[1204.4321] The optimal schedule for pulsar timing array observations

Authors: K. J. Lee, C. G. Bassa, R. Karuppusamy, M. Kramer, R. Smits, B. W.Stappers

Date: 19 Apr 2012

Abstract: In order to maximize the sensitivity of pulsar timing arrays to a stochastic gravitational wave background, we present computational techniques to optimize observing schedules. The techniques are applicable to both single and multi-telescope experiments. The observing schedule is optimized for each telescope by adjusting the observing time allocated to each pulsar while keeping the total amount of observing time constant. The optimized schedule depends on the timing noise characteristics of each individual pulsar as well as the performance of instrumentation. Several examples are given to illustrate the effects of different types of noise. A method to select the most suitable pulsars to be included in a pulsar timing array project is also presented.

abs pdf

Apr 23, 2012

1204.4321 (/preprints)
2012-04-23, 16:40 [edit]


[1204.3858] Binary Neutron Star Mergers

Authors: Joshua A. Faber, Frederic A. Rasio

Date: 17 Apr 2012

Abstract: We review the current status of studies of the coalescence of binary neutron star systems. We begin with a discussion of the formation channels of merging binaries and we discuss the most recent theoretical predictions for merger rates. Next, we turn to the quasi-equilibrium formalisms that are used to study binaries prior to the merger phase and to generate initial data for fully dynamical simulations. The quasi-equilibrium approximation has played a key role in developing our understanding of the physics of binary coalescence and, in particular, of the orbital instability processes that can drive binaries to merger at the end of their lifetimes. We then turn to the numerical techniques used in dynamical simulations, including relativistic formalisms, (magneto-)hydrodynamics, gravitational-wave extraction techniques, and nuclear microphysics treatments. This is followed by a summary of the simulations performed across the field to date, including the most recent results from both fully relativistic and microphysically detailed simulations. Finally, we discuss the likely directions for the field as we transition from the first to the second generation of gravitational-wave interferometers and while supercomputers reach the petascale frontier.

abs pdf

Apr 19, 2012

1204.3858 (/preprints)
2012-04-19, 11:19 [edit]


[1204.4110] Gravitational waves and gamma-ray bursts

Authors: Alessandra Corsi, for the LIGO Scientific Collaboration, for the Virgo Collaboration

Date: 18 Apr 2012

Abstract: Gamma-Ray Bursts are likely associated with a catastrophic energy release in stellar mass objects. Electromagnetic observations provide important, but indirect information on the progenitor. On the other hand, gravitational waves emitted from the central source, carry direct information on its nature. In this context, I give an overview of the multi-messenger study of gamma-ray bursts that can be carried out by using electromagnetic and gravitational wave observations. I also underline the importance of joint optical and gravitational wave searches, in the absence of a gamma-ray trigger. Finally, I discuss how multi-messenger observations may probe alternative gamma-ray burst progenitor models, such as the magnetar scenario.

abs pdf

Apr 19, 2012

1204.4110 (/preprints)
2012-04-19, 11:18 [edit]


[1204.3781] Gravitational waves from color-magnetic `mountains' in neutron stars

Authors: K. Glampedakis, D.I. Jones, L. Samuelsson

Date: 17 Apr 2012

Abstract: Neutron stars may harbour the true ground state of matter in the form of strange quark matter. If present, this type of matter is expected to be a color superconductor, a consequence of quark pairing with respect to the color/flavor degrees of freedom. The stellar magnetic field threading the quark core becomes a color-magnetic admixture and, in the event that superconductivity is of type II, leads to the formation of color-magnetic vortices. In this Letter we show that the volume-averaged color-magnetic vortex tension force should naturally lead to a significant degree of non-axisymmetry in systems like radio pulsars. We show that gravitational radiation from such color-magnetic ‘mountains’ in young pulsars like the Crab and Vela could be observable by the future Einstein Telescope, thus becoming a probe of paired quark matter in neutron stars. The detectability threshold can be pushed up toward the sensitivity level of Advanced LIGO if we invoke an interior magnetic field about a factor ten stronger than the surface polar field.

abs pdf

Apr 19, 2012

1204.3781 (/preprints)
2012-04-19, 11:18 [edit]


[1204.3948] The relativistic pulsar-white dwarf binary PSR J1738 0333 I. Mass determination and evolutionary history

Authors: J. Antoniadis, M. H. van Kerkwijk, D. Koester, P. C. C. Freire, N. Wex, T. M. Tauris, M. Kramer, C. G. Bassa

Date: 18 Apr 2012

Abstract: PSR J1738+0333 is one of the four millisecond pulsars known to be orbited by a white dwarf companion bright enough for optical spectroscopy. Of these, it has the shortest orbital period, making it especially interesting for a range of astrophysical and gravity related questions. We present a spectroscopic and photometric study of the white dwarf companion and infer its radial velocity curve, effective temperature, surface gravity and luminosity. We find that the white dwarf has properties consistent with those of low-mass white dwarfs with thick hydrogen envelopes, and use the corresponding mass-radius relation to infer its mass; M_WD = 0.181 +/- +0.007/-0.005 solar masses. Combined with the mass ratio q=8.1 +/- 0.2 inferred from the radial velocities and the precise pulsar timing ephemeris, the neutron star mass is constrained to M_PSR = 1.47 +/- +0.07/-0.06 solar masses. Contrary to expectations, the latter is only slightly above the Chandrasekhar limit. We find that, even if the birth mass of the neutron star was only 1.20 solar masses, more than 60% of the matter that left the surface of the white dwarf progenitor escaped the system. The accurate determination of the component masses transforms this system in a laboratory for fundamental physics by constraining the orbital decay predicted by general relativity. Currently, the agreement is within 1 sigma of the observed decay. Further radio timing observations will allow precise tests of white dwarf models, assuming the validity of general relativity.

abs pdf

Apr 19, 2012

1204.3948 (/preprints)
2012-04-19, 11:17 [edit]


[1204.2507] Support for the thermal origin of the Pioneer anomaly

Authors: Slava G. Turyshev, Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok, Jordan Ellis

Date: 11 Apr 2012

Abstract: We investigate the possibility that the anomalous acceleration of the Pioneer 10 and 11 spacecraft is due to the recoil force associated with an anisotropic emission of thermal radiation off the vehicles. To this end, relying on the project and spacecraft design documentation, we constructed a comprehensive finite-element thermal model of the two spacecraft. Then, we numerically solve thermal conduction and radiation equations using the actual flight telemetry as boundary conditions. We use the results of this model to evaluate the effect of the thermal recoil force on the Pioneer 10 spacecraft at various heliocentric distances. We found that the magnitude, temporal behavior, and direction of the resulting thermal acceleration are all similar to the properties of the observed anomaly. As a novel element of our investigation, we develop a parameterized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains.

abs pdf

Apr 16, 2012

1204.2507 (/preprints)
2012-04-16, 12:11 [edit]


[1204.2585] Model-Independent Test of General Relativity: An Extended post-Einsteinian Framework with Complete Polarization Content

Authors: Katerina Chatziioannou, Nicolas Yunes, Neil Cornish

Date: 11 Apr 2012

Abstract: We develop a model-independent test of General Relativity that allows for the constraint of the gravitational wave (GW) polarization content with GW detections of binary compact object inspirals. We first consider three modified gravity theories (Brans-Dicke theory, Rosen's theory and Lightman-Lee theory) and calculate the response function of ground-based detectors to gravitational waves in the inspiral phase. This allows us to see how additional polarizations predicted in these theories modify the General Relativistic prediction of the response function. We then consider general power-law modifications to the Hamiltonian and radiation-reaction force and study how these modify the time-domain and Fourier response function when all polarizations are present. From these general arguments and specific modified gravity examples, we infer an improved parameterized post-Einsteinian template family with complete polarization content. This family enhances General Relativity templates through the inclusion of new theory parameters, reducing to the former when these parameters acquire certain values, and recovering modified gravity predictions for other values, including all polarizations. We conclude by discussing detection strategies to constrain these new, polarization theory parameters by constructing certain null channels through the combination of output from multiple detectors.

abs pdf

Apr 16, 2012

1204.2585 (/preprints)
2012-04-16, 12:11 [edit]


[1204.0028] The ELM Survey. IV. 24 White Dwarf Merger Systems

Authors: Mukremin Kilic, Warren R. Brown, Carlos Allende Prieto, S. J. Kenyon, Craig O. Heinke, M. A. Agueros, S. J. Kleinman

Date: 30 Mar 2012

Abstract: We present new radial velocity and X-ray observations of extremely low-mass (ELM, 0.2 Msol) white dwarf candidates in the Sloan Digital Sky Survey (SDSS) Data Release 7 area. We identify seven new binary systems with 1-18 h orbital periods. Five of the systems will merge due to gravitational wave radiation within 10 Gyr, bringing the total number of merger systems found in the ELM Survey to 24. The ELM Survey has now quintupled the known merger white dwarf population. It has also discovered the eight shortest period detached binary white dwarf systems currently known. We discuss the characteristics of the merger and non-merger systems observed in the ELM Survey, including their future evolution. About half of the systems have extreme mass ratios. These are the progenitors of the AM Canum Venaticorum systems and supernovae .Ia. The remaining targets will lead to the formation of extreme helium stars, subdwarfs, or massive white dwarfs. We identify three targets that are excellent gravitational wave sources. These should be detected by the Laser Interferometer Space Antenna (LISA)-like missions within the first year of operation. The remaining targets are important indicators of what the Galactic foreground may look like for gravitational wave observatories.

abs pdf

Apr 16, 2012

1204.0028 (/preprints)
2012-04-16, 08:53 [edit]


[1204.1073] Circumbinary MHD Accretion into Inspiraling Binary Black Holes

Authors: Scott C. Noble (1), Bruno C. Mundim (1), Hiroyuki Nakano (1), Julian H. Krolik (2), Manuela Campanelli (1), Yosef Zlochower (1), Nicolás Yunes (3) ((1) Rochester Institute of Technology, (2) Johns Hopkins University, (3) Montana State University)

Date: 4 Apr 2012

Abstract: As 2 black holes bound to each other in a close binary approach merger their inspiral time becomes shorter than the characteristic inflow time of surrounding orbiting matter. Using an innovative technique in which we represent the changing spacetime in the region occupied by the orbiting matter with a 2.5PN approximation and the binary orbital evolution with 3.5PN, we have simulated the MHD evolution of a circumbinary disk surrounding an equal-mass non-spinning binary. Prior to the beginning of the inspiral, the structure of the circumbinary disk is predicted well by extrapolation from Newtonian results. The binary opens a low-density gap whose radius is roughly two binary separations, and matter piles up at the outer edge of this gap as inflow is retarded by torques exerted by the binary; nonetheless, the accretion rate is diminished relative to its value at larger radius by only about a factor of 2. During inspiral, the inner edge of the disk at first moves inward in coordination with the shrinking binary, but as the orbital evolution accelerates, the rate at which the inner edge moves toward smaller radii falls behind the rate of binary compression. In this stage, the rate of angular momentum transfer from the binary to the disk slows substantially, but the net accretion rate decreases by only 10-20%. When the binary separation is tens of gravitational radii, the rest-mass efficiency of disk radiation is a few percent, suggesting that supermassive binary black holes in galactic nuclei could be very luminous at this stage of their evolution. If the luminosity were optically thin, it would be modulated at a frequency that is a beat between the orbital frequency of the disk's surface density maximum and the binary orbital frequency. However, a disk with sufficient surface density to be luminous should also be optically thick; as a result, the periodic modulation may be suppressed.

abs pdf

Apr 11, 2012

1204.1073 (/preprints)
2012-04-11, 09:27 [edit]


[1204.1888] Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers

Authors: A. Bauswein (1), H.-T. Janka (1), K. Hebeler (2), A. Schwenk (3,4) ((1) MPI for Astrophysics, Garching, (2) Ohio State University, Columbus, (3) ExtreMe Matter Institute EMMI, Darmstadt, (4) Technische Universitaät Darmstadt)

Date: 9 Apr 2012

Abstract: Neutron-star (NS) merger simulations are conducted for 38 representative microphysical descriptions of high-density matter in order to explore the equation-of-state dependence of the postmerger ring-down phase. The formation of a deformed, oscillating, differentially rotating very massive NS is the typical outcome of the coalescence of two stars with 1.35 $M_{\odot}$ for most candidate EoSs. The oscillations of this object imprint a pronounced peak in the gravitational-wave (GW) spectra, which is used to characterize the emission for a given model. The peak frequency of this postmerger GW signal correlates very well with the radii of nonrotating NSs, and thus allows to constrain the high-density EoS by a GW detection. In the case of 1.35-1.35 $M_{\odot}$ mergers the peak frequency scales particularly well with the radius of a NS with 1.6 $M_{\odot}$, where the maximum deviation from this correlation is only 60 meters for fully microphysical EoSs which are compatible with NS observations. Combined with the uncertainty in the determination of the peak frequency it appears likely that a GW detection can measure the radius of a 1.6 $M_{\odot}$ NS with an accuracy of about 100 to 200 meters. We also uncover relations of the peak frequency with the radii of nonrotating NSs with 1.35 $M_{\odot}$ or 1.8 $M_{\odot}$, with the radius or the central energy density of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the pressure or sound speed at a fiducial rest-mass density of about twice nuclear saturation density. Furthermore, it is found that a determination of the dominant postmerger GW frequency can provide an upper limit for the maximum mass of nonrotating NSs. The prospects for a detection of the postmerger GW signal and a determination of the dominant GW frequency are estimated to be in the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO detector.

abs pdf

Apr 11, 2012

1204.1888 (/preprints)
2012-04-11, 09:25 [edit]


[1204.1859] Equivalence Principles, Lense-Thirring Effects, and Solar-System Tests of Cosmological Models

Authors: Wei-Tou Ni

Date: 9 Apr 2012

Abstract: In this talk, we review the empirical status for modern gravitational theories with emphases on (i) Equivalence Principles; (ii) Lense-Thirring effects and the implications of Gravity Probe B experiment; (iii) Solar-System Tests of Cosmological Models.

abs pdf

Apr 11, 2012

1204.1859 (/preprints)
2012-04-11, 09:25 [edit]


[1204.1670] Probing the Inhomogeneous Universe with Gravitational Wave Cosmology

Authors: Kent Yagi, Atsushi Nishizawa, Chul-Moon Yoo

Date: 7 Apr 2012

Abstract: If we assume that we live in the center of a spherical inhomogeneous universe, we can explain the apparent accelerating expansion of the universe without introducing the unknown dark energy or modifying gravitational theory. Direct measurement of the cosmic acceleration can be a powerful tool in distinguishing $\Lambda$CDM and the inhomogeneous models. If $\Lambda$CDM is the correct model, we have shown that DECIGO/BBO has sufficient ability to detect the positive redshift drift of the source by observing gravitational waves from neutron star binaries for 5-10 years. This enables us to rule out any Lema\ˆitre-Tolman-Bondi (LTB) void model with monotonically increasing density profile. Furthermore, by detecting the positive redshift drift at $z\sim 0$, we can even rule out generic LTB models unless we allow unrealistically steep density gradient at $z\sim 0$. We also show that the measurement accuracy is slightly improved when we consider the joint search of DECIGO/BBO and the third generation Einstein Telescope. This test can be performed with GW observations alone without any reference to electromagnetic observations.

abs pdf

Apr 11, 2012

1204.1670 (/preprints)
2012-04-11, 09:24 [edit]


[1204.2000] Gravitational Wave Astronomy: Needle in a Haystack

Authors: Neil J. Cornish

Date: 9 Apr 2012

Abstract: A world-wide array of highly sensitive interferometers stands poised to usher in a new era in astronomy with the first direct detection of gravitational waves. The data from these instruments will provide a unique perspective on extreme astrophysical phenomena such as neutron stars and black holes, and will allow us to test Einstein's theory of gravity in the strong field, dynamical regime. To fully realize these goals we need to solve some challenging problems in signal processing and inference, such as finding rare and weak signals that are buried in non-stationary and non-Gaussian instrument noise, dealing with high-dimensional model spaces, and locating what are often extremely tight concentrations of posterior mass within the prior volume. Gravitational wave detection using space based detectors and Pulsar Timing Arrays bring with them the additional challenge of having to isolate individual signals that overlap one another in both time and frequency. Promising solutions to these problems will be discussed, along with some of the challenges that remain.

abs pdf

Apr 11, 2012

1204.2000 (/preprints)
2012-04-11, 09:23 [edit]


[1203.6603] Compact binary coalescence parameter estimations for 2.5 post-Newtonian aligned spinning waveforms

Authors: Alex B. Nielsen

Date: 29 Mar 2012

Abstract: We examine the parameter accuracy that can be achieved by advanced ground-based detectors for binary inspiralling black holes and neutron stars. We use the 2.5 PN spinning waveforms of Arun et al. (2009). Our main result is that the errors are noticeably different from existing 2PN studies for aligned spins. While the masses can be determined more accurately, the individual spins are measured less accurately compared to previous work at lower PN order. We also examine several regions of parameter space relevant to expected sources and the impact of simple priors. A combination of the spins is measurable to higher accuracy and we examine what this can tell us about spinning systems.

abs pdf

Apr 05, 2012

1203.6603 (/preprints)
2012-04-05, 09:28 [edit]


[1203.6443] Continuum and Discrete Initial-Boundary-Value Problems and Einstein's Field Equations

Authors: Olivier Sarbach, Manuel Tiglio

Date: 29 Mar 2012

Abstract: Many evolution problems in physics are described by partial differential equations on an infinite domain; therefore, one is interested in the solutions to such problems for a given initial dataset. A prominent example is the binary black hole problem within Einstein's theory of gravitation, in which one computes the gravitational radiation emitted from the inspiral of the two black holes, merger and ringdown. Powerful mathematical tools can be used to establish qualitative statements about the solutions, such as their existence, uniqueness, continuous dependence on the initial data, or their asymptotic behavior over large time scales. However, one is often interested in computing the solution itself, and unless the partial differential equation is very simple, or the initial data possesses a high degree of symmetry, this computation requires approximation by numerical discretization. When solving such discrete problems on a machine, one is faced with a finite limit to computational resources, which leads to the replacement of the infinite continuum domain with a finite computer grid. This, in turn, leads to a discrete initial-boundary value problem. The hope is to recover, with high accuracy, the exact solution in the limit where the grid spacing converges to zero with the boundary being pushed to infinity.
The goal of this article is to review some of the theory necessary to understand the continuum and discrete initial-boundary value problems arising from hyperbolic partial differential equations and to discuss its applications to numerical relativity; in particular, we present well-posed initial and initial-boundary value formulations of Einstein's equations, and we discuss multi-domain high-order finite difference and spectral methods to solve them.

abs pdf

Apr 05, 2012

1203.6443 (/preprints)
2012-04-05, 09:28 [edit]


[1203.6352] No Evidence for Intermediate-Mass Black Holes in Globular Clusters: Strong Constraints from the JVLA

Authors: Jay Strader, Laura Chomiuk, Thomas Maccarone, James Miller-Jones, Anil Seth, Craig Heinke, Gregory Sivakoff

Date: 28 Mar 2012

Abstract: With a goal of searching for accreting intermediate-mass black holes (IMBHs), we report the results of ultra-deep Jansky VLA radio continuum observations of the cores of three Galactic globular clusters: M15, M19, and M22. We reach rms noise levels of 1.5-2.1 uJy/beam at an average frequency of 6 GHz. No sources are observed at the center of any of the clusters. For a conservative set of assumptions about the properties of the accretion, we set 3-sigma upper limits on IMBHs from 360-980 M_sun. These limits are among the most stringent obtained for any globular cluster. They add to a growing body of work that suggests either (a) IMBHs ~> 1000 M_sun are rare in globular clusters, or (b) when present, IMBHs accrete in an extraordinarily inefficient manner.

abs pdf

Apr 05, 2012

1203.6352 (/preprints)
2012-04-05, 09:28 [edit]


[1204.1043] The third and a half post-Newtonian gravitational wave quadrupole mode for quasi-circular inspiralling compact binaries

Authors: Guillaume Faye, Sylvain Marsat, Luc Blanchet, Bala R. Iyer

Date: 4 Apr 2012

Abstract: We compute the quadrupole mode of the gravitational waveform of inspiralling compact binaries at the third and a half post-Newtonian (3.5PN) approximation of general relativity. The computation is performed using the multipolar post-Newtonian formalism, and restricted to binaries without spins moving on quasi-circular orbits. The new inputs mainly include the 3.5PN terms in the mass quadrupole moment of the source, and the control of required subdominant corrections to the contributions of hereditary integrals (tails and non-linear memory effect). The result is given in the form of the quadrupolar mode (2,2) in a spin-weighted spherical harmonic decomposition of the waveform, and will allow a more accurate comparison with the outcome of numerical relativity simulations.

abs pdf

Apr 05, 2012

1204.1043 (/preprints)
2012-04-05, 09:25 [edit]


[1203.6685] Binary Disruption by Massive Black Holes: Hypervelocity Stars, S Stars, and Tidal Disruption Events

Authors: Benjamin C. Bromley (1), Scott J. Kenyon (2), Margaret J. Geller (2), Warren R. Brown (2) ((1) University of Utah (2) Smithsonian Astrophysical Observatory)

Date: 29 Mar 2012

Abstract: We examine whether disrupted binary stars can fuel black hole growth. In this mechanism, tidal disruption produces a single hypervelocity star (HVS) ejected at high velocity and a former companion star bound to the black hole. After a cluster of bound stars forms, orbital diffusion allows the black hole to accrete stars by tidal disruption at a rate comparable to the capture rate. In the Milky Way, HVSs and the S star cluster imply similar rates of 10ˆ{-5}--10ˆ{-3} yrˆ{-1} for binary disruption. These rates are consistent with estimates for the tidal disruption rate in nearby galaxies and imply significant black hole growth from disrupted binaries on 10 Gyr time scales.

abs pdf

Apr 01, 2012

1203.6685 (/preprints)
2012-04-01, 23:03 [edit]


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