[1107.4322] Binary dynamics from spin1-spin2 coupling at fourth post-Newtonian order

Authors: Michele Levi

Date: 21 Jul 2011

Abstract: We calculate the next-to-next-to-leading order (NNLO) spin1-spin2 dynamics of a compact binary evaluated at fourth post-Newtonian (PN) order. We use an effective field theory (EFT) approach, and first demonstrate here the ability of the EFT approach to go at NNLO in the PN corrections of spin effects. The NNLO spin1-spin2 interaction sector includes contributions from diagrams, which are not pure spin1-spin2 diagrams, as they arise from other sectors. These diagrams contribute through the leading order spin accelerations and precessions, that should be first taken into account here. The EFT calculation is carried out in terms of the nonrelativistic gravitational (NRG) fields. The fact that the spin is derivative-coupled adds significantly to the complexity of computations. In particular, for the irreducible two-loop diagrams, which are the most complicated in this sector, irreducible two-loop tensor integrals up to order 4 are encountered. Moreover, not all of the benefits of the NRG fields apply to spin interactions, as all possible diagram topologies are realized at each order of G included. Still, the NRG fields remain advantageous, and thus there was no use of automated computations in this work. Our final result can be reduced, and a NNLO spin1-spin2 Hamiltonian can be derived from it.

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Jul 31, 2011

1107.4322 (/preprints)
2011-07-31, 08:06 [edit]


[1107.5725] Measuring a cosmological distance-redshift relationship using only gravitational wave observations of binary neutron star coalescences

Authors: Chris Messenger, Jocelyn Read

Date: 28 Jul 2011

Abstract: Detection of gravitational waves from the inspiral phase of binary neutron star coalescence will allow us to measure the effects of the tidal coupling in such systems. These effects will be measurable using 3rd generation gravitational wave detectors, e.g. the Einstein Telescope, which will be capable of detecting inspiralling binary neutron star systems out to redshift z=4. Tidal effects provide additional contributions to the phase evolution of the gravitational wave signal that break a degeneracy between the system's mass parameters and redshift and thereby allowing for the simultaneous measurement of both the effective distance and the redshift for individual sources. Using the population of O(10ˆ3-10ˆ7) detectable binary neutron star systems predicted for the Einstein Telescope the luminosity distance--redshift relation can be probed independently of the cosmological distance ladder and independently of electromagnetic observations. We present the results of a Fisher information analysis applied to waveforms assuming a subset of possible neutron equations of state. We conclude that the redshift of such systems can be determined to O(10%) for z>1 and in the most optimistic case accuracies of 2% can be achieved.

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Jul 31, 2011

1107.5725 (/preprints)
2011-07-31, 08:06 [edit]


[1107.5056] The final parsec problem: aligning a binary with an external accretion disc

Authors: Chris Nixon, Andrew King, Jim Pringle

Date: 25 Jul 2011

Abstract: We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in galaxy merger events involving supermassive black holes, and in the formation of stellar--mass binaries in star clusters. We work out the gravitational torque between the binary and disc, and show that their angular momenta J_b, J_d stably counteralign if their initial orientation is sufficiently retrograde, specifically if the angle theta between them obeys cos(theta) < -J_d/2J_b, on a time short compared with the mass gain time of the central accretor(s). The magnitude J_b remains unchanged in this process. Counteralignment can promote the rapid merger of supermassive black hole binaries, and possibly the formation of coplanar but retrograde planets around stars in binary systems.

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Jul 31, 2011

1107.5056 (/preprints)
2011-07-31, 08:06 [edit]


[1107.4987] Tests of gravity at the solar system scale

Authors: Marc-Thierry Jaekel, Serge Reynaud

Date: 25 Jul 2011

Abstract: As confirmed by tests performed in the solar system, General Relativity (GR) presently represents the best description of gravitation. It is however challenged by observations at very large length scales, and already at the solar system scale, tracking of the Pioneer 10/11 probes has failed to confirm their expected behavior according to GR. Metric extensions of GR, which are presented here, have the quality of preserving the fundamental properties of GR while introducing scale dependent modifications. We show that they moreover represent an appropriate family of gravitation theories to be compared with observations when analysing gravity tests. We also discuss different tests which could allow one to determine the metric extension of GR prevailing in the solar system.

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Jul 31, 2011

1107.4987 (/preprints)
2011-07-31, 08:06 [edit]


[1107.4583] The Candidate Intermediate-Mass Black Hole in the Globular Cluster M54

Authors: J.M. Wrobel, J.E. Greene, L.C. Ho

Date: 22 Jul 2011

Abstract: Ibata et al. reported evidence for density and kinematic cusps in the Galactic globular cluster M54, possibly due to the presence of a 9400 solar-mass black hole. Radiative signatures of accretion onto M54's candidate intermediate-mass black hole (IMBH) could bolster the case for its existence. Analysis of new Chandra and recent Hubble Space Telescope astrometry rules out the X-ray counterpart to the candidate IMBH suggested by Ibata et al. If an IMBH exists in M54, then it has an Eddington ratio of L(0.3-8 keV) / L(Edd) < 1.4 x 10ˆ(-10), more similar to that of the candidate IMBH in M15 than that in G1. From new imaging with the NRAO Very Large Array, the luminosity of the candidate IMBH is L(8.5 GHz) < 3.6 x 10ˆ29 ergs/s (3 sigma). Two background active galaxies discovered toward M54 could serve as probes of its intracluster medium.

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Jul 31, 2011

1107.4583 (/preprints)
2011-07-31, 08:06 [edit]


[1107.4294] Next-to-next-to-leading order post-Newtonian spin(1)-spin(2) Hamiltonian for self-gravitating binaries

Authors: Johannes Hartung, Jan Steinhoff

Date: 21 Jul 2011

Abstract: We present the next-to-next-to-leading order post-Newtonian (PN) spin(1)-spin(2) Hamiltonian for two self-gravitating spinning compact objects. If both objects are rapidly rotating, then the corresponding interaction is comparable in strength to a 4PN effect. The Hamiltonian is checked via the global Poincare algebra with the center-of-mass vector uniquely determined by an ansatz.

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Jul 31, 2011

1107.4294 (/preprints)
2011-07-31, 08:06 [edit]


[1107.5064] Revealing the physics of r-modes in low-mass X-ray binaries

Authors: Wynn C. G. Ho, Nils Andersson (U Southampton), Brynmor Haskell (U Amsterdam)

Date: 25 Jul 2011

Abstract: We consider the astrophysical constraints on the gravitational-wave driven r-mode instability in accreting neutron stars in low-mass X-ray binaries. We use recent results on superfluid and superconducting properties to infer the core temperature in these neutron stars and show the diversity of the observed population. Simple theoretical models indicate that many of these systems reside inside the r-mode instability region. However, this is in clear disagreement with expectations, especially for the systems containing the most rapidly rotating neutron stars. The inconsistency highlights the need to re-evaluate our understanding of the many areas of physics relevant to the r-mode instability. We summarize the current status of our understanding, and we discuss directions for future research which could resolve this dilemma.

abs pdf

Jul 31, 2011

1107.5064 (/preprints)
2011-07-31, 08:06 [edit]


[1107.4106] The fate of Cyg X-1: an empirical limit on BH-NS merger rate

Authors: Krzysztof Belczynski, Tomasz Bulik, Charles Bailyn

Date: 20 Jul 2011

Abstract: The recent distance determination allowed precise estimation of the orbital parameters of Cyg X-1, which contains a massive 14.8 Msun BH with a 19.2 Msun O star companion. This system appears to be the clearest example of a potential progenitor of a BH-NS system. We follow the future evolution of Cyg X-1, and show that it will soon encounter a Roche lobe overflow episode, followed shortly by a Type Ib/c supernova and the formation of a NS. It is demonstrated that in majority of cases (>70%) the supernova and associated natal kick disrupts the binary due to the fact that the orbit expanded significantly in the Roche lobe overflow episode. In the reminder of cases (<30%) the newly formed BH-NS system is too wide to coalesce in the Hubble time. Only sporadically (1%) a Cyg X-1 like binary may form a coalescing BH-NS system given a favorable direction and magnitude of the natal kick. If Cyg X-1 like channel (X-ray active phase shorter than 10 Myr) is the only or dominant way to form BH-NS binaries in the Galaxy we can estimate the empirical BH-NS merger rate in the Galaxy at the level of 0.001 per Myr. This rate is so low that the detection of BH-NS systems in gravitational radiation is highly unlikely, generating Advanced LIGO/VIRGO detection rates at the level of only 1 per century. If BH-NS inspirals are in fact detected, it will indicate that the formation of these systems proceeds via some alternative and yet unobserved channels.

abs pdf

Jul 31, 2011

1107.4106 (/preprints)
2011-07-31, 08:06 [edit]


[1107.5707] The existence of a two-solar mass neutron star constrains the gravitational constant G_N at strong field

Authors: Antonio Dobado, Felipe J. Llanes-Estrada (U. Complutense Madrid), Jose Antonio Oller (U. Murcia)

Date: 28 Jul 2011

Abstract: In General Relativity there is a maximum mass allowed for neutron stars that, if exceeded, entails their collapse into a black hole. Its precise value depends on details of the nuclear matter equation of state about which we are much more certain thanks to recent progress in low-energy effective theories.
The discovery of a two-solar mass neutron star, near that maximum mass, when analyzed with modern equations of state, implies that Newton's gravitational constant in the star cannot exceed its value on Earth by more than 8% at 95% confidence level. This is a remarkable leap of ten orders of magnitude in the gravitational field intensity at which the constant has been constrained.

abs pdf

Jul 31, 2011

1107.5707 (/preprints)
2011-07-31, 08:06 [edit]


[1107.4243] Kinematic signature of an intermediate-mass black hole in the globular cluster NGC 6388

Authors: N. L&#xfc;tzgendorf, M. Kissler-Patig, E. Noyola, B. Jalali, P. T. de Zeeuw, K. Gebhardt, H. Baumgardt

Date: 21 Jul 2011

Abstract: Intermediate-mass black holes (IMBHs) are of interest in a wide range of astrophysical fields. In particular, the possibility of finding them at the centers of globular clusters has recently drawn attention. IMBHs became detectable since the quality of observational data sets, particularly those obtained with HST and with high resolution ground based spectrographs, advanced to the point where it is possible to measure velocity dispersions at a spatial resolution comparable to the size of the gravitational sphere of influence for plausible IMBH masses. We present results from ground based VLT/FLAMES spectroscopy in combination with HST data for the globular cluster NGC 6388. The aim of this work is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and to compare the results with the expected value predicted by the $M_{\bullet} - \sigma$ scaling relation. The spectroscopic data, containing integral field unit measurements, provide kinematic signatures in the center of the cluster while the photometric data give information of the stellar density. Together, these data sets are compared to dynamical models and present evidence of an additional compact dark mass at the center: a black hole. Using analytical Jeans models in combination with various Monte Carlo simulations to estimate the errors, we derive (with 68% confidence limits) a best fit black-hole mass of $ (17 \pm 9) \times 10ˆ3 M_{\odot}$ and a global mass-to-light ratio of $M/L_V = (1.6 \pm 0.3) \ M_{\odot}/L_{\odot}$.

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Jul 31, 2011

1107.4243 (/preprints)
2011-07-31, 08:05 [edit]


[1107.3967] An Introduction to String Theory

Authors: James Bedford

Date: 20 Jul 2011

Abstract: These notes are based on lectures given by Michael Green during Part III of the Mathematics Tripos (the Certificate for Advanced Study in Mathematics) in the Spring of 2003. The course provided an introduction to string theory, focussing on the Bosonic string, but treating the superstring as well. A background in quantum field theory and general relativity is assumed. Some background in particle physics, group theory and conformal field theory is useful, though not essential. A number of appendices on more advanced topics are also provided, including an introduction to orientifolds in various brane configurations which helps to populate a relatively sparse part of the literature.

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Jul 21, 2011

1107.3967 (/preprints)
2011-07-21, 04:36 [edit]


[1107.3866] Unexpected Formation Modes of the First Hard Binary in Core Collapse

Authors: Ataru Tanikawa, Piet Hut, Junichiro Makino

Date: 20 Jul 2011

Abstract: The conventional wisdom for the formation of the first hard binary in core collapse is that three-body interactions of single stars form many soft binaries, most of which are quickly destroyed, but eventually one of them survives. We report on direct N-body simulations to test these ideas, for the first time. We find that both assumptions are often incorrect: 1) quite a few three-body interactions produce a hard binary from scratch; 2) and in many cases there are more than three bodies directly and simultaneously involved in the production of the first binary. The main reason for the discrepancies is that the core of a star cluster, at the first deep collapse, contains typically only five or so stars. Therefore, the homogeneous background assumption, which still would be reasonable for, say, 25 stars, utterly breaks down. There have been some speculations in this direction, but we demonstrate this result here explicitly, for the first time.

abs pdf

Jul 21, 2011

1107.3866 (/preprints)
2011-07-21, 04:36 [edit]


[1107.2937] Electromagnetic counterparts of supermassive black hole binaries resolved by pulsar timing arrays

Authors: Takamitsu Tanaka, Kristen Menou, Zolt&#xe1;n Haiman (Columbia University)

Date: 14 Jul 2011

Abstract: Pulsar timing arrays (PTAs) are expected to detect gravitational waves (GWs) from individual low-redshift (z<1.5) compact supermassive (M>10ˆ9 Msun) black hole (SMBH) binaries with orbital periods of approx. 0.1 - 10 yrs. Identifying the electromagnetic (EM) counterparts of these sources would provide confirmation of putative direct detections of GWs, present a rare opportunity to study the environments of compact SMBH binaries, and could enable the use of these sources as standard sirens for cosmology. Here we consider the feasibility of such an EM identification. We show that because the host galaxies of resolved PTA sources are expected to be exceptionally massive and rare, it should be possible to find unique hosts of resolved sources out to redshift z=0.2. At higher redshifts, the PTA error boxes are larger, and may contain as many as 100 massive-galaxy interlopers. The number of candidates, however, remains tractable for follow-up searches in upcoming wide-field EM surveys. We develop a toy model to characterize the dynamics and the thermal emission from a geometrically thin, gaseous disc accreting onto a PTA-source SMBH binary. Our model predicts that at optical and infrared frequencies, the source should appear similar to a typical luminous active galactic nucleus (AGN). However, owing to the evacuation of the accretion flow by the binary's tidal torques, the source might have an unusually low soft X-ray luminosity and weak UV and broad optical emission lines, as compared to an AGN powered by a single SMBH with the same total mass. For sources near z=1, the decrement in the rest-frame UV should be observable as an extremely red optical color. These properties would make the PTA sources stand out among optically luminous AGN, and could allow their unique identification.

abs pdf

Jul 21, 2011

1107.2937 (/preprints)
2011-07-21, 04:36 [edit]


[1107.4014] The impact of a stochastic gravitational-wave background on pulsar timing parameters

Authors: J. Ellis, M. A. McLaughlin, J. P. W. Verbiest

Date: 20 Jul 2011

Abstract: Gravitational waves are predicted by Einstein's theory of general relativity as well as other theories of gravity. The rotational stability of the fastest pulsars means that timing of an array of these objects can be used to detect and investigate gravitational waves. Simultaneously, however, pulsar timing is used to estimate spin period, period derivative, astrometric, and binary parameters. Here we calculate the effects that a stochastic background of gravitational waves has on pulsar timing parameters through the use of simulations and data from the millisecond pulsars PSR J0437--4715 and PSR J1713+0747. We show that the reported timing uncertainties become underestimated with increasing background amplitude by up to a factor of $\sim10$ for a stochastic gravitational-wave background amplitude of $A=5\times 10ˆ{-15}$, where $A$ is the amplitude of the characteristic strain spectrum at one-year gravitational wave periods. We find evidence for prominent low-frequency spectral leakage in simulated data sets including a stochastic gravitational-wave background. We use these simulations along with independent Very Long Baseline Interferometry (VLBI) measurements of parallax to set a 2--sigma upper limit of $A\le9.1\times 10ˆ{-14}$. We find that different supermassive black hole assembly scenarios do not have a significant effect on the calculated upper limits. We also test the effects that ultralow--frequency (10$ˆ{-12}$--10$ˆ{-9}$ Hz) gravitational waves have on binary pulsar parameter measurements and find that the corruption of these parameters is less than those due to $10ˆ{-9}$--$10ˆ{-7}$ Hz gravitational waves.

abs pdf

Jul 21, 2011

1107.4014 (/preprints)
2011-07-21, 04:36 [edit]


[1107.3460] S. Chandrasekhar: White Dwarfs, $H^-$ ion,.., Black holes

Authors: Patrick Das Gupta

Date: 18 Jul 2011

Abstract: This is a concise review, addressed to undergraduate students, of S. Chandrasekhar's oeuvre in astrophysics, ranging from his early studies on white dwarfs using relativistic quantum statistics to topics as diverse as dynamical friction, negative hydrogen ion, fluid dynamical instabilities, black holes and gravitational waves. The exposition is based on simple physical explanations in the context of observational astronomy. Black holes and their role as central engines of active, compact, high energy sources have been discussed.

abs pdf

Jul 20, 2011

1107.3460 (/preprints)
2011-07-20, 04:24 [edit]


[1107.3528] Graviton mass bounds from space-based gravitational-wave observations of massive black hole populations

Authors: Emanuele Berti, Jonathan Gair, Alberto Sesana

Date: 18 Jul 2011

Abstract: Space-based gravitational-wave detectors, such as LISA or a similar ESA-led mission, will offer unique opportunities to test general relativity. We study the bounds that space-based detectors could place on the graviton Compton wavelength \lambda_g=h/(m_g c) by observing multiple inspiralling black hole binaries. We show that while observations of individual inspirals will yield mean bounds \lambda_g~3x10ˆ15 km, the combined bound from observing several events in a two-year mission is about ten times better: \lambda_g~3x10ˆ16 km (m_g~4x10ˆ-26 eV). This result is only mildly dependent on details of black hole formation and detector characteristics. The bound achievable in practice should be one order of magnitude better than this figure (and hence almost competitive with the static, model-dependent bounds from gravitational effects on cosmological scales), because our calculations ignore the merger/ringdown portion of the waveform. The observation that an ensemble of events can sensibly improve the bounds that individual binaries set on \lambda_g applies to any theory whose deviations from general relativity are parametrized by a set of global parameters.

abs pdf

Jul 20, 2011

1107.3528 (/preprints)
2011-07-20, 04:24 [edit]


[1107.3703] Recent Advances in the Numerical Simulations of Binary Black Holes

Authors: Pedro Marronetti, Wolfgang Tichy

Date: 19 Jul 2011

Abstract: Since the breakthrough papers from 2005/2006, the field of numerical relativity has experienced a growth spurt that took the two-body problem in general relativity from the category of "really-hard-problems" to the realm of "things-we-know-how-to-do". Simulations of binary black holes in circular orbits, the holy grail of numerical relativity, are now tractable problems that lead to some of the most spectacular results in general relativity in recent years. We cover here some of the latest achievements and highlight the field's next challenges.

abs pdf

Jul 20, 2011

1107.3703 (/preprints)
2011-07-20, 04:18 [edit]


[1107.2872] Optimizing the Earth-LISA "rendez-vous"

Authors: Fabrizio De Marchi, Giuseppe Pucacco, Massimo Bassan

Date: 14 Jul 2011

Abstract: We present a general survey of heliocentric LISA orbits, hoping it might help in the exercise of rescoping the mission. We try to semi-analytically optimize the orbital parameters in order to minimize the disturbances coming from the Earth-LISA interaction. In a set of numerical simulations we include nonautonomous perturbations and provide an estimate of Doppler shift and breathing as a function of the trailing angle.

abs pdf

Jul 15, 2011

1107.2872 (/preprints)
2011-07-15, 06:48 [edit]


[1107.2819] Numerical simulations of black-hole binaries and gravitational wave emission

Authors: Ulrich Sperhake, Emanuele Berti, Vitor Cardoso

Date: 14 Jul 2011

Abstract: We review recent progress in numerical relativity simulations of black-hole (BH) spacetimes. Following a brief summary of the methods employed in the modeling, we summarize the key results in three major areas of BH physics: (i) BHs as sources of gravitational waves (GWs), (ii) astrophysical systems involving BHs, and (iii) BHs in high-energy physics. We conclude with a list of the most urgent tasks for numerical relativity in these three areas.

abs pdf

Jul 15, 2011

1107.2819 (/preprints)
2011-07-15, 06:48 [edit]


[1107.2665] Toward Early-Warning Detection of Gravitational Waves from Compact Binary Coalescence

Authors: Kipp Cannon, Romain Cariou, Adrian Chapman, Mireia Crisp&#xed;n-Ortuzar, Nickolas Fotopoulos, Melissa Frei, Chad Hanna, Erin Kara, Drew Keppel, Laura Liao, Stephen Privitera, Antony Searle, Leo Singer, Alan Weinstein

Date: 13 Jul 2011

Abstract: Rapid detection of compact binary coalescence with a network of advanced gravitational-wave detectors will offer a unique opportunity for multi-messenger astronomy. Prompt detection alerts for the astronomical community might make it possible to observe the onset of electromagnetic emission from compact binary coalescence. We demonstrate a computationally practical filtering strategy that could produce early-warning triggers before gravitational radiation from the final merger has arrived at the detectors.

abs pdf

Jul 15, 2011

1107.2665 (/preprints)
2011-07-15, 06:48 [edit]


[1107.2904] Extending the effective-one-body Hamiltonian of black-hole binaries to include next-to-next-to-leading spin-orbit couplings

Authors: Enrico Barausse, Alessandra Buonanno (Univ. of Maryland)

Date: 14 Jul 2011

Abstract: In the effective-one-body (EOB) approach the dynamics of two compact objects of masses m1 and m2 and spins S1 and S2 is mapped into the dynamics of one test particle of mass mu = m1 m2/(m1+m2) and spin S* moving in a deformed Kerr metric with mass M = m1+m2 and spin Skerr. In a previous paper we computed an EOB Hamiltonian for spinning black-hole binaries that (i) when expanded in post-Newtonian orders, reproduces the leading order spin-spin coupling and the leading and next-to-leading order spin-orbit couplings for any mass ratio, and (iii) reproduces all spin-orbit couplings in the test-particle limit. Here we extend this EOB Hamiltonian to include next-to-next-to-leading spin-orbit couplings for any mass ratio. We discuss two classes of EOB Hamiltonians that differ by the way the spin variables are mapped between the effective and real descriptions. We also investigate the main features of the dynamics when the motion is equatorial, such as the existence of the innermost stable circular orbit and of a peak in the orbital frequency during the plunge subsequent to the inspiral.

abs pdf

Jul 15, 2011

1107.2904 (/preprints)
2011-07-15, 06:48 [edit]


[1107.1946] Assessing the redshift evolution of massive black holes and their hosts

Authors: Marta Volonteri, Daniel P. Stark

Date: 11 Jul 2011

Abstract: Motivated by recent observational results that focus on high redshift black holes, we explore the effect of scatter and observational biases on the ability to recover the intrinsic properties of the black hole population at high redshift. We find that scatter and selection biases can hide the intrinsic correlations between black holes and their hosts, with 'observable' subsamples of the whole population suggesting, on average, positive evolution even when the underlying population is characterized by no- or negative evolution. We create theoretical mass functions of black holes convolving the mass function of dark matter halos with standard relationships linking black holes with their hosts. Under these assumptions, we find that the local MBH - sigma correlation is unable to fit the z = 6 black hole mass function proposed by Willott et al. (2010), overestimating the number density of all but the most massive black holes. Positive evolution or including scatter in the MBH - sigma correlation makes the discrepancy worse, as it further increases the number density of observable black holes. We notice that if the MBH - sigma correlation at z = 6 is steeper than today, then the mass function becomes shallower. This helps reproducing the mass function of z = 6 black holes proposed by Willott et al. (2010). Alternatively, it is possible that very few halos (of order 1/1000) host an active massive black hole at z = 6, or that most AGN are obscured, hindering their detection in optical surveys. Current measurements of the high redshift black hole mass function might be underestimating the density of low mass black holes if the active fraction or luminosity are a function of host or black hole mass. Finally, we discuss physical scenarios that can possibly lead to a steeper MBH - sigma relation at high redshift.

abs pdf

Jul 15, 2011

1107.1946 (/preprints)
2011-07-15, 06:47 [edit]


[1107.1711] Accretion disks around kicked black holes: Post-kick Dynamics

Authors: Marcelo Ponce, Joshua A. Faber, James C. Lombardi Jr

Date: 8 Jul 2011

Abstract: Numerical calculations of merging black hole binaries indicate that asymmetric emission of gravitational radiation can kick the merged black hole at up to thousands of km/s, and a number of systems have been observed recently whose properties are consistent with an active galactic nucleus containing a supermassive black hole moving with substantial velocity with respect to its broader accretion disk. We study here the effect of an impulsive kick delivered to a black hole on the dynamical evolution of its accretion disk using a smoothed particle hydrodynamics code, focusing attention on the role played by the kick angle with respect to the orbital angular momentum vector of the pre-kicked disk. We find that for more vertical kicks, for which the angle between the kick and the normal vector to the disk $\theta\lesssim 30ˆ\circ$, a gap remains present in the inner disk, in accordance with the prediction from an analytic collisionless Keplerian disk model, while for more oblique kicks with $\theta\gtrsim 45ˆ\circ$, matter rapidly accretes toward the black hole. There is a systematic trend for higher potential luminosities for more oblique kick angles for a given black hole mass, disk mass and kick velocity, and we find large amplitude oscillations in time in the case of a kick oriented $60ˆ\circ$ from the vertical.

abs pdf

Jul 11, 2011

1107.1711 (/preprints)
2011-07-11, 03:24 [edit]


[1106.5056] Testing the black hole no-hair theorem at the galactic center: Perturbing effects of stars in the surrounding cluster

Authors: Laleh Sadeghian, Clifford M. Will

Date: 24 Jun 2011

Abstract: Observations of the precessing orbits of stars very near the massive black hole in the galactic center could provide measurements of the spin and quadrupole moment of the hole and thereby test the no-hair theorem of general relativity. Since the galactic center is likely to be populated by a distribution of stars and small black holes, their gravitational interactions will perturb the orbit of any given star. We estimate the effects of such perturbations using analytic orbital perturbation theory, and show that for a range of possible stellar distributions, and for an observed star sufficiently close to the black hole, the relativistic spin and quadrupole effects will be larger than the effects of stellar cluster perturbations. Our results are consistent those from recent numerical N-body simulations by Merritt et al.

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Jul 11, 2011

1106.5056 (/preprints)
2011-07-11, 03:23 [edit]


[1106.6355] A detection pipeline for galactic binaries in LISA data

Authors: Tyson B. Littenberg

Date: 30 Jun 2011

Abstract: The Galaxy is suspected to contain hundreds of millions of binary white dwarf systems, a large fraction of which will have sufficiently small orbital period to emit gravitational radiation in band for space-based gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA). LISA's main science goal is the detection of cosmological events (supermassive black hole mergers, etc.) however the gravitational signal from the galaxy will be the dominant contribution to the data -- including instrumental noise -- over approximately two decades in frequency. The catalogue of detectable binary systems will serve as an unparalleled means of studying the Galaxy. Furthermore, to maximize the scientific return from the mission, the data must be "cleansed" of the galactic foreground. We will present an algorithm that can accurately resolve and subtract >10000 of these sources from simulated data supplied by the Mock LISA Data Challenge Task Force. Using the time evolution of the gravitational wave frequency, we will reconstruct the position of the recovered binaries and show how LISA will sample the entire compact binary population in the Galaxy.

abs pdf

Jul 10, 2011

1106.6355 (/preprints)
2011-07-10, 01:40 [edit]


[1106.6313] Approximate Waveforms for Extreme-Mass-Ratio Inspirals in Modified Gravity Spacetimes

Authors: Jonathan R. Gair, Nicolas Yunes

Date: 30 Jun 2011

Abstract: Extreme-mass-ratio inspirals, in which a stellar-mass compact object spirals into a supermassive black hole, are prime candidates for detection with space-borne milliHertz gravitational wave detectors, similar to the Laser Interferometer Space Antenna. The gravitational waves generated during such inspirals encode information about the background in which the small object is moving, providing a tracer of the spacetime geometry and a probe of strong-field physics. In this paper, we construct approximate, "analytic-kludge" waveforms for such inspirals with parameterized post-Einsteinian corrections that allow for generic, model-independent deformations of the supermassive black hole background away from the Kerr metric. These approximate waveforms include all of the qualitative features of true waveforms for generic inspirals, including orbital eccentricity and relativistic precession. The deformations of the Kerr metric are modeled using a recently proposed, modified gravity bumpy metric, which parametrically deforms the Kerr spacetime while ensuring that three approximate constants of the motion remain for geodesic orbits: a conserved energy, azimuthal angular momentum and Carter constant. The deformations represent modified gravity effects and have been analytically mapped to several modified gravity black hole solutions in four dimensions. In the analytic kludge waveforms, the conservative motion is modeled by a post-Newtonian expansion of the geodesic equations in the deformed spacetimes, which in turn induce modifications to the radiation-reaction force. These analytic-kludge waveforms serve as a first step toward complete and model-independent tests of General Relativity with extreme-mass-ratio inspirals.

abs pdf

Jul 10, 2011

1106.6313 (/preprints)
2011-07-10, 01:40 [edit]


[1107.0766] A nonlinear scalar model of extreme mass ratio inspirals in effective field theory II. Scalar perturbations and a master source

Authors: Chad R. Galley

Date: 5 Jul 2011

Abstract: The motion of a small compact object (SCO) in a background spacetime is investigated further in a class of model nonlinear scalar field theories having a perturbative structure analogous to the General Relativistic description of extreme mass ratio inspirals (EMRIs). We derive regular expressions for the scalar perturbations generated by the SCO's motion valid through third order in $\epsilon$, the size of the SCO to the background curvature length scale. Our expressions are compared to those calculated through second order in $\epsilon$ by Rosenthal in [E. Rosenthal, CQG 22, S859 (2005)] and found to agree but our procedure for regularizing the scalar perturbations is considerably simpler. Following the Detweiler-Whiting (DW) scheme, we use our regular expressions for the field and derive the regular self-force corrections through third order. We find agreement with our previous derivation based on a variational principle of an effective action for the worldline associated with the SCO thus demonstrating the internal consistency of our formalism. This also explicitly demonstrates that the DW decomposition of Green's functions is a valid and practical method of self force computation at higher orders in perturbation theory and, as we show in an appendix, at all orders in perturbation theory. Finally, we identify a master source from which all other physically relevant quantities are derivable. Knowing the master source perturbatively allows one to construct the waveform measured by an observer, the regular part of the field on the worldline, the regular part of the self force, and orbital quantities such as shifts of the innermost stable circular orbit, etc. The existence of a master source together with the regularization methods implemented in this series should be indispensable for derivations of higher-order gravitational self force corrections.

abs pdf

Jul 10, 2011

1107.0766 (/preprints)
2011-07-10, 01:37 [edit]


[1107.0996] Reliability of complete gravitational waveform models for compact binary coalescences

Authors: Frank Ohme, Mark Hannam, Sascha Husa

Date: 5 Jul 2011

Abstract: With recent advances in post-Newtonian (PN) theory and numerical relativity (NR) it has become possible to construct inspiral-merger-ringdown gravitational waveforms from coalescing compact binaries by combining both descriptions into one complete hybrid signal. It is important to estimate the error of such waveforms. Previous studies have identified the PN contribution as the dominant source of error, which can be reduced by incorporating longer NR simulations. There are two outstanding issues that make it difficult to determine the minimum simulation length necessary to produce suitably accurate hybrids: (1) the relevant criteria for a signal search is the mismatch between the true waveform and a set of model waveforms, optimized over all waveforms in the model. For discrete hybrids this optimization is not possible. (2) these calculations require that NR waveforms already exist, while ideally we would like to know the necessary length before performing the simulation. Here we overcome these difficulties by developing a general procedure that allows us to estimate hybrid mismatch errors without numerical data, and to optimize them over all physical parameters. Using this procedure we find that, contrary to some earlier studies, ~10 NR orbits before merger allow for the construction of waveform families that are accurate enough for detection in a broad range of parameters, only excluding highly spinning, unequal-mass systems. Nonspinning binaries, even with high mass-ratio (>20) are well modeled for astrophysically reasonable component masses. In addition, the parameter bias is only of the order of 1% for total mass and symmetric mass-ratio and less than 0.1 for the dimensionless spin magnitude. We take the view that similar NR waveform lengths will remain the state of the art in the Advanced detector era, and begin to assess the limits of the science that can be done with them.

abs pdf

Jul 10, 2011

1107.0996 (/preprints)
2011-07-10, 01:37 [edit]


[1107.1181] Mergers of black-hole binaries with aligned spins: Waveform characteristics

Authors: Bernard J. Kelly, John G. Baker, William D. Boggs, Sean T. McWilliams, Joan Centrella

Date: 6 Jul 2011

Abstract: We conduct a descriptive analysis of the multipolar structure of gravitational-radiation waveforms from equal-mass aligned-spin mergers, following an approach first presented in the complementary context of nonspinning black holes of varying mass ratio [Baker et al., Phys. Rev. D 78:044046 (2008)]. We find that, as with the nonspinning mergers, the dominant waveform mode phases evolve together in lock-step through inspiral and merger, supporting the previous waveform description in terms of an adiabatically rigid rotator driving gravitational-wave emission -- an implicit rotating source (IRS). We further apply the late-time merger-ringdown model for the rotational frequency introduced in Baker et al. (2008), along with an improved amplitude model appropriate for the dominant (2,+/- 2) modes. This provides a quantitative description of the merger-ringdown waveforms, and suggests that the major features of these waveforms can be described with reference only to the intrinsic parameters associated with the state of the final black hole formed in the merger. We provide an explicit model for the merger-ringdown radiation, and demonstrate that this model agrees to fitting factors better than 95% with the original numerical waveforms for system masses above ~ 150 MSun. This model may be directly applicable to gravitational-wave detection of intermediate-mass black hole mergers.

abs pdf

Jul 10, 2011

1107.1181 (/preprints)
2011-07-10, 01:37 [edit]


[1107.1267] Addressing the spin question in gravitational-wave searches: Waveform templates for inspiralling compact binaries with nonprecessing spins

Authors: P. Ajith

Date: 6 Jul 2011

Abstract: This paper presents a post-Newtonian (PN) template family of gravitational waveforms from inspiralling compact binaries with non-precessing spins, where the spin effects are described by a single "reduced-spin" parameter. This template family, which reparametrizes all the spin-dependent PN terms in terms of the leading-order (1.5PN) spin-orbit coupling term in an approximate way, has very high overlaps (fitting factor > 0.99) with non-precessing binaries with arbitrary mass ratios and spins. We also show that this template family is "effectual" towards a significant fraction of generic spinning binaries in the comparable-mass regime (m_2/m_1 < 10), providing an attractive and feasible way of searching for gravitational waves (GWs) from spinning low-mass binaries. We also show that the secular (non-oscillatory) spin-dependent effects in the phase evolution (which are taken into account by the non-precessing templates) are more important than the oscillatory effects of precession in the comparable-mass (m_1 ~= m_2) regime. Hence the effectualness of non-spinning templates is particularly poor in this case, as compared to non-precessing-spin templates. For the case of binary neutron stars observable by Advanced LIGO, even small spins (L_N . S/mˆ2 ~= 0.1) will cause significant de-phasing with non-spinning templates. This is contrary to the expectation that neutron-star spins may not be relevant for GW detection.

abs pdf

Jul 10, 2011

1107.1267 (/preprints)
2011-07-10, 01:37 [edit]


[1107.0854] Black-hole hair loss: learning about binary progenitors from ringdown signals

Authors: Ioannis Kamaretsos (Cardiff), Mark Hannam (Cardiff), Sascha Husa (Palma), B.S. Sathyaprakash (Cardiff)

Date: 5 Jul 2011

Abstract: Perturbed Kerr black holes emit gravitational radiation, which (for the practical purposes of gravitational-wave astronomy) consists of a superposition of damped sinusoids termed quasi-normal modes. The frequencies and time-constants of the modes depend only on the mass and spin of the black hole - a consequence of the no-hair theorem. It has been proposed that a measurement of two or more quasi-normal modes could be used to confirm that the source is a black hole and to test if general relativity continues to hold in ultra-strong gravitational fields. In this paper we propose a practical approach to testing general relativity with quasi-normal modes. We will also argue that the relative amplitudes of the various quasi-normal modes encode important information about the origin of the perturbation that caused them. This helps in inferring the nature of the perturbation from an observation of the emitted quasi-normal modes. In particular, we will show that the relative amplitudes of the different quasi-normal modes emitted in the process of the merger of a pair of nonspinning black holes can be used to measure the component masses of the progenitor binary.

abs pdf

Jul 10, 2011

1107.0854 (/preprints)
2011-07-10, 01:37 [edit]


[1107.1075] The case for testing MOND using LISA Pathfinder

Authors: Joao Magueijo, Ali Mozaffari

Date: 6 Jul 2011

Abstract: We quantify the potential for testing MOdified Newtonian Dynamics (MOND) with LISA Pathfinder (LPF), should a saddle point flyby be incorporated into the mission. We forecast the expected signal to noise ratio (SNR) for a variety of instrument noise models and trajectories past the saddle. For standard theoretical parameters the SNR reaches middle to high double figures even with modest assumptions about instrument performance and saddle approach. Obvious concerns, like systematics arising from LPF self-gravity, or the Newtonian background, are examined and shown not to be a problem. We also investigate the impact of a negative observational result upon the free-function determining the theory. We demonstrate that, if Newton's gravitational constant is constrained not be re-normalized by more than a few percent, only very contrived MONDian free-functions would survive a negative result. Finally we scan the structure of all proposed relativistic MONDian theories. We conclude that only the Einstein-Aether formulation would survive a negative result.

abs pdf

Jul 10, 2011

1107.1075 (/preprints)
2011-07-10, 01:37 [edit]


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