[1104.4751] Numerical relativity simulations of binary neutron stars

Authors: Marcus Thierfelder, Sebastiano Bernuzzi, Bernd Bruegmann

Date: 25 Apr 2011

Abstract: We present a new numerical relativity code designed for simulations of compact binaries involving matter. The code is an upgrade of the BAM code to include general relativistic hydrodynamics and implements state-of-the-art high-resolution-shock-capturing schemes on a hierarchy of mesh refined Cartesian grids with moving boxes. We test and validate the code in a series of standard experiments involving single neutron star spacetimes. We present test evolutions of quasi-equilibrium equal-mass irrotational binary neutron star configurations in quasi-circular orbits which describe the late inspiral to merger phases. Neutron star matter is modeled as a zero-temperature fluid; thermal effects can be included by means of a simple ideal-gas prescription. We analyze the impact that the use of different values of damping parameter in the Gamma-driver shift condition has on the dynamics of the system. The use of different reconstruction schemes and their impact in the post-merger dynamics is investigated. We compute and characterize the gravitational radiation emitted by the system. Self-convergence of the waves is tested, and we consistently estimate error-bars on the numerically generated waveforms in the inspiral phase.

Apr 26, 2011

1104.4751 (/preprints)
2011-04-26, 16:42 

[1104.4608] Nordstrom's scalar theory of gravity and the equivalence principle

Authors: Nathalie Deruelle

Date: 24 Apr 2011

Abstract: Nordstrom's theory of gravity, which describes gravity by a scalar field in flat spacetime, is observationally ruled out. It is however the only theory of gravity with General Relativity to obey the strong equivalence principle. I show in this paper that this remarkable property is true beyond post-newtonian level and can be related to the existence of a 'Nordstrom-Katz' superpotential.

Apr 26, 2011

1104.4608 (/preprints)
2011-04-26, 16:42 

[1104.3565] Stochastic Gravitational Wave Background from Coalescing Binary Black Holes

Authors: Xing-Jiang Zhu, Eric Howell, Tania Regimbau, David Blair, Zong-Hong Zhu

Date: 18 Apr 2011

Abstract: We estimate the stochastic gravitational wave (GW) background signal from the field population of coalescing binary stellar mass black holes (BHs) throughout the Universe. This study is motivated by recent observations of BH-Wolf-Rayet star systems and by new estimates in the metallicity abundances of star forming galaxies that imply BH-BH systems are more common than previously assumed. Using recent analytical results of the inspiral-merger-ringdown waveforms for coalescing binary BH systems, we estimate the resulting stochastic GW background signal. Assuming average quantities for the single source energy emissions, we explore the parameter space of chirp mass and local rate density required for detection by advanced and third generation interferometric GW detectors. For an average chirp mass of 8.7$M_{\odot}$, we find that detection through 3 years of cross-correlation by two advanced detectors will require a rate density, $r_0 \geq 0.5 \rm{Mpc}ˆ{-3} \rm{Myr}ˆ{-1}$. Combining data from multiple pairs of detectors can reduce this limit by up to 40%. Investigating the full parameter space we find that detection could be achieved at rates $r_0 \sim 0.1 \rm{Mpc}ˆ{-3} \rm{Myr}ˆ{-1}$ for populations of coalescing binary BH systems with average chirp masses of $\sim 15M_{\odot}$ which are predicted by recent studies of BH-Wolf-Rayet star systems \citep{Bulik08}. While this scenario is at the high end of theoretical estimates, cross-correlation of data by two Einstein Telescopes could detect this signal under the condition $r_0 \geq 10ˆ{-3} \rm{Mpc}ˆ{-3} \rm{Myr}ˆ{-1}$. Such a signal could potentially mask a primordial GW background signal of dimensionless energy density, $\Omega_{\rm{GW}}\sim 10ˆ{-10}$, around the (1--500) Hz frequency range.

Apr 22, 2011

1104.3565 (/preprints)
2011-04-22, 15:08 

[1104.3303] Random Walks in Curved Spacetimes

Authors: Matteo Smerlak

Date: 17 Apr 2011

Abstract: We introduce the master equation describing random walks in curved spacetimes, and derive the corresponding Fokker-Planck equation. By a combination of redshift and spatial curvature effects, the latter generates subleading corrections to Einstein's square-root law for the RMS displacement. We compute the first correction explicitely, and evaluate it for the cases of the Schwarzschild constant-density star, the Kerr black hole and the Friedmann-Robertson-Walker universe: in the first two cases, gravity turns out to enhance diffusion at small times, while in the third case the sign of the correction depends on the curvature of space, and diverges at the Big Bang - unless space is flat.

Apr 22, 2011

1104.3303 (/preprints)
2011-04-22, 15:07 

[1104.3741] Key problems in black hole physics today

Authors: Pankaj S. Joshi

Date: 19 Apr 2011

Abstract: We review here some of the major open issues and challenges in black hole physics today, and the current progress on the same. It is pointed out that to secure a concrete foundation for the basic theory as well as astrophysical applications for black hole physics, it is essential to gain a suitable insight into these questions. In particular, we discuss the recent results investigating the final fate of a massive star within the framework of the Einstein gravity, and the stability and genericity aspects of the gravitational collapse outcomes in terms of black holes and naked singularities. Recent developments such as spinning up a black hole by throwing matter into it, and physical effects near naked singularities are considered. It is pointed out that some of the new results obtained in recent years in the theory of gravitational collapse imply interesting possibilities and understanding for the theoretical advances in gravity as well as towards new astrophysical applications.

Apr 22, 2011

1104.3741 (/preprints)
2011-04-22, 15:06 

[1104.3079] Next-to-next-to-leading order post-Newtonian spin-orbit Hamiltonian for self-gravitating binaries

Authors: Johannes Hartung, Jan Steinhoff

Date: 15 Apr 2011

Abstract: We present the next-to-next-to-leading order post-Newtonian (PN) spin-orbit Hamiltonian for two self-gravitating spinning compact objects. If at least one of the objects is rapidly rotating, then the corresponding interaction is comparable in strength to a 3.5PN effect. The result in the present paper in fact completes the knowledge of the post-Newtonian Hamiltonian for binary spinning black holes up to and including 3.5PN. The Hamiltonian is checked via known results for the test-spin case and via the global Poincaré algebra with the center-of-mass vector uniquely determined by an ansatz.

Apr 18, 2011

1104.3079 (/preprints)
2011-04-18, 15:29 

[1104.2968] Gravitational wave astronomy - astronomy of the 21st century

Authors: S. V. Dhurandhar

Date: 15 Apr 2011

Abstract: An enigmatic prediction of Einstein's general theory of relativity is gravitational waves. With the observed decay in the orbit of the Hulse-Taylor binary pulsar agreeing within a fraction of a percent with the theoretically computed decay from Einstein's theory, the existence of gravitational waves was firmly established. Currently there is a worldwide effort to detect gravitational waves with interferometric gravitational wave observatories or detectors and several such detectors have been built or being built. The initial detectors have reached their design sensitivities and now the effort is on to construct advanced detectors which are expected to detect gravitational waves from astrophysical sources. The era of gravitational wave astronomy has arrived. This article describes the worldwide effort which includes the effort on the Indian front - the IndIGO project -, the principle underlying interferometric detectors both on ground and in space, the principal noise sources that plague such detectors, the astrophysical sources of gravitational waves that one expects to detect by these detectors and some glimpse of the data analysis methods involved in extracting the very weak gravitational wave signals from detector noise.

Apr 18, 2011

1104.2968 (/preprints)
2011-04-18, 15:22 

[1104.2938] An improved, "phase-relaxed" F-statistic for gravitational-wave data analysis

Authors: Curt Cutler

Date: 14 Apr 2011

Abstract: Rapidly rotating, slightly non-axisymmetric neutron stars emit nearly periodic gravitational waves (GWs), quite possibly at levels detectable by ground-based GW interferometers. We refer to these sources as "GW pulsars". For any given sky position and frequency evolution, the F-statistic is the optimal (frequentist) statistic for the detection of GW pulsars. However, in "all-sky" searches for previously unknown GW pulsars, it would be computationally intractable to calculate the (fully coherent) F-statistic at every point of a (suitably fine) grid covering the parameter space: the number of gridpoints is many orders of magnitude too large for that. Here we introduce a "phase-relaxed" F-statistic, which we denote F_pr, for incoherently combining the results of fully coherent searches over short time intervals. We estimate (very roughly) that for realistic searches, our F_pr is ~10-15% more sensitive than the "semi-coherent" F-statistic that is currently used. Moreover, as a byproduct of computing F_pr, one obtains a rough determination of the time-evolving phase offset between one's template and the true signal imbedded in the detector noise. Almost all the ingredients that go into calculating F_pr are already implemented in LAL, so we expect that relatively little additional effort would be required to develop a search code that uses F_pr.

Apr 18, 2011

1104.2938 (/preprints)
2011-04-18, 14:10 

[1104.2700] N-body Simulation for Astronomical Collisional Systems with a New SIMD Instruction Set Extension to the x86 Architecture, Advanced Vector Extensions

Authors: Ataru Tanikawa, Kohji Yoshikawa, Takashi Okamoto, Keigo Nitadori

Date: 14 Apr 2011

Abstract: We present a high-performance N-body code for astronomical collisional systems accelerated with the aid of a new SIMD instruction set extension of the x86 architecture: Advanced Vector eXtensions (AVX), an enhanced version of the Streaming SIMD Extensions (SSE). With one processor core of Intel Core i7-2600 processor (8MB cache and 3.40 GHz) based on Sandy Bridge micro-architecture, we achieved the performance of ~ 20 giga floating point number operations per second (GFlops) for double-precision accuracy, which is two times and five times higher than that of the previously developed code implemented with the SSE instructions (Nitadori et al., 2006b), and that of a code implemented without any explicit use of SIMD instructions with the same processor core. We have parallelized the collisional N-body code by using so-called NINJA scheme (Nitadori et al., 2006a), and achieved ~ 90 GFlops for a system containing more than N = 8192 particles with 8 MPI processes on four cores. We can expect to achieve about 10 tera Flops (TFlops) for an astronomical collisional system with N ~ 10ˆ5 on massively parallel systems with at most 800 cores with Sandy Bridge micro-architecture. This performance will be comparable to that of Graphic Processing Unit (GPU) cluster systems. This paper offers an alternative to collisional N-body simulations with GRAPEs and GPUs.

Apr 15, 2011

1104.2700 (/preprints)
2011-04-15, 13:52 

[1104.2712] Beating the spin-down limit on gravitational wave emission from the Vela pulsar

Date: 14 Apr 2011

Abstract: We present direct upper limits on continuous gravitational wave emission from the Vela pulsar using data from the Virgo detector's second science run. These upper limits have been obtained using three independent methods that assume the gravitational wave emission follows the radio timing. Two of the methods produce frequentist upper limits for an assumed known orientation of the star's spin axis and value of the wave polarization angle of, respectively, $1.9\ee{-24}$ and $2.2\ee{-24}$, with 95% confidence. The third method, under the same hypothesis, produces a Bayesian upper limit of $2.1\ee{-24}$, with 95% degree of belief. These limits are below the indirect {\it spin-down limit} of $3.3\ee{-24}$ for the Vela pulsar, defined by the energy loss rate inferred from observed decrease in Vela's spin frequency, and correspond to a limit on the star ellipticity of $\sim 10ˆ{-3}$. Slightly less stringent results, but still well below the spin-down limit, are obtained assuming the star's spin axis inclination and the wave polarization angles are unknown.

Apr 15, 2011

1104.2712 (/preprints)
2011-04-15, 13:51 

[1104.2650] A blind hierarchical coherent search for gravitational-wave signals from coalescing compact binaries in a network of interferometric detectors

Authors: Sukanta Bose, Thilina Dayanga, Shaon Ghosh, Dipongkar Talukder

Date: 14 Apr 2011

Abstract: We describe a hierarchical data analysis pipeline for coherently searching for gravitational wave (GW) signals from non-spinning compact binary coalescences (CBCs) in the data of multiple earth-based detectors. It assumes no prior information on the sky position of the source or the time of occurrence of its transient signals and, hence, is termed "blind". The pipeline computes the coherent network search statistic that is optimal in stationary, Gaussian noise, and allows for the computation of a suite of alternative statistics and signal-based discriminators that can improve its performance in real data. Unlike the coincident multi-detector search statistics employed so far, the coherent statistics are different in the sense that they check for the consistency of the signal amplitudes and phases in the different detectors with their different orientations and with the signal arrival times in them. The first stage of the hierarchical pipeline constructs coincidences of triggers from the multiple interferometers, by requiring their proximity in time and component masses. The second stage follows up on these coincident triggers by computing the coherent statistics. The performance of the hierarchical coherent pipeline on Gaussian data is shown to be better than the pipeline with just the first (coincidence) stage.

Apr 15, 2011

1104.2650 (/preprints)
2011-04-15, 13:51 

[1104.2044] Gravitational wave background from coalescence of black hole binaries population

Authors: Stefania Marassi, Raffaella Schneider, Giovanni Corvino, Valeria Ferrari, Simon Portergies Zwart

Date: 11 Apr 2011

Abstract: We compute the stochastic gravitational wave background(GWB) generated by a cosmological population of (BH-BH) binaries. Using an updated version of the SeBa population synthesis code, we simulate a large sample of binary systems. Adopting a set of "standard" conservative assumptions calibrated to reproduce the observed properties of single Wolf-Rayet stars and double pulsars, we extract fundamental statistical information on (BH-BH) physical parameters (primary and secondary BH masses, orbital separations and eccentricities, formation and merger timescales). We then derive the binary birth rate from the cosmic star formation history obtained from a numerical study which reproduces the available observations at redshifts $z < 8$. Making a significant step forward to previous calculations, where only the inspiral signal was considered, we include the contribution to the GWB coming from the merging of the two BHs and from the ring-down of the final BH. We find that the GWB from the inspiral phase is characterized by a maximum amplitude in the range $\Omega_{\rm GW} \sim [0.88-1.7]\times 10ˆ{-7}$ at frequencies $[80 - 100]$ Hz; this signal could be detected with a (S/N)$> 100$ by a second generation interferometer, such as Advanced LIGO/VIRGO, with 1-3 years of integration. Third generation detectors, such as the Einstein Telescope, could easily detect the GWB generated by the emission during all the three phases of the evolution. The frequency dependence and amplitude of the GWB generated during the merger and ring-down is very sensitive to the adopted core mass threshold for BH formation. This opens up the possibility to better understand the final stages of the evolution of massive stellar binaries using observational constraints on the associated gravitational wave emission.

Apr 14, 2011

1104.2044 (/preprints)
2011-04-14, 21:35 

[1104.2322] Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks

Authors: Bence Kocsis, Nicolas Yunes, Abraham Loeb

Date: 12 Apr 2011

Abstract: We examine the electromagnetic (EM) and gravitational wave (GW) signatures of stellar-mass compact objects (COs) spiraling into a supermassive black hole (extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure dominated, accretion disk. At large separations, the tidal effect of the secondary CO clears a gap. We show that the gap refills during the late GW-driven phase of the inspiral, leading to a sudden EM brightening of the source. The accretion disk leaves an imprint on the GW through its angular momentum exchange with the binary, the mass increase of the binary members due to accretion, and its gravity. We compute the disk-modified GWs both in an analytical Newtonian approximation and in a numerical effective-one-body approach. We find that disk-induced migration provides the dominant perturbation to the inspiral, with weaker effects from the mass accretion onto the CO and hydrodynamic drag. Depending on whether a gap is present, the perturbation of the GW phase is between 10 and 1000 radians per year, detectable with the future Laser Interferometer Space Antenna (LISA) at high significance. The Fourier transform of the disk-modified GW in the stationary phase approximation is sensitive to disk parameters with a frequency trend different from post-Newtonian vacuum corrections. Our results suggest that observations of EMRIs may place new sensitive constraints on the physics of accretion disks.

Apr 14, 2011

1104.2322 (/preprints)
2011-04-14, 21:35 

[1012.4869] Frame-Dragging Vortexes and Tidal Tendexes Attached to Colliding Black Holes: Visualizing the Curvature of Spacetime

Authors: Robert Owen, Jeandrew Brink, Yanbei Chen, Jeffrey D. Kaplan, Geoffrey Lovelace, Keith D. Matthews, David A. Nichols, Mark A. Scheel, Fan Zhang, Aaron Zimmerman, Kip S. Thorne

Date: 22 Dec 2010

Abstract: When one splits spacetime into space plus time, the spacetime curvature (Weyl tensor) gets split into an "electric" part E_{jk} that describes tidal gravity and a "magnetic" part B_{jk} that describes differential dragging of inertial frames. We introduce tools for visualizing B_{jk} (frame-drag vortex lines, their vorticity, and vortexes) and E_{jk} (tidal tendex lines, their tendicity, and tendexes), and also visualizations of a black-hole horizon's (scalar) vorticity and tendicity. We use these tools to elucidate the nonlinear dynamics of curved spacetime in merging black-hole binaries.

Apr 14, 2011

1012.4869 (/preprints)
2011-04-14, 21:35 

[1104.1183] Gravitational waves from extreme mass-ratio inspirals in Dynamical Chern-Simons gravity

Authors: Paolo Pani, Vitor Cardoso, Leonardo Gualtieri

Date: 6 Apr 2011

Abstract: Dynamical Chern-Simons gravity is an interesting extension of General Relativity, which finds its way in many different contexts, including string theory, cosmological settings and loop quantum gravity. In this theory, the gravitational field is coupled to a scalar field by a parity-violating term, which gives rise to characteristic signatures. Here we investigate how Chern-Simons gravity would affect the quasi-circular inspiralling of a small, stellar-mass object into a large non-rotating supermassive black hole, and the accompanying emission of gravitational and scalar waves. We find the relevant equations describing the perturbation induced by the small object, and we solve them through the use of Green's function techniques. Our results show that for a wide range of coupling parameters, the Chern-Simons coupling gives rise to an increase in total energy flux, which translates into a fewer number of gravitational-wave cycles over a certain bandwidth. For space-based gravitational-wave detectors such as LISA, this effect can be used to constrain the coupling parameter effectively.

Apr 12, 2011

1104.1183 (/preprints)
2011-04-12, 14:30 

[1104.1977] Cosmography with strong lensing of LISA gravitational wave sources

Authors: M. Sereno (POLITO), Ph. Jetzer, A. Sesana, M. Volonteri

Date: 11 Apr 2011

Abstract: LISA might detect gravitational waves from mergers of massive black hole binaries strongly lensed by intervening galaxies (Sereno et al. 2010). The detection of multiple gravitational lensing events would provide a new tool for cosmography. Constraints on cosmological parameters could be placed by exploiting either lensing statistics of strongly lensed sources or time delay measurements of lensed gravitational wave signals. These lensing methods do not need the measurement of the redshifts of the sources and the identification of their electromagnetic counterparts. They would extend cosmological probes to redshift z <= 10 and are then complementary to other lower or higher redshift tests, such as type Ia supernovae or cosmic microwave background. The accuracy of lensing tests strongly depends on the formation history of the merging binaries, and the related number of total detectable multiple images. Lensing amplification might also help to find the host galaxies. Any measurement of the source redshifts would allow to exploit the distance-redshift test in combination with lensing methods. Time-delay analyses might measure the Hubble parameter H_0 with accuracy of >= 10 km sˆ{-1}Mpcˆ{-1}. With prior knowledge of H_0, lensing statistics and time delays might constrain the dark matter density (delta Omega_M >= 0.08, due to parameter degeneracy). Inclusion of our methods with other available orthogonal techniques might significantly reduce the uncertainty contours for Omega_M and the dark energy equation of state.

Apr 12, 2011

1104.1977 (/preprints)
2011-04-12, 14:30 

[1104.1704] Search method for long-duration gravitational-wave transients from neutron stars

Authors: Reinhard Prix, Stefanos Giampanis, Chris Messenger

Date: 9 Apr 2011

Abstract: We introduce a search method for a new class of gravitational-wave signals, namely long-duration O(hours - weeks) transients from spinning neutron stars. We discuss the astrophysical motivation from glitch relaxation models and we derive a rough estimate for the maximal expected signal strength based on the superfluid excess rotational energy. The transient signal model considered here extends the traditional class of infinite-duration continuous-wave signals by a finite start-time and duration. We derive a multi-detector Bayes factor for these signals in Gaussian noise using $\F$-statistic amplitude priors, which simplifies the detection statistic and allows for an efficient implementation. We consider both a fully coherent statistic, which is computationally limited to directed searches for known pulsars, and a cheaper semi-coherent variant, suitable for wide parameter-space searches for transients from unknown neutron stars. We have tested our method by Monte-Carlo simulation, and we find that it outperforms orthodox maximum-likelihood approaches both in sensitivity and in parameter-estimation quality.

Apr 12, 2011

1104.1704 (/preprints)
2011-04-12, 14:29 

[1104.1122] Effective field theory calculation of conservative binary dynamics at third post-Newtonian order

Authors: S. Foffa, R. Sturani

Date: 6 Apr 2011

Abstract: We reproduce the two-body gravitational conservative dynamics at third post-Newtonian order for spin-less sources by using the effective field theory methods for the gravitationally bound two-body system, proposed by Goldberger and Rothstein. This result has been obtained by automatizing the computation of Feynman amplitudes within a Mathematica algorithm, paving the way for higher-order computations not yet performed by traditional methods.

Apr 07, 2011

1104.1122 (/preprints)
2011-04-07, 13:25 

[1104.0387] Systematics of black hole binary inspiral kicks and the slowness approximation

Authors: Richard H. Price, Gaurav Khanna, Scott A. Hughes

Date: 3 Apr 2011

Abstract: During the inspiral and merger of black holes, the interaction of gravitational wave multipoles carries linear momentum away, thereby providing an astrophysically important recoil, or "kick" to the system and to the final black hole remnant. It has been found that linear momentum during the last stage (quasinormal ringing) of the collapse tends to provide an "antikick" that in some cases cancels almost all the kick from the earlier (quasicircular inspiral) emission. We show here that this cancellation is not due to peculiarities of gravitational waves, black holes, or interacting multipoles, but simply to the fact that the rotating flux of momentum changes its intensity slowly. We show furthermore that an understanding of the systematics of the emission allows good estimates of the net kick for numerical simulations started at fairly late times, and is useful for understanding qualitatively what kinds of systems provide large and small net kicks.

Apr 06, 2011

1104.0387 (/preprints)
2011-04-06, 12:17 

[1104.0819] Linearized f(R) Gravity: Gravitational Radiation & Solar System Tests

Authors: Christopher P. L. Berry, Jonathan R. Gair

Date: 5 Apr 2011

Abstract: We investigate the linearized form of metric f(R)-gravity, assuming that f(R) is analytic about R = 0 so it may be expanded as f(R) = R + a_2 Rˆ2/2 + … Gravitational radiation is modified, admitting an extra mode of oscillation, that of the Ricci scalar. We derive an effective energy-momentum tensor for the radiation. We also present weak-field metrics for simple sources. These demonstrate that Kerr (or Schwarzschild) black holes do not exist in f(R)-gravity. We apply the metrics to tests that could constrain f(R). We show that light deflection experiments cannot distinguish f(R)-gravity from general relativity as both have an effective post-Newtonian parameter gamma = 1. We find that planetary precession rates are enhanced relative to general relativity; from the orbit of Mercury we derive the bound |a_2| < 1.2 \times 10ˆ{18} mˆ2. Gravitational wave astronomy may be more useful: considering the phase of a gravitational waveform we estimate deviations from general relativity could be measurable for an extreme-mass-ratio inspiral about a 10ˆ6 M_sol black hole if |a_2| > 10ˆ{17} mˆ2. However Eot-Wash experiments provide the strictest bound |a_2| < 2 \times 10ˆ{-9} mˆ2. Although the astronomical bounds are weaker, they are still of interest in the case that the effective form of f(R) is modified in different regions, perhaps through the chameleon mechanism. Assuming the laboratory bound is universal, we conclude that the propagating Ricci scalar mode cannot be excited by astrophysical sources.

Apr 06, 2011

1104.0819 (/preprints)
2011-04-06, 12:16 

[1104.0385] Formation scenarios and mass-radius relation for neutron stars

Authors: J.L. Zdunik, P. Haensel

Date: 3 Apr 2011

Abstract: Neutron star crust, formed via accretion of matter from a companion in a low-mass X-ray binary (LMXB), has an equation of state (EOS) stiffer than that of catalyzed matter. At a given neutron star mass, M, the radius of a star with an accreted crust is therefore larger, by DR(M), than for usually considered star built of catalyzed matter. Using a compressible liquid drop model of nuclei, we calculate, within the one-component plasma approximation, the EOSs corresponding to different nuclear compositions of ashes of X-ray bursts in LMXB. These EOSs are then applied for studying the effect of different formation scenarios on the neutron-star mass-radius relation. Assuming the SLy EOS for neutron star's liquid core, derived by Douchin & Haensel (2001), we find that at M=1.4 M_sun the star with accreted crust has a radius more than 100 m larger that for the crust of catalyzed matter. Using smallness of the crust mass compared to M, we derive a formula that relates DR(M) to the difference in the crust EOS. This very precise formula gives also analytic dependence of DR on M and R of the reference star built of catalyzed matter. The formula is valid for any EOS of the liquid core. Rotation of neutron star makes DR(M) larger. We derive an approximate but very precise formula that gives difference in equatorial radii, DR_eq(M), as a function of stellar rotation frequency.

Apr 06, 2011

1104.0385 (/preprints)
2011-04-06, 12:16 

[1104.0060] Astrodynamical Space Test of Relativity using Optical Devices I (ASTROD I) - A class-M fundamental physics mission proposal for Cosmic Vision 2015-2025: 2010 Update

Authors: Claus Braxmaier, Hansj&#xf6;rg Dittus, Bernard Foulon, Ertan G&#xf6;kl&#xfc;, Catia Grimani, Jian Guo, Sven Herrmann, Claus L&#xe4;mmerzahl, Wei-Tou Ni, Achim Peters, Benny Rievers, &#xc9;tienne Samain, Hanns Selig, Diana Shaul, Drazen Svehla, Pierre Touboul, Gang Wang, An-Ming Wu, Alexander F. Zakharov

Date: 1 Apr 2011

Abstract: This paper on ASTROD I is based on our 2010 proposal submitted for the ESA call for class-M mission proposals, and is a sequel and an update to our previous paper [Experimental Astronomy 23 (2009) 491-527; designated as Paper I] which was based on our last proposal submitted for the 2007 ESA call. In this paper, we present our orbit selection with one Venus swing-by together with orbit simulation. In Paper I, our orbit choice is with two Venus swing-bys. The present choice takes shorter time (about 250 days) to reach the opposite side of the Sun. We also present a preliminary design of the optical bench, and elaborate on the solar physics goals with the radiation monitor payload. We discuss telescope size, trade-offs of drag-free sensitivities, thermal issues and present an outlook. ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals.

Apr 04, 2011

1104.0060 (/preprints)
2011-04-04, 10:08 

[1103.5867] Prospects of observing continuous gravitational waves from known pulsars

Authors: Matthew Pitkin

Date: 30 Mar 2011

Abstract: Several searches for gravitational waves from a selection of known pulsars have been performed with data from the science runs of the LIGO gravitational wave detectors. So far these have lead to no detection, but upper limits on the gravitational wave amplitudes have been set. Here we study our intrinsic ability to detect, and estimate the gravitational wave amplitude for non-accreting pulsars. Using spin-down limits on emission as a guide we examine amplitudes that would be required to observe known pulsars with future detectors (Advanced LIGO, Advanced Virgo and the Einstein Telescope), assuming that they are triaxial stars emitting at precisely twice the known rotation frequency. Maximum allowed amplitudes depend on the stars' equation of state (e.g. a normal neutron star, a quark star, a hybrid star) and the theoretical mass quadrupoles that they can sustain. We study what range of quadrupoles, and therefore equations of state, would be consistent with being able to detect these sources. For globular cluster pulsars, with spin-downs masked by accelerations within the cluster, we examine what spin-down values gravitational wave observations would be able to set. For all pulsars we also alternatively examine what internal magnetic fields they would need to sustain observable ellipticities.

Apr 02, 2011

1103.5867 (/preprints)
2011-04-02, 22:33 

[1103.6041] Accuracy of the post-Newtonian approximation. II. Optimal asymptotic expansion of the energy flux for quasicircular, extreme mass-ratio inspirals into a Kerr black hole

Authors: Zhongyang Zhang, Nicolas Yunes, Emanuele Berti

Date: 30 Mar 2011

Abstract: We study the effect of black hole spin on the accuracy of the post-Newtonian approximation. We focus on the gravitational energy flux for the quasicircular, equatorial, extreme mass-ratio inspiral of a compact object into a Kerr black hole of mass M and spin J. For a given dimensionless spin a=J/Mˆ2 (in geometrical units), the energy flux depends only on the orbital velocity v or (equivalently) on the Boyer-Lindquist orbital radius r. We investigate the formal region of validity of the Taylor post-Newtonian expansion of the energy flux (which is known up to order vˆ8 beyond the quadrupole formula), generalizing previous work by two of us. The "error function" used to determine the region of validity of the post-Newtonian expansion can have two qualitatively different kinds of behavior, and we deal with these two cases separately. We find that, at any fixed post-Newtonian order, the edge of the region of validity (as measured by v/v_{ISCO}, where v_{ISCO} is the orbital velocity at the innermost stable circular orbit) is only weakly dependent on a. Unlike in the nonspinning case, the lack of sufficiently high order terms does not allow us to determine if there is a convergent to divergent transition at order vˆ6. Independently of a, the inclusion of angular multipoles up to and including l=5 in the numerical flux is necessary to achieve the level of accuracy of the best-known (N=8) PN expansion of the energy flux.

Apr 02, 2011

1103.6041 (/preprints)
2011-04-02, 22:26