[1201.1508] An observational test of the Vainshtein mechanism

Authors: Lam Hui, Alberto Nicolis

Date: 6 Jan 2012

Abstract: Modified gravity theories capable of genuine self-acceleration typically invoke a galileon scalar which mediates a long range force, but is screened by the Vainshtein mechanism on small scales. In such theories, non-relativistic stars carry the full scalar charge (proportional to their mass), while black holes carry none. Thus, for a galaxy free-falling in some external gravitational field, its central massive black hole is expected to lag behind the stars. To look for this effect, and to distinguish it from other astrophysical effects, one can correlate the gravitational pull from the surrounding structure with the offset between the stellar center and the black hole. The expected offset depends on the central density of the galaxy, and ranges up to ~0.1 kpc for small galaxies. The observed offset in M87 cannot be explained by this effect unless the scalar force is significantly stronger than gravity. We also discuss the systematic offset of compact objects from the galactic plane as another possible signature.

abs pdf

Mar 29, 2012

1201.1508 (/preprints)
2012-03-29, 09:49 [edit]


[1203.6075] The Formation of the First Massive Black Holes

Authors: Zoltán Haiman

Date: 27 Mar 2012

Abstract: Supermassive black holes (SMBHs) are common in local galactic nuclei, and SMBHs as massive as several billion solar masses already exist at redshift z=6. These earliest SMBHs may grow by the combination of radiation-pressure-limited accretion and mergers of stellar-mass seed BHs, left behind by the first generation of metal-free stars, or may be formed by more rapid direct collapse of gas in rare special environments where dense gas can accumulate without first fragmenting into stars. This chapter offers a review of these two competing scenarios, as well as some more exotic alternative ideas. It also briefly discusses how the different models may be distinguished in the future by observations with JWST, (e)LISA and other instruments.

abs pdf

Mar 29, 2012

1203.6075 (/preprints)
2012-03-29, 09:49 [edit]


[1203.6108] General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes

Authors: Bruno Giacomazzo, John G. Baker, M. Coleman Miller, Christopher S. Reynolds, James R. van Meter

Date: 27 Mar 2012

Abstract: Coalescing supermassive black hole binaries are produced by the mergers of galaxies and are the most powerful sources of gravitational waves accessible to space-based gravitational observatories. Some such mergers may occur in the presence of matter and magnetic fields and hence generate an electromagnetic counterpart. In this paper we present the first general relativistic simulations of magnetized plasma around merging supermassive black holes using the general relativistic magnetohydrodynamic code Whisky. By considering different magnetic field strengths, going from non-magnetically dominated to magnetically dominated regimes, we explore how magnetic fields affect the dynamics of the plasma and the possible emission of electromagnetic signals. In particular we observe a total amplification of the magnetic field of ~2 orders of magnitude which is driven by the accretion onto the binary and that leads to much stronger electromagnetic signals, more than a factor of 10ˆ4 larger than comparable calculations done in the force-free regime where such amplifications are not possible.

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Mar 29, 2012

1203.6108 (/preprints)
2012-03-29, 09:49 [edit]


[1203.6060] Nonspinning searches for spinning binaries in ground-based detector data: Amplitude and mismatch predictions in the constant precession cone approximation

Authors: D. Brown (1), A. Lundgren (1,2,3), R. O'Shaughnessy (2,4) ((1) Syracuse University, (2) Penn State University, (3) Albert Einstein Institute, Hannover, (4) University of Wisconsin-Milwaukee)

Date: 27 Mar 2012

Abstract: Current searches for compact binary mergers by ground-based gravitational-wave detectors assume for simplicity the two bodies are not spinning. If the binary contains compact objects with significant spin, then this can reduce the sensitivity of these searches, particularly for black hole--neutron star binaries. In this paper we investigate the effect of neglecting precession on the sensitivity of searches for spinning binaries using non-spinning waveform models. We demonstrate that in the sensitive band of Advanced LIGO, the angle between the binary's orbital angular momentum and its total angular momentum is approximately constant. Under this \emph{constant precession cone} approximation, we show that the gravitational-wave phasing is modulated in two ways: a secular increase of the gravitational-wave phase due to precession and an oscillation around this secular increase. We show that this secular evolution occurs in precisely three ways, corresponding to physically different apparent evolutions of the binary's precession about the line of sight. We estimate the best possible fitting factor between \emph{any} non-precessing template model and a single precessing signal, in the limit of a constant precession cone. Our closed form estimate of the fitting-factor depends only the geometry of the in-band precession cone; it does not depend explicitly on binary parameters, detector response, or details of either signal model. The precessing black hole--neutron star waveforms least accurately matched by nonspinning waveforms correspond to viewing geometries where the precession cone sweeps the orbital plane repeatedly across the line of sight, in an unfavorable polarization alignment.

abs pdf

Mar 29, 2012

1203.6060 (/preprints)
2012-03-29, 09:49 [edit]


[1203.5892] Determining gravitational wave radiation from close galaxy pairs using a binary population synthesis approach

Authors: Jinzhong Liu, Yu Zhang, Hailong Zhang, Yutao Sun, Na Wang

Date: 27 Mar 2012

Abstract: Context. The early phase of the coalescence of supermassive black hole (SMBH) binaries from their host galaxies provides a guaranteed source of low-frequency (nHz-$\mu$Hz) gravitational wave (GW) radiation by pulsar timing observations. These types of GW sources would survive the coalescing and be potentially identifiable. Aims. We aim to provide an outline of a new method for detecting GW radiation from individual SMBH systems based on the Sloan Digital Sky Survey (SDSS) observational results, which can be verified by future observations. Methods. Combining the sensitivity of the international Pulsar Timing Array (PTA) and the Square Kilometer Array (SKA) detectors, we used a binary population synthesis (BPS) approach to determine GW radiation from close galaxy pairs under the assumption that SMBHs formed at the core of merged galaxies. We also performed second post-Newtonian approximation methods to estimate the variation of the strain amplitude with time. Results. We find that the value of the strain amplitude \emph{h} varies from about $10ˆ{-14}$ to $10ˆ{-17}$ using the observations of 20 years, and we estimate that about 100 SMBH sources can be detected with the SKA detector.

abs pdf

Mar 29, 2012

1203.5892 (/preprints)
2012-03-29, 09:48 [edit]


[1203.5192] Multimessenger astronomy with gravitational waves and high-energy neutrinos

Authors: S. Ando, B. Baret (APC), B. Bouhou (APC), E. Chassande-Mottin (APC), A. Kouchner (APC), L. Moscoso (APC, SEDI), Veronique Van Elewyck (APC), I. Bartos, S. Márka, Z. Márka, A. Corsi, I. Di Palma, M. A. Papa, A. Dietz (LAPP), C. Donzaud (APC), D. Eichler, C. Finley, D. Guetta, F. Halzen, G. Jones, P. J. Sutton, S. Kandhasamy, V. Mandic, E. Thrane, K. Kotake, T. Piran, T. Pradier (IPHC), G. E. Romero, E. Waxman

Date: 23 Mar 2012

Abstract: Many of the astrophysical sources and violent phenomena observed in our Universe are potential emitters of gravitational waves (GW) and high-energy neutrinos (HEN). Both GWs and HENs may escape very dense media and travel unaffected over cosmological distances, carrying information from the innermost regions of the astrophysical engines. Such messengers could also reveal new, hidden sources that have not been observed by conventional photon-based astronomy. Coincident observation of GWs and HENs may thus play a critical role in multimessenger astronomy. This is particularly true at the present time owing to the advent of a new generation of dedicated detectors: IceCube, ANTARES, VIRGO and LIGO. Given the complexity of the instruments, a successful joint analysis of this data set will be possible only if the expertise and knowledge of the data is shared between the two communities. This review aims at providing an overview of both theoretical and experimental state-of-the-art and perspectives for such a GW+HEN multimessenger astronomy.

abs pdf

Mar 29, 2012

1203.5192 (/preprints)
2012-03-29, 09:48 [edit]


[1203.4663] Observable Spectra of Induced Gravitational Waves from Inflation

Authors: Laila Alabidi, Kazunori Kohri, Misao Sasaki, Yuuiti Sendouda

Date: 21 Mar 2012

Abstract: Measuring the primordial power spectrum on small scales is a powerful tool in inflation model building, yet constraints from Cosmic Microwave Background measurements alone are insufficient to place bounds stringent enough to be appreciably effective. For the very small scale spectrum, those which subtend angles of less than 0.3 degrees on the sky, an upper bound can be extracted from the astrophysical constraints on the possible production of primordial black holes in the early universe. A recently discovered observational by-product of an enhanced power spectrum on small scales, induced gravitational waves, have been shown to be within the range of proposed space based gravitational wave detectors; such as NASA's LISA and BBO detectors, and the Japanese DECIGO detector. In this paper we explore the impact such a detection would have on models of inflation known to lead to an enhanced power spectrum on small scales, namely the Hilltop-type and running mass models. We find that the Hilltop-type model can produce observable induced gravitational waves within the range of BBO and DECIGO for integral and fractional powers of the potential within a reasonable number of e-folds. We also find that the running mass model can produce a spectrum within the range of these detectors, but require that inflation terminates after an unreasonably small number of e-folds. Finally, we argue that if the thermal history of the Universe were to accomodate such a small number of e-folds the Running Mass Model can produce Primordial Black Holes within a mass range compatible with Dark Matter, i.e. within a mass range 10ˆ{20} g< M_{BH}<10ˆ{27} g.

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Mar 27, 2012

1203.4663 (/preprints)
2012-03-27, 13:12 [edit]


[1203.5166] Numerical simulations of compact object binaries

Authors: Harald P. Pfeiffer

Date: 23 Mar 2012

Abstract: Coalescing compact object binaries consisting of black holes and/or Neutron stars are a prime target for ground-based gravitational wave detectors. This article reviews the status of numerical simulations of these systems, with an emphasis on recent progress.

abs pdf

Mar 26, 2012

1203.5166 (/preprints)
2012-03-26, 12:03 [edit]


[1203.4105] Lorentz breaking Effective Field Theory and observational tests

Authors: Stefano Liberati

Date: 19 Mar 2012

Abstract: Analogue models of gravity have provided an experimentally realizable test field for our ideas on quantum field theory in curved spacetimes but they have also inspired the investigation of possible departures from exact Lorentz invariance at microscopic scales. In this role they have joined, and sometime anticipated, several quantum gravity models characterized by Lorentz breaking phenomenology. A crucial difference between these speculations and other ones associated to quantum gravity scenarios, is the possibility to carry out observational and experimental tests which have nowadays led to a broad range of constraints on departures from Lorentz invariance. We shall review here the effective field theory approach to Lorentz breaking in the matter sector, present the constraints provided by the available observations and finally discuss the implications of the persisting uncertainty on the composition of the ultra high energy cosmic rays for the constraints on the higher order, analogue gravity inspired, Lorentz violations.

abs pdf

Mar 21, 2012

1203.4105 (/preprints)
2012-03-21, 13:46 [edit]


[1203.3914] Prospects for an advanced Kennedy-Thorndike experiment in low Earth orbit

Authors: J. A. Lipa, S. Buchman, S. Saraf, J. Zhou, A. Alfauwaz, J. Conklin, G. D. Cutler, R. L. Byer

Date: 18 Mar 2012

Abstract: We discuss the potential for a small space mission to perform an advanced Kennedy-Thorndike test of Special Relativity using the large and rapid velocity modulation available in low Earth orbit. An improvement factor of ~100 over present ground results is expected, with an additional factor of 10 possible using more advanced technology.

abs pdf

Mar 21, 2012

1203.3914 (/preprints)
2012-03-21, 13:45 [edit]


[1203.4352] Measurability of the tidal polarizability of neutron stars in late-inspiral gravitational-wave signals

Authors: Thibault Damour, Alessandro Nagar, Loic Villain

Date: 20 Mar 2012

Abstract: The gravitational wave signal from a binary neutron star inspiral contains information on the nuclear equation of state. This information is contained in a combination of the tidal polarizability parameters of the two neutron stars and is clearest in the late inspiral, just before merger. We use the recently defined tidal extension of the effective one-body formalism to construct a controlled analytical description of the frequency-domain phasing of neutron star inspirals up to merger. Exploiting this analytical description we find that the tidal polarizability parameters of neutron stars can be measured by the advanced LIGO-Virgo detector network from gravitational wave signals having a reasonable signal-to-noise ratio of $\rho=16$. This measurability result seems to hold for all the nuclear equations of state leading to a maximum mass larger than $1.97M_\odot$. We also propose a promising new way of extracting information on the nuclear equation of state from a coherent analysis of an ensemble of gravitational wave observations of separate binary merger events.

abs pdf

Mar 21, 2012

1203.4352 (/preprints)
2012-03-21, 13:44 [edit]


[1203.3077] Investigating stellar-mass black hole kicks

Authors: Serena Repetto, Melvyn B. Davies, Steinn Sigurdsson

Date: 14 Mar 2012

Abstract: We investigate whether stellar-mass black holes have to receive natal kicks in order to explain the observed distribution of low-mass X-ray binaries containing black holes within our Galaxy. Such binaries are the product of binary evolution, where the massive primary has exploded forming a stellar-mass black hole, probably after a common envelope phase where the system contracted down to separations of order 10-30 Rsun. We perform population synthesis calculations of these binaries, applying both kicks due to supernova mass-loss and natal kicks to the newly-formed black hole. We then integrate the trajectories of the binary systems within the Galactic potential. We find that natal kicks are in fact necessary to reach the large distances above the Galactic plane achieved by some binaries. Further, we find that the distribution of natal kicks would seem to be similar to that of neutron stars, rather than one where the kick velocities are reduced by the ratio of black hole to neutron-star mass (i.e. where the kicks have the same momentum). This result is somewhat surprising; in many pictures of stellar-mass black-hole formation, one might have expected black holes to receive kicks having the same momentum (rather than the same speed) as those given to neutron stars.

abs pdf

Mar 16, 2012

1203.3077 (/preprints)
2012-03-16, 18:13 [edit]


[1203.2920] Effects of post-Newtonian Spin Alignment on the Distribution of Black-Hole Recoils

Authors: Emanuele Berti, Michael Kesden, Ulrich Sperhake

Date: 13 Mar 2012

Abstract: Recent numerical relativity simulations have shown that the final black hole produced in a binary merger can recoil with a velocity as large as 5,000 km/s. Because of enhanced gravitational-wave emission in the so-called "hang-up" configurations, this maximum recoil occurs when the black-hole spins are partially aligned with the orbital angular momentum. We revisit our previous statistical analysis of post-Newtonian evolutions of black-hole binaries in the light of these new findings. We demonstrate that despite these new configurations with enhanced recoil velocities, spin alignment during the post-Newtonian stage of the inspiral will still significantly suppress (or enhance) kick magnitudes when the initial spin of the more massive black hole is more (or less) closely aligned with the orbital angular momentum than that of the smaller hole. We present a preliminary study of how this post-Newtonian spin alignment affects the ejection probabilities of supermassive black holes from their host galaxies with astrophysically motivated mass ratio and initial spin distributions. We find that spin alignment suppresses (enhances) ejection probabilities by ~ 40% (20%) for an observationally motivated mass-dependent galactic escape velocity, and by an even greater amount for a constant escape velocity of 1,000 km/s. Kick suppression is thus at least a factor two more efficient than enhancement.

abs pdf

Mar 16, 2012

1203.2920 (/preprints)
2012-03-16, 18:12 [edit]


[1203.3090] Thoughts about a conceptual framework for relativistic gravity

Authors: Bernard F. Schutz

Date: 14 Mar 2012

Abstract: I consider the isolation of general relativity research from the rest of theoretical physics during the 1930s-1950s, and the subsequent reinvigoration of the field. I suggest that the main reason for the isolation was that relativists of the time did not develop heuristic concepts about the physics of the theory with which they could communicate with other physicists, and that the revival happened when they began to develop such concepts. A powerful heuristic today is the concept of a black hole, which is a robust and stable component of many astronomical systems. During the 1930s relativists could only offer the "Schwarzschild singularity". I argue that the change occurred at least partly because key theoretical physicists schooled in quantum theory entered relativity research and began to approach problematic issues by asking questions about observable effects and the outcomes of thought experiments. The result was the development of a physical intuition about such things as black holes, which could then be communicated to non-specialists. Only then was it possible to integrate general relativity fully into the rest of physics.

abs pdf

Mar 16, 2012

1203.3090 (/preprints)
2012-03-16, 18:12 [edit]


[1203.2856] A new code for parameter estimation in searches for gravitational waves from known pulsars

Authors: M. Pitkin, C. Gill, J. Veitch, E. Macdonald, G. Woan

Date: 13 Mar 2012

Abstract: We describe the consistency testing of a new code for gravitational wave signal parameter estimation in known pulsar searches. The code uses an implementation of nested sampling to explore the likelihood volume. Using fake signals and simulated noise we compare this to a previous code that calculated the signal parameter posterior distributions on both a grid and using a crude Markov chain Monte Carlo (MCMC) method. We define a new parameterisation of two orientation angles of neutron stars used in the signal model (the initial phase and polarisation angle), which breaks a degeneracy between them and allows more efficient exploration of those parameters. Finally, we briefly describe potential areas for further study and the uses of this code in the future.

abs pdf

Mar 16, 2012

1203.2856 (/preprints)
2012-03-16, 18:12 [edit]


[1203.2962] Spin induced multipole moments for the gravitational wave amplitude from binary inspirals to 2.5 Post-Newtonian order

Authors: Rafael A. Porto, Andreas Ross, Ira Z. Rothstein

Date: 13 Mar 2012

Abstract: Using the NRGR effective field theory formalism we calculate the remaining source multipole moments necessary to obtain the spin contributions to the gravitational wave amplitude to 2.5 Post-Newtonian (PN) order. We also reproduce the tail contribution to the waveform linear in spin at 2.5PN arising from the nonlinear interaction between the current quadrupole and the mass monopole.

abs pdf

Mar 16, 2012

1203.2962 (/preprints)
2012-03-16, 18:12 [edit]


[1203.2938] Secular evolution of compact binaries near massive black holes: Gravitational wave sources and other exotica

Authors: Fabio Antonini, Hagai Perets

Date: 13 Mar 2012

Abstract: The environment near super massive black holes (SMBHs) in galactic nuclei contain a large number of stars and compact objects. A fraction of these are likely to be members of binaries. Here we discuss the binary population of stellar black holes and neutron stars near SMBHs and focus on the secular evolution of such binaries, due to the perturbation by the SMBH. Binaries with highly inclined orbits in respect to their orbit around the SMBH are strongly affected by secular Kozai processes, which periodically change their eccentricities and inclinations (Kozai-cycles). During periapsis approach, at the highest eccentricities during the Kozai-cycles, gravitational wave emission becomes highly efficient. Some binaries in this environment can inspiral and coalesce at timescales much shorter than a Hubble time and much shorter than similar binaries which do not reside near a SMBH. The close environment of SMBHs could therefore serve as catalyst for the inspiral and coalescence of binaries, and strongly affect their orbital properties. Such compact binaries would be detectable as gravitational wave (GW) sources by the next generation of GW detectors (e.g. advanced- LIGO). About 0.5% of such nuclear merging binaries will enter the LIGO observational window while on orbit that are still very eccentric (e>~0.5). The efficient gravitational wave analysis for such systems would therefore require the use of eccentric templates. We also find that binaries very close to the MBH could evolve through a complex dynamical (non-secular) evolution leading to emission of several GW pulses during only a few yrs (though these are likely to be rare). Finally, we note that the formation of close stellar binaries, X-ray binaries and their merger products could be induced by similar secular processes, combined with tidal friction rather than GW emission as in the case of compact object binaries.

abs pdf

Mar 16, 2012

1203.2938 (/preprints)
2012-03-16, 18:10 [edit]


[1203.2150] Lunar Laser Ranging Tests of the Equivalence Principle

Authors: James G. Williams, Slava G. Turyshev, Dale Boggs

Date: 9 Mar 2012

Abstract: The Lunar Laser Ranging (LLR) experiment provides precise observations of the lunar orbit that contribute to a wide range of science investigations. In particular, time series of highly accurate measurements of the distance between the Earth and Moon provide unique information that determine whether, in accordance with the Equivalence Principle (EP), both of these celestial bodies are falling towards the Sun at the same rate, despite their different masses, compositions, and gravitational self-energies. Analyses of precise laser ranges to the Moon continue to provide increasingly stringent limits on any violation of the EP. Current LLR solutions give (-0.8 +/- 1.3) x 10ˆ{-13} for any possible inequality in the ratios of the gravitational and inertial masses for the Earth and Moon, (m_G/m_I)_E - (m_G/m_I)_M. Such an accurate result allows other tests of gravitational theories. Focusing on the tests of the EP, we discuss the existing data and data analysis techniques. The robustness of the LLR solutions is demonstrated with several different approaches to solutions. Additional high accuracy ranges and improvements in the LLR data analysis model will further advance the research of relativistic gravity in the solar system, and will continue to provide highly accurate tests of the Equivalence Principle.

abs pdf

Mar 12, 2012

1203.2150 (/preprints)
2012-03-12, 18:32 [edit]


[1203.0317] Gravitational Wave Heating of Stars and Accretion Disks

Authors: Gongjie Li, Bence Kocsis, Abraham Loeb

Date: 1 Mar 2012

Abstract: We investigate the electromagnetic (EM) counterpart of gravitational waves (GWs) emitted by a supermassive black hole binary (SMBHB) through the viscous dissipation of the GW energy in an accretion disk and stars surrounding the SMBHB. We account for the suppression of the heating rate if the forcing period is shorter than the turnover time of the largest turbulent eddies. We find that the viscous heating luminosity in 0.1 solar mass stars can be significantly higher than their intrinsic luminosity. The relative brightening is small for accretion disks.

abs pdf

Mar 06, 2012

1203.0317 (/preprints)
2012-03-06, 11:11 [edit]


[1202.4901] Double Compact Objects I: The Significance Of The Common Envelope On Merger Rates

Authors: Michal Dominik, Krzysztof Belczynski, Christopher Fryer, Daniel Holz, Emanuele Berti, Tomasz Bulik, Ilya Mandel, Richard O&#x27;Shaughnessy

Date: 22 Feb 2012

Abstract: The development of gravitational wave observatories (Advanced LIGO/Virgo, Einstein Telescope) is proceeding apace, and the direct detection of gravitational waves should be imminent. The last decade of observational and theoretical developments in stellar and binary evolution provides us with improvements to the predictions from populations synthesis models. Among the most important revisions in the formation and evolution of double compact objects are: updated wind mass loss rates (allowing for stellar mass black holes up to 80 Msun), a realistic treatment of the common envelope phase (that can affect merger rates by 2--3 orders of magnitude), and a qualitatively new neutron star/black hole mass distribution (consistent with the observed "mass gap"). We present a parameter study with these major physical updates included, focusing on the most important factors that set the DCO merger rates. A few of our more interesting findings are: the binding energy of the envelope and our description of natal kicks from supernovae play an important role in determining the formation and merger rate of DCOs. Also, models incorporating delayed (SASI) supernovae do not agree with the observed NS/BH "mass gap", in accordance with our previous work. And, finally, we find enhanced rates for BH-BH mergers as compared to previous estimates, with an expectation of ~100 such mergers per year in Advanced LIGO/Virgo detectors (although this rate is sensitive to factors, such as the natal kick distribution). This is the first in a series of three papers. The second paper will study the merger rates of double compact objects as a function of cosmological redshift, star formation rate, and metallicity. In the third paper we will present the detection rates for future gravitational wave observatories, using up-to-date signal waveforms and sensitivity curves. (abridged)

abs pdf

Mar 06, 2012

1202.4901 (/preprints)
2012-03-06, 11:09 [edit]


[1202.5336] Pseudo-Newtonian Potentials for Nearly Parabolic Orbits

Authors: Christopher Wegg

Date: 23 Feb 2012

Abstract: We describe a pseudo-Newtonian potential which, to within 1% error at all angular momenta, reproduces the precession due to general relativity of particles whose specific orbital energy is small compared to cˆ2 in the Schwarzschild metric. For bound orbits the constraint of low energy is equivalent to requiring the apoapsis of a particle to be large compared to the Schwarzschild radius. Such low energy orbits are ubiquitous close to supermassive black holes in galactic nuclei, but the potential is relevant in any context containing particles on low energy orbits. Like the more complex post-Newtonian expressions, the potential correctly reproduces the precession in the far-field, but also correctly reproduces the position and magnitude of the logarithmic divergence in precession for low angular momentum orbits. An additional advantage lies in its simplicity, both in computation and implementation. We also provide two simpler, but less accurate potentials, for cases where orbits always remain at large angular momenta, or when the extra accuracy is not needed. In all of the presented cases the accuracy in precession in low energy orbits exceeds that of the well known potential of Paczynski & Wiita (1980), which has ~30% error in the precession at all angular momenta.

abs pdf

Mar 06, 2012

1202.5336 (/preprints)
2012-03-06, 11:09 [edit]


[1202.5932] Understanding and analysing time-correlated stochastic signals in pulsar timing

Authors: Rutger van Haasteren (AEI), Yuri Levin (Monash)

Date: 27 Feb 2012

Abstract: Although it is widely understood that pulsar timing observations generally contain time-correlated stochastic signals (TCSSs; red timing noise is of this type), most data analysis techniques that have been developed make an assumption that the stochastic uncertainties in the data are uncorrelated, i.e. "white". Recent work has pointed out that this can introduce severe bias in determination of timing-model parameters, and that better analysis methods should be used. This paper presents a detailed investigation of timing-model fitting in the presence of TCSSs, and gives closed expressions for the post-fit signals in the data. This results in a Bayesian technique to obtain timing-model parameter estimates in the presence of TCSSs, as well as computationally more efficient expressions of their marginalised posterior distribution. A new method to analyse hundreds of mock dataset realisations simultaneously without significant computational overhead is presented, as well as a statistically rigorous method to check the internal consistency of the results. As a by-product of the analysis, closed expressions of the rms introduced by a stochastic background of gravitational-waves in timing-residuals are obtained. Using $T$ as the length of the dataset, and $h_c(1\rm{yr}ˆ{-1})$ as the characteristic strain, this is: $\sigma_{\rm GWB}ˆ2 = h_{c}(1\rm{yr}ˆ{-1})ˆ2 (9\sqrt[3]{2\piˆ4}\Gamma(-10/3) / 8008) \rm{yr}ˆ{-4/3} Tˆ{10/3}$.

abs pdf

Mar 06, 2012

1202.5932 (/preprints)
2012-03-06, 11:08 [edit]


[1202.5911] Generic bounds on dipolar gravitational radiation from inspiralling compact binaries

Authors: K. G. Arun (Chennai Mathematical Instt)

Date: 27 Feb 2012

Abstract: Various alternative theories of gravity predict dipolar gravitational radiation in addition to quadrupolar radiation. We show that gravitational wave (GW) observations of inspiralling compact binaries can put interesting constraints on the strengths of the dipole modes of GW polarizations. We put forward a physically motivated gravitational waveform for dipole modes, in the Fourier domain, in terms of two parameters: one which captures the relative amplitude of the dipole mode with respect to the quadrupole mode ($\alpha$) and the other a dipole term in the phase ($\beta$). We then use this two parameter representation to discuss typical bounds on their values using GW measurements. We obtain the expected bounds on the amplitude parameter $\alpha$ and the phase parameter $\beta$ for Advanced LIGO (AdvLIGO) and Einstein Telescope (ET) noise power spectral densities using Fisher information matrix. AdvLIGO and ET may at best bound $\alpha$ to an accuracy of $\sim10ˆ{-2}$ and $\sim10ˆ{-3}$ and $\beta$ to an accuracy of $\sim10ˆ{-5}$ and $\sim10ˆ{-6}$ respectively.

abs pdf

Mar 06, 2012

1202.5911 (/preprints)
2012-03-06, 11:08 [edit]


[1202.5699] The physical foundations for the geometric structure of relativistic theories of gravitation. From General Relativity to Extended Theories of Gravity through Ehlers-Pirani-Schild approach

Authors: S. Capozziello, M. De Laurentis, L. Fatibene, M. Francaviglia

Date: 25 Feb 2012

Abstract: We discuss in a critical way the physical foundations of geometric structure of relativistic theories of gravity by the so-called Ehlers-Pirani-Schild formalism. This approach provides a natural interpretation of the observables showing how relate them to General Relativity and to a large class of Extended Theories of Gravity. In particular we show that, in such a formalism, geodesic and causal structures of space-time can be safely disentangled allowing a correct analysis in view of observations and experiment. As specific case, we take into account the case of f(R) gravity.

abs pdf

Mar 06, 2012

1202.5699 (/preprints)
2012-03-06, 11:07 [edit]


[1202.6311] Theoretical Aspects of the Equivalence Principle

Authors: Thibault Damour

Date: 28 Feb 2012

Abstract: We review several theoretical aspects of the Equivalence Principle (EP). We emphasize the unsatisfactory fact that the EP maintains the absolute character of the coupling constants of physics while General Relativity, and its generalizations (Kaluza-Klein,…, String Theory), suggest that all absolute structures should be replaced by dynamical entities. We discuss the EP-violation phenomenology of dilaton-like models, which is likely to be dominated by the linear superposition of two effects: a signal proportional to the nuclear Coulomb energy, related to the variation of the fine-structure constant, and a signal proportional to the surface nuclear binding energy, related to the variation of the light quark masses. We recall the various theoretical arguments (including a recently proposed anthropic argument) suggesting that the EP be violated at a small, but not unmeasurably small level. This motivates the need for improved tests of the EP. These tests are probing new territories in physics that are related to deep, and mysterious, issues in fundamental physics.

abs pdf

Mar 06, 2012

1202.6311 (/preprints)
2012-03-06, 11:07 [edit]


[1202.6069] Testing the No-Hair Theorem with Observations in the Electromagnetic Spectrum. IV. Relativistically Broadened Iron Lines

Authors: Tim Johannsen, Dimitrios Psaltis (Arizona)

Date: 27 Feb 2012

Abstract: According to the no-hair theorem, astrophysical black holes are fully characterized by their masses and spins and are described by the Kerr metric. This theorem can be tested observationally by measuring (at least) three different multipole moments of the spacetimes of black holes. In this paper, we calculate the profiles of fluorescent iron lines emitted from the accretion flows around black holes within a framework that allows us to perform the calculation as a function of the mass and spin of a black hole as well as of a free parameter that measures potential deviations from the Kerr metric. We show that such deviations lead to line profiles that are significantly altered and exhibit a modified flux ratio of the two peaks in their characteristic double-peaked shape. We estimate the precision that near-future X-ray missions such as Astro-H and ATHENA are required to achieve in order to resolve deviations from the Kerr metric in iron line profiles and show that constraints on such deviations will be strongest for rapidly spinning black holes. More generally, we show that measuring the line profile with a precision of ~5% at a disk inclination of 30{\deg} constraints the deviation parameter to order unity irrespectively of the spin of the black hole.

abs pdf

Mar 06, 2012

1202.6069 (/preprints)
2012-03-06, 11:06 [edit]


[1202.6063] Evolution of binary black holes in self gravitating discs: dissecting the torques

Authors: Constanze Roedig, Alberto Sesana, Massimo Dotti, Jorge Cuadra, Pau Amaro-Seoane, Francesco Haardt

Date: 27 Feb 2012

Abstract: We analyse 3D SPH simulations of the evolution of initially quasi-circular massive black hole binaries (BHBs) residing in the central hollow (cavity) of self-gravitating circumbinary discs. We perform a set of simulations adopting different thermodynamics for the gas within the cavity and for the 'numerical size' of the black holes. We study the interplay between gas accretion and gravity torques in changing the binary elements (semi-major axis and eccentricity) and its total angular momentum budget. We pay special attention to the gravity torques, by analysing their physical origin and location. We show that (i) the BHB eccentricity grows due to gravity torques from the inner edge of the disc, independently of the accretion and the adopted thermodynamics; (ii) the semi-major axis decay depends not only on the gravity torques but also on their subtle interplay with the disc-binary angular momentum transfer due to accretion; (iii) the spectral structure of the gravity torques is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc; (iv) the net gravity torque changes sign across the BHB corotation radius: gas inside this radius exerts a net positive torque, while streams located outside this radius (but within the cavity) exert a net negative torque. The relative importance of the two might depend on the thermodynamical properties of the instreaming gas and is crucial in assessing the disc--binary angular momentum transfer; (v) the net torque manifests as a purely kinematic effect as it stems from the low density cavity, where the material flows in and out in highly eccentric orbits. Thus both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the binary.

abs pdf

Mar 06, 2012

1202.6063 (/preprints)
2012-03-06, 11:05 [edit]


[1203.0401] Prospects for transient gravitational waves at r-mode frequencies associated with pulsar glitches

Authors: I. Santiago-Prieto, I. S. Heng, D. I. Jones, J. Clark

Date: 2 Mar 2012

Abstract: Glitches in pulsars are likely to trigger oscillation modes in the fluid interior of neutron stars. We examined these oscillations specifically at r-mode frequencies. The excited r-modes will emit gravitational waves and can have long damping time scales O(minutes - days). We use simple estimates of how much energy the glitch might put into the r-mode and assess the detectability of the emitted gravitational waves with future interferometers.

abs pdf

Mar 06, 2012

1203.0401 (/preprints)
2012-03-06, 11:03 [edit]


[1203.0398] Effective action approach to cosmological perturbations in dark energy and modified gravity

Authors: Richard A. Battye, Jonathan A. Pearson

Date: 2 Mar 2012

Abstract: In light of upcoming observations modelling perturbations in dark en- ergy and modified gravity models has become an important topic of research. We develop an effective action to construct the components of the perturbed dark energy momentum tensor which appears in the perturbed generalized gravitational field equations, {\delta}G_{\mu\nu} = 8{\pi}G{\delta}T_{\mu\nu} + {\delta}U_{\mu\nu} for linearized perturbations. Our method does not require knowledge of the Lagrangian density of the dark sector to be provided, only its field content. The method is based on the fact that it is only necessary to specify the perturbed Lagrangian to quadratic order and couples this with the assumption of global statistical isotropy of spatial sections to show that the model can be specified completely in terms of a finite number of background dependent functions. We present our formalism in a coordinate independent fashion and provide explicit formulae for the perturbed conservation equation and the components of {\delta}U_{\mu\nu} for two explicit generic examples: (i) the dark sector does not contain extra fields, L = L(g_{\mu\nu}) and (ii) the dark sector contains a scalar field and its first derivative L = L(g_{\mu\nu}, {\phi}, \nabla_{\mu}{\phi}). We discuss how the formalism can be applied to modified gravity models containing derivatives of the metric, curvature tensors, higher derivatives of the scalar fields and vector fields.

abs pdf

Mar 06, 2012

1203.0398 (/preprints)
2012-03-06, 11:03 [edit]


[1203.1010] Periodic Orbits and Escapes in Dynamical Systems

Authors: George Contopoulos, Mirella Harsoula, Georgios Lukes-Gerakopoulos

Date: 5 Mar 2012

Abstract: We study the periodic orbits and the escapes in two different dynamical systems, namely (1) a classical system of two coupled oscillators, and (2) the Manko-Novikov metric (1992) which is a perturbation of the Kerr metric (a general relativistic system). We find their simple periodic orbits, their characteristics and their stability. Then we find their ordered and chaotic domains. As the energy goes beyond the escape energy, most chaotic orbits escape. In the first case we consider escapes to infinity, while in the second case we emphasize escapes to the central "bumpy" black hole. When the energy reaches its escape value a particular family of periodic orbits reaches an infinite period and then the family disappears (the orbit escapes). As this family approaches termination it undergoes an infinity of equal period and double period bifurcations at transitions from stability to instability and vice versa. The bifurcating families continue to exist beyond the escape energy. We study the forms of the phase space for various energies, and the statistics of the chaotic and escaping orbits. The proportion of these orbits increases abruptly as the energy goes beyond the escape energy.

abs pdf

Mar 05, 2012

1203.1010 (/preprints)
2012-03-05, 18:12 [edit]


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Tantum in modicis, quantum in maximis