bursts sourced from cusp formation and evaporation. We notice that the radiation is anisotropic due to the properties of the string cusps. Using a model for the gravitational backreaction force, we predict that the strength of subsequent bursts might fade with the string energy. A rough estimate the waveform h+, hx of the radiation is computed numerically.
Three-Body Interactions of Black Holes in Globular Clusters
Kayhan Gultekin, M. Coleman Miller, and Douglas P. Hamilton
Evidence has been mounting for the existence of black holes with masses from 100 to 10000 M_sun associated with stellar clusters. Such intermediate-mass black holes will encounter other black holes in the dense cores of these clusters. The binaries produced in these encounters will encounter other objects as well thus changing the orbital characteristics of the binaries. These binaries and their subsequent mergers due to gravitational radiation are important sources of gravitational waves. We will present the results of numerical simulations of high mass ratio encounters, which will help clarify the nature of the interactions of intermediate-mass black holes in globular clusters and help determine what types of detectable gravitational wave signatures are likely.
Nearby Quasar Remnants: Dark and Well Fed
Timothy S. Hamilton
Present-epoch giant elliptical galaxies, apparently the dormant remnants of previously active quasars, now harbor SMBH (super massive black hole) nuclei built-up via early galactic mergers followed by the steadier growth associated with accretion of matter supplied by the galactic bulge. We here focus on the growth arising from accretion. In order for these SMBH nuclei to serve as viable compact dynamos for generating ultra-relativistic cosmic rays we have seen that it is necessary for them to have a substantial accretion-supported magnetic field, an especially dark radiation environment, and near maximal spin. In addition to using the luminosity of the galactic bulge to estimate its mass and exploiting the velocity dispersion of stars in the bulge to infer the core SMBH mass, we are now employing UBV colors to determine the galaxy HMA (heuristic merger age) since its last major galactic merger. Based on a large sample of nearby giant ellipticals located at z<0.02 (with median HMA=6.6 Gyr) we find a statistically significant systematic increase with HMA for the ratio of SMBH mass to bulge mass. Those galaxies that have merged farther in the past have a larger present SMBH mass than those that have merged more recently. Although the bulge mass itself is essentially HMA invariant, its distribution is statistically consistent with the relatively small mass loss needed to account for the concomitant HMA build-up of SMBH mass. We find that the inferred growth since the last galactic merger corresponds to a suitably high rate of accretion that can readily account for at least half the SMBH mass evident at present. If this final growth stage is associated with disk accretion, the mass transferred is then sufficient to assure a spun-up canonical Kerr hole (Thorne 1974).
Posters Session #8
Coalescence of Spinning Black Holes
Manuela Campanelli, Carlos Lousto
We present here the results of the numerical evolution of a series of binary black hole configurations with modest spins aligned and counter-aligned with the orbital angular momentum from the innermost stable circular orbit (ISCO) down to the final single rotating (Kerr) black hole. Thereby we obtain complete waveforms, and estimates of plunging times, the energy, and momentum radiated to infinity, and we discuss the dependence of the final black hole's spin on initial spins. The resulting waveforms have still the same qualitative simple looking of the nonspinning binaries. We finally discuss the relevance of our results to assist detection and interpretation of forthcoming data from laser interferometric observatories.
Adaptive Evolutions of Strong Gravitational Waves
Calculation of accurate waveforms generated by the sources likely to be detected by several worldwide gravitational wave (GW) observatories such as LIGO, VIRGO and LISA will play an essential role for the successful detection and interpretation of the signals. Such calculations involve the full Einstein equations in 3-D that must be solved to follow both the dynamics of the sources and the generation and propagation of gravitational waves into the wave zone. Our research program is aimed at calculating such waveforms for one of the most promising sources--inspiralling black hole binaries. One of the crucial requirements for this kind of simulation is adaptive mesh refinement. My talk is based on work-in-progress that solves the vacuum Einstein equations with strong gravitational waves (Brill waves) as initial data. This work constitutes a first step towards full simulations and provides us with an important test problem to verify various components of our AMR code. Apart from astrophysical motivations, evolutions of strong wave data can also lead to a study of interesting physics problems such as critical phenomena. Some preliminary results are presented.
Evolving a Black Hole with Mesh Refinement
J. David Brown
John Baker, and Joan Centrella
We carry out evolutions of a single black hole in 3-D with mesh refinement. We evolve the black hole as a "puncture" using a BSSN system with various slicing conditions. We explore the behavior of the solution near the mesh refinement boundary, and the evolution of the puncture at high resolution.
Simulations of sources of Gravitational Waves become more and more needed to analyze the first (and future) incoming gravitational wave data. A full simulation of the merger of a binary Black Hole system is not yet done. In this poster we show methods and first results evolving Binary Black Hole data.
Low Frequency Astrophysical Gravitational Wave Background
Scott E. Pollack, Peter L. Bender
Three simplified merger tree models for galaxy formation have been explored by Menou, Haiman and Narayanan (2000) and by Menou (2003). One model had massive black holes (MBHs) present in almost all halos at the start of the simulation at z = 5, and the other two had MBHs in only 3% of the halos. For one of these two models, the MBHs were in the most massive halos, and for the other they were randomly distributed. The main purpose of the simulations was to investigate whether most halos merging at low z would have MBHs, even if a small percentage did initially, and the answer was that they would. We have used the merger trees for a very preliminary investigation of what range of GW background levels at low frequencies might be expected from MBH-MBH binary coalescences after mergers of galaxies or pre-galactic structures. The variation of MBH mass with halo mass was that employed by Menou (2003). The GW background levels were calculated for the three models and for very simple assumptions about whether the two MBHs would succeed in getting down near the center of the merged object, avoid getting stalled in the last parsec, and succeed in coalescing. A simple cutoff model was used, where all MBHs with masses below the cutoff were assumed not to coalesce. The sensitivity of the results to the cutoff mass for the three merger trees will be described.
Collisional Dynamics of Binary Black Holes in Galactic Centres
We follow the sinking of two massive black holes in a spherical stellar systemi by means of high precision direct Nbody simulation using NBODY6++ and Eurostar. The massive particles become bound under the regime of dynamical friction. Once bound, the binary hardens by superelastic three body encounters with surrounding stars. It is found that the cumulative effect of many of such resonant encounters keeps the black hole binary at a very high eccentricity and helps to bring the black holes close enough together that they can merge by gravitational radiation in a time scale of the order of 108 years (avoiding the stalling problem). While most of our study presently uses an idealized system (equal black hole masses, flat galactic core) more simulations are under way which vary black hole mass ratios. We discuss the situation in the recently discovered double black hole nucleus (see this conference) in light of our results.
Presentation Title: A Fast Apparent Horizon Finder for 3-D Cartesian Grids in Numerical Relativity
In 3+1 numerical simulations of dynamic black hole spacetimes, it's useful to be able to find the apparent horizon(s) in each slice of a time evolution. A number of apparent horizon finders are available, but in general they're too slow to be practically usable at each time step.Here I present a new apparent-horizon finder, AHFinderDirect, which is very fast and accurate: It typically takes only 1-3 seconds to find an apparent horizon on a single GHz-class processor, and given accurate values of g_ij and K_ij on the underlying Cartesian grid, it typically finds apparent horizons to accuracies on the order of 1e-5M. I assume that an apparent horizon to be searched for is a Strahlkoerper ("star-shaped region") with respect to some local origin, and accordingly parameterize the horizon by r = h(angle) for some single-valued function h defined on the 2-sphere S^2. The apparent horizon equation then becomes a nonlinear elliptic PDE in h on S^2, whose coefficients are algebraic functions of g_ij, K_ij, and the (xyz) spatial derivatives of g_ij. I discretize S^2 using 6 angular patches (one each in the neighborhood of the +/- x, +/- y, and +/- z axes) to avoid coordinate singularities, and finite difference the apparent horizon equation in the angular coordinates using 4th order finite differencing. I solve the resulting system of nonlinear algebraic equations (for h at the angular grid points) by Newton's method, using a "symbolic differentiation" technique to compute the Jacobian matrix.
AHFinderDirect is implemented as a thorn in the Cactus computational toolkit (http://www.cactuscode.org), and will be generally available (GNU GPL license) in early 2003.
I thank the Alexander von Humboldt foundation and the Max Planck Institut fuer Gravitationsphysik for financial support.
Dynamical Interactions of Binary Black Holes at High Redshift
Motivated by the recent discovery of luminous quasars around redshift z=6 suggesting a very early assembly epoch, we have modeled the evolution of a SMBH population from primordial seeds: the massive BHs end-product of PopIII stars, likely to be hosted in the highest peaks of cosmological fluctuations. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. We track the merger history of dark matter halos and associated SMBHs, from early times to the present, via cosmological Monte Carlo realizations of the merger hierarchy, following all BH dynamical interactions. Triple BH interactions will inevitably take place at early times if the formation route for the assembly of SMBHs goes back to the very first generation of stars. The three BHs are likely to undergo a complicated resonance scattering interaction, leading to the final expulsion of one of the three bodies and to a more tightly bound binary. We have investigated their effect on binary BHs cosmic merger rate and global evolution.
Posters Session 9
Radius Constraint For A Magnetized Neutron Star
Maria Cristina Babiuc
"Gh. Asachi" T. U.
Isolated neutron stars undergoing non-radial oscillations are expected to emit gravitational waves. Radius determinations have improved in recent years with the discovery of a class of isolated neutron stars. New models of neutron star dynamics emerged, which demonstrate that the neutron star radius is primarily determined by the behavior of the pressure in the vicinity of nuclear matter equilibrium density. The direct and indirect influence of the magnetic fields on gravitational wave perturbations has been also investigated. Starting from the Einstein-Maxwell equations, we develop a general-relativistic model for the interior of a magnetized neutron star, generalizing the interior Schwarzschild solution. We show that for a source constituted on a magneto-fluid distribution, there exists a class of interior solutions for which the metric and consequently the physical quantities depend on the magnetic field. For a neutron star with uniform density, we find a direct correspondence between the neutron star radius and the internal magnetic field. Following the dependence of the metric and of the physical parameters on the magnetic field we conclude that the mass is magnetic in origin and state the contribution of the magnetic field, as a magnetic mass, to the neutron star stability. From the requirements over the star pressure, at the origin and at the surface, we obtain a limitation of the magnetized star radius, which entirely depends on the magnetic field, standing for the essential contribution of the magnetic field to the stellar geometry. We further find an exact Schwarzschild-type interior solution for the simplest case of a static neutron star. Using the Darmois-Israel matching condition at the stellar surface, we also study the magnetic field imprint onto the Schwarzschild-type asymptotically flat vacuum exterior metric of a uniform neutron star.
Rapidly Rotating Neutron Stars as Steady Detectable Gravitational Sources
Axel Bonacic M.
Pontificia Universidad Catolica de Chile (PUC)
Rapidly rotating neutron stars are expected to radiate gravitational waves caused by r-modes, which are made unstable through the so-called Chandrasekhar-Friedman-Schutz mechanism. Internal dissipation through weak interaction processes involving Λ0 and Σ- hyperons, reheats the neutron star core and balances direct Urca neutrino cooling. This leads to an r-mode amplitude alpha ~ 10-6 and a core temperature T ~ 107 K) quasi-equilibrium mediated only by rotation frequency. Thus, as the former parameters evolution timescales are set by the spin-down rate, milisecond pulsars become extremely steady gravitational radiators for millions of years, and very young neutron stars with moderate magnetic fields ~ 1012 G will do so for hundreds of years. According to recent sensitivity curves, these signals are almost within the reach of the advanced LIGO with signal recycling, tuned at the appropriate frequency and integrating for 1/3 yr.
Estimating Gravitational Wave Source Parameters: The Effect of Eccentricity
Key parameters of gravitational wave sources such as mass, angular momentum and quadrupole moment can be estimated from the gravitational waves themselves, particularly in the case of high mass-ratio inspiralling binary sources. Continuing our study of the accuracy with which parameterized descriptions of compact astrophysical sources can be extracted from inspiral observations, we extend our calculations to include the effects on the analysis of the eccentricity of the orbit of the inspiralling body. Primarily using phase information from the inspiral, we directly match the computed gravitational waves from two different sources to demonstrate the extent to which duplications and ambiguity can occur, and thereby deduce limits on the accuracy with which the source parameters can be measured by this technique. Adding eccentricity increases the search space by one more dimension and otherwise complicates the analysis, but it makes the simulation much more realistic! Increasing eccentricity probably also decreases the signal to noise ratio for objects of a given mass and distance. We here investigate the extent to which it increases the number of cycles required to constrain the quadrupole moment and other source parameters.
Gravitational Wave Observations of Pulsars
D. I. Jones
In this poster I will describe the problem of extracting useful physics from the gravitational wave detection of a spinning-down pulsar. In particular, I will ask whether or not the information we gain from studying details of the electromagnetic signal enables us to test physical models in ways not possible via gravitational wave detection alone.
Gravitational Waves from Accretion-Induced Magnetic Field Burial in X-Ray Millisecond Pulsars
E. Sterl Phinney
The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated numerically as a function of accreted mass, starting from a centered magnetic dipole and evolving through a quasistatic sequence of two-dimensional Grad-Shafranov equilibria, in a process termed magnetic burial. Our calculation is the first to solve self-consistently (by respecting flux freezing) for the distribution of accreted matter and magnetic flux as a function of stellar latitude. We find that 10^-5 M_solar must be accreted before the magnetic field is distorted enough to appreciably reduce the magnetic dipole moment, well above earlier estimates of 10^-10 M_solar. Moreover, the magnetic field is compressed locally up to 10^5 times its initial strength, deforming the crust significantly and creating a sizeable, inclined mass quadrupole (epsilon ~ 10^-7 for 10^-1 M_solar accreted). We compute the gravitational wave amplitude expected from objects undergoing accretion-induced magnetic field burial, principally X-ray millisecond pulsars.
The LISA Simulator
Using the results of Cornish and Rubbo (PRD 67, 022001, 2003) we have built a full simulation of the Laser Interferometer Space Antenna (LISA) that is valid at all frequencies and for arbitrary gravitational waveforms. The simulator takes an input waveform and returns the fully modulated LISA response, complete with realistic instrument noise. The presented poster describes the physics behind the simulator and demonstrate its use by simulating the response of the LISA detector to a monochromatic binary and a chiping binary system. The simulator codes are available for download from www.physics.montana.edu/LISA/
Detectability of Gravitational Waves from SN 1987A Remnant
We discuss the potential for detection of gravitational waves from a rapidly spinning neutron star produced by supernova 1987A taking the parameters claimed by Middleditch et al. (2000) at face value. Assuming that the dominant mechanism for spin down is gravitational waves emitted by a freely precessing neutron star, it is possible to constrain the wobble angle, the effective moment of inertia of the precessing crust and the crust cracking stress limit. Our analysis suggests that, if the interpretation of the Middleditch data is correct, the compact remnant of SN 1987A may well provide a predictable source of gravitational waves well within the capabilities of LIGO II. The computational task required for the data analysis is within the capabilities of current computers if performed offline and could be accomplished online using techniques such as demodulation and decimation.
The Triggering of Electromagnetic Observations by Gravitational Wave Events
I will discuss the prospects for the observation of electromagnetic emission by gravitational wave sources first detected using current or future networks of interferometers. Various emission mechanisms and detection techniques for compact binary inspirals are studied to show that the pointing ability of gravitational wave detectors and the efficacy of electromagnetic detectors at scanning large areas of the sky for transient sources render a counterpart detection improbable for the Initial interferometers, possible with Advanced LIGO detectors, and likely with an Advanced detector in Europe. I will also present results from a new position estimation algorithm for unmodeled sources, and from the study of the prospects for the observation of counterparts to unmodeled bursts of gravitational radiation (such as the bursts from supernovae, for instance).