[1210.7173] Data analysis challenges in transient gravitational-wave astronomy

Authors: Eric Chassande-Mottin, for the LIGO Scientific Collaboration, for the Virgo Collaboration

Date: 26 Oct 2012

Abstract: Gravitational waves are radiative solutions of space-time dynamics predicted by Einstein's theory of General Relativity. A world-wide array of large-scale and highly-sensitive interferometric detectors constantly scrutinizes the geometry of the local space-time with the hope to detect deviations that would signal an impinging gravitational wave from a remote astrophysical source. Finding the rare and weak signature of gravitational waves buried in non-stationary and non-Gaussian instrument noise is a particularly challenging problem. We will give an overview of the data-analysis techniques and associated observational results obtained so far by Virgo (in Europe) and LIGO (in the US), along with the prospects offered by the up-coming advanced versions of those detectors.

abs pdf

Oct 30, 2012

1210.7173 (/preprints)
2012-10-30, 11:46 [edit]


[1210.6983] Fake plunges are very eccentric real EMRIs in disguise ... they dominate the rates and are blissfully ignorant of angular momentum barriers

Authors: Pau Amaro-Seoane, Carlos Sopuerta, Patrick Brem

Date: 25 Oct 2012

Abstract: The capture of a compact object in a galactic nucleus by a massive black hole (MBH) is the best way to map space and time around it. It is well established that the event rate of stars kicked directly through the horizon (referred to as direct plunges) is much larger than the gradual inspiral due to the emission of gravitational waves. We prove that it is actually very difficult to get a compact object such a stellar black hole to be swallowed whole. A plunge will most likely be deflected into an EMRI orbit. They are simply very eccentric EMRIs and dominate the event rate. Moreover, if the central MBH is spinning, the net result on the rates is an enhancement, on both kinds of EMRIs. On the other hand, recent work on stellar dynamics has demonstrated that there seems to be a conspiracy in phase space, since rates decrease significantly by the presence of a blockade in the rate at which orbital angular momenta change takes place. This so-called "Schwarzschild barrier" is a result of the impact of relativistic precession on to the stellar potential torques and was first investigated by Merritt and collaborators. We confirm and quantify the existence of this barrier using a statistical sample of 2,500 direct-summation N-body simulations using both a post-Newtonian but also, and for the first time in a direct-summation integrator, a geodesic approximation for the relativistic orbits. Although the existence of the barrier prevents "traditional EMRIs" (i.e. EMRIs which are not very eccentric) from approaching the central MBH, very eccentric EMRIs, wrongly classified as plunges, insolently ignore the presence of the barrier. The combinations of these effects leads to the result that very eccentric orbits will dominate the rates.

abs pdf

Oct 30, 2012

1210.6983 (/preprints)
2012-10-30, 11:45 [edit]


[1210.7293] Creep events and creep noise in gravitational-wave interferometers: basic formalism and stationary limit

Authors: Yuri Levin (Monash University and Leiden Observatory)

Date: 27 Oct 2012

Abstract: In gravitational-wave interferometers, test masses are suspended on thin fibers which experience considerable tension stress. Sudden microscopic stress release in a suspension fiber, which I call a 'creep event', would excite motion of the test mass that would be coupled to the interferometer's readout. The random test-mass motion due to a time-sequence of creep events is referred to as 'creep noise'. In this paper I present an elasto-dynamic calculation for the test-mass motion due to a creep event. I show that within a simple suspension model, the main coupling to the optical readout occurs via a combination of a "dc" horizontal displacement of the test mass, and excitation of the violin and pendulum modes, and not, as was thought previously, via lengthening of the fiber. When the creep events occur sufficiently frequently and their statistics is time-independent, the creep noise can be well-approximated by a stationary Gaussian random process. I derive the functional form of the creep noise spectral density in this limit, with the restrictive assumption that the creep events are statistically independent from each other.

abs pdf

Oct 30, 2012

1210.7293 (/preprints)
2012-10-30, 11:44 [edit]


[1210.7518] Prospects for probing strong gravity with a pulsar-black hole system

Authors: N. Wex, K. Liu, R. P. Eatough, M. Kramer, J. M. Cordes, T. J. W. Lazio

Date: 28 Oct 2012

Abstract: The discovery of a pulsar (PSR) in orbit around a black hole (BH) is expected to provide a superb new probe of relativistic gravity and BH properties. Apart from a precise mass measurement for the BH, one could expect a clean verification of the dragging of space-time caused by the BH spin. In order to measure the quadrupole moment of the BH for testing the no-hair theorem of general relativity (GR), one has to hope for a sufficiently massive BH. In this respect, a PSR orbiting the super-massive BH in the center of our Galaxy would be the ultimate laboratory for gravity tests with PSRs. But even for gravity theories that predict the same properties for BHs as GR, a PSR-BH system would constitute an excellent test system, due to the high grade of asymmetry in the strong field properties of these two components. Here we highlight some of the potential gravity tests that one could expect from different PSR-BH systems, utilizing present and future radio telescopes, like FAST and SKA.

abs pdf

Oct 30, 2012

1210.7518 (/preprints)
2012-10-30, 11:44 [edit]


[1210.7388] Fifth ASTROD Symposium and Outlook of Direct Gravitational-Wave Detection

Authors: K.G. Arun, Bala R. Iyer, Wei-Tou Ni

Date: 28 Oct 2012

Abstract: From July 11 to July 13, 2012, Raman Research Institute (Bangalore, India) hosted the Fifth International ASTROD Symposium on Laser Astrodynamics, Space Test of Relativity and Gravitational-Wave Astronomy. This is a report on the symposium with an exposition of the outlook of direct gravitational-wave detection.

abs pdf

Oct 30, 2012

1210.7388 (/preprints)
2012-10-30, 11:44 [edit]


[1210.6807] Gravitational waveforms for black hole binaries with unequal masses

Authors: Márton Tápai, Zoltán Keresztes, László Árpád Gergely

Date: 25 Oct 2012

Abstract: We derived a post-Newtonian (PN) inspiral only gravitational waveform for unequal mass, spinning black hole binaries. Towards the end of the inspiral the larger spin dominates over the orbital angular momentum (while the smaller spin is negligible), hence the name Spin-Dominated Waveforms (SDW). Such systems are common sources for future gravitational wave detectors and during the inspiral the largest amplitude waves are emitted exactly in its last part. The SDW waveforms emerge as a double expansion in the PN parameter and the ratio of the orbital angular momentum to the dominant spin.

abs pdf

Oct 26, 2012

1210.6807 (/preprints)
2012-10-26, 08:31 [edit]


[1210.6666] An effectual template bank for the detection of gravitational waves from inspiralling compact binaries with generic spins

Authors: P. Ajith, N. Fotopoulos, S. Privitera, A. Neunzert, A. J. Weinstein

Date: 24 Oct 2012

Abstract: We report the construction of a three-dimensional template bank for the search for gravitational waves from inspiralling binaries consisting of spinning compact objects. The parameter space consists of two dimensions describing the mass parameters and one "reduced-spin parameter", which describes the secular (non-precessing) spin effects in the waveform. The template placement is based on an efficient stochastic algorithm and makes use of the semi-analytical computation of a metric in the parameter space. We demonstrate that such a template family is "effectual" (effective fitting factor ~ 0.93--0.99) for the detection of generic spinning "low-mass" (m_1 + m_2 <~ 12 M_sun) binaries in the advanced detector era, over the entire parameter space of interest (including binary neutron stars, binary black holes, and black hole-neutron star binaries). This provides a powerful and practical method for searching for gravitational waves from generic spinning low-mass compact binaries. The expected improvement in the average detection volume (at a fixed signal-to-noise-ratio threshold) of this spinning search, as compared to a non-spinning search is ~19--58%.

abs pdf

Oct 26, 2012

1210.6666 (/preprints)
2012-10-26, 08:30 [edit]


[1210.6674] General-Relativistic Simulations of Three-Dimensional Core-Collapse Supernovae

Authors: C. D. Ott (1), E. Abdikamalov (1), P. Moesta (1), R. Haas (1), S. Drasco (1,2), E. O&#x27;Connor (3), C. Reisswig (1), C. Meakin (4), E. Schnetter (5) ((1) TAPIR, Caltech, (2) Grinnell College, (3) CITA, (4) Theoretical Division, LANL, (5) Perimeter Institute)

Date: 24 Oct 2012

Abstract: We study the three-dimensional (3D) hydrodynamics of the post-core-bounce phase of the collapse of a 27 solar-mass star and pay special attention to the development of the standing accretion shock instability (SASI) and neutrino-driven convection. To this end, we perform 3D general-relativistic simulations with a 3-species neutrino leakage scheme with neutrino heating. Unlike "light-bulb" heating/cooling schemes, the leakage scheme captures the essential aspects of neutrino cooling, heating, and lepton number exchange as predicted by radiation-hydrodynamics simulations. The 27 solar-mass progenitor was studied in 2D by B. Mueller et al. (2012; arXiv:1205.7078), who observed strong growth of the SASI while neutrino-driven convection was suppressed. In our 3D simulations, neutrino-driven convection grows from numerical perturbations imposed by our Cartesian grid. It becomes the dominant instability and leads to large-scale non-oscillatory deformations of the shock front. These will result in strongly aspherical explosions without the need for large-scale SASI shock oscillations. Low-l-mode SASI oscillations are present in our models, but saturate at small amplitudes that decrease with increasing neutrino heating and vigor of convection. Our results suggest that once neutrino-driven convection is started, it is likely to become the dominant instability in 3D. Whether it is the primary instability after bounce will ultimately depend on the physical seed perturbations present in the cores of massive stars. The gravitational wave signal, which we extract and analyze for the first time from 3D general-relativistic models, will serve as an observational probe of the postbounce dynamics and, in combination with neutrinos, may allow us to determine the primary hydrodynamic instability.

abs pdf

Oct 26, 2012

1210.6674 (/preprints)
2012-10-26, 08:30 [edit]


[1210.2396] Resolving multiple supermassive black hole binaries with pulsar timing arrays II: genetic algorithm implementation

Authors: Antoine Petiteau, Stanislav Babak, Alberto Sesana, Mariana de Araujo

Date: 8 Oct 2012

Abstract: Pulsar timing arrays (PTAs) might detect gravitational waves (GWs) from massive black hole (MBH) binaries within this decade. The signal is expected to be an incoherent superposition of several nearly-monochromatic waves of different strength. The brightest sources might be individually resolved, and the overall deconvolved, at least partially, in its individual components. In this paper we extend the maximum-likelihood based method developed in Babak & Sesana 2012, to search for individual MBH binaries in PTA data. We model the signal as a collection of circular monochromatic binaries, each characterized by three free parameters: two angles defining the sky location, and the frequency. We marginalize over all other source parameters and we apply an efficient multi-search genetic algorithm to maximize the likelihood function and look for sources in synthetic datasets. On datasets characterized by white Gaussian noise plus few injected sources with signal-to-noise ratio (SNR) in the range 10-60, our search algorithm performs well, recovering all the injections with no false positives. Individual source SNRs are estimated within few % of the injected values, sky locations are recovered within few degrees, and frequencies are determined with sub-Fourier bin precision.

abs pdf

Oct 25, 2012

1210.2396 (/preprints)
2012-10-25, 14:25 [edit]


[1210.6362] Identifying Elusive Electromagnetic Counterparts to Gravitational Wave Mergers: an end-to-end simulation

Authors: Samaya Nissanke, Mansi Kasliwal, Alexandra Georgieva

Date: 23 Oct 2012

Abstract: Combined gravitational-wave (GW) and electromagnetic (EM) observations of compact binary mergers should enable detailed studies of astrophysical processes in the strong-field gravity regime. Networks of GW interferometers have poor angular resolution on the sky and their EM signatures are predicted to be faint. Therefore, a challenging goal will be to unambiguously pinpoint the EM counterparts to GW mergers. We perform the first comprehensive end-to-end simulation that focuses on: i) GW sky localization, distance measures and volume errors with two compact binary populations and four different GW networks, ii) subsequent detectability by a slew of multiwavelength telescopes and, iii) final identification of the merger counterpart amidst a sea of possible astrophysical false-positives. First, we find that double neutron star (NS) binary mergers can be detected out to a maximum distance of 400 Mpc (or 750 Mpc) by three (or five) detector GW networks respectively. NS -- black-hole (BH) mergers can be detected a factor of 1.5 further out. The sky localization uncertainties for NS-BH mergers are 50--170 sq. deg. (or 6--65 sq. deg.) for a three (or five detector) GW network respectively. Second, we quantify relative fractions of optical counterparts that are detectable by different size telescopes. Third, we present five case studies to illustrate the diversity of challenges in secure identification of the EM counterpart at low and high Galactic latitudes. For the first time, we demonstrate how construction of low-latency GW volumes in conjunction with local universe galaxy catalogs can help solve the problem of false positives.

abs pdf

Oct 24, 2012

1210.6362 (/preprints)
2012-10-24, 18:55 [edit]


[1210.6014] Weighing The Evidence For A Gravitational-Wave Background In The First International Pulsar Timing Array Data Challenge

Authors: Stephen R. Taylor, Jonathan R. Gair, L. Lentati

Date: 22 Oct 2012

Abstract: We describe an analysis of the First International Pulsar Timing Array Data Challenge. We employ a robust, unbiased Bayesian framework developed by van Haasteren to study the three Open and Closed datasets, testing various models for each dataset and using MultiNest to recover the evidence for the purposes of Bayesian model-selection. The parameter constraints of the favoured model are confirmed using an adaptive MCMC technique. Our results for Closed1 favoured a gravitational-wave background with strain amplitude at f=1 yr-1, A, of (1.1 +/- 0.1) x 10ˆ{-14}, power spectral-index gamma=4.30 +/- 0.15 and no evidence for red-timing noise or single-sources. The evidence for Closed2 favours a gravitational-wave background with A=(6.1 +/- 0.3) x 10ˆ{-14}, gamma=4.34 +/- 0.09 with no red-timing noise or single-sources. Finally, the evidence for Closed3 favours the presence of red-timing noise and a gravitational-wave background, with no single-sources. The properties of the background were A=(5 +/- 1) x 10ˆ{-15} and gamma=4.23 +/- 0.35, while the properties of the red-noise were N_{red}=(12 +/- 4) ns and gamma_{red}=1.5 +/- 0.3. In all cases the redness of the recovered background is consistent with a source-population of inspiraling supermassive black-hole binaries. We also investigate the effect that down-sampling of the datasets has on parameter constraints and run-time. Finally we provide a proof-of-principle study of the ability of the Bayesian framework used in this paper to reconstruct the angular correlation of gravitational-wave background induced timing-residuals, comparing this to the Hellings and Downs curve.

abs pdf

Oct 23, 2012

1210.6014 (/preprints)
2012-10-23, 20:15 [edit]


[1210.5998] Why Gravitational Wave Science Needs Pulsar Timing Arrays And Why Pulsar Timing Arrays Need Both Arecibo and the GBT: A Response to the NSF-AST Portfolio Review from the NANOGrav Collaboration

Authors: The NANOGrav Collaboration

Date: 22 Oct 2012

Abstract: Gravitational waves (GWs) are ripples in space-time that are known to exist but have not yet been detected directly. Once they are, a key feature of any viable theory of gravity will be demonstrated and a new window on the Universe opened. GW astronomy was named as one of five key discovery areas in the New Worlds, New Horizons Decadal Report. Pulsar timing probes GW frequencies, and hence source classes, that are inaccessible to any other detection method and can uniquely constrain the nonlinear nature of General Relativity. Pulsar timing is therefore a critical capability with its own discovery space and potential. Fulfilling this capability requires the complementary enabling features of both the Green Bank Telescope (GBT) and the Arecibo Observatory.

abs pdf

Oct 23, 2012

1210.5998 (/preprints)
2012-10-23, 20:15 [edit]


[1210.6091] Gravitational wave emission and spindown of young pulsars

Authors: Mark G. Alford, Kai Schwenzer

Date: 23 Oct 2012

Abstract: The rotation frequencies of young pulsars are systematically below their theoretical Kepler limit. R-modes have been suggested as a possible explanation for this observation. With the help of semi-analytic expressions that make it possible to assess the uncertainties of the r-mode scenario due to the impact of uncertainties in underlying microphysics, we perform a quantitative analysis of the the spin-down and the emitted gravitational waves of young pulsars. We find that the frequency to which r-modes spin down a young neutron star as well as the characteristic gravitational wave strain amplitude are extremely insensitive both to the microscopic details and the saturation amplitude. Comparing our result to astrophysical data, we show that for a range of saturation amplitudes r-modes provide a viable spindown scenario and that all observed young pulsars are very likely already outside the r-mode instability region. Taking into account the finite observation time, we find that the signal to noise ratio for gravitational waves is smaller than previous estimates, but large enough to detect such sources with next generation detectors.

abs pdf

Oct 23, 2012

1210.6091 (/preprints)
2012-10-23, 19:59 [edit]


[1210.4159] Stellar transits in active galactic nuclei

Authors: Bence B&#xe9;ky, Bence Kocsis (Harvard)

Date: 15 Oct 2012

Abstract: Supermassive black holes (SMBH) are typically surrounded by a dense stellar population in galactic nuclei. Stars crossing the line of site in active galactic nuclei (AGN) produce a characteristic transit lightcurve, just like extrasolar planets do when they transit their host star. We examine the possibility of finding such AGN transits in deep optical, UV, and X-ray surveys. We calculate transit lightcurves using the Novikov--Thorne thin accretion disk model, including general relatistic effects. Based on the expected properties of stellar cusps, we find that around 10ˆ6 solar mass SMBHs, transits of red giants are most common for stars on close orbits with transit durations of a few weeks and orbital periods of a few years. We find that detecting AGN transits requires repeated observations of thousands of low mass AGNs to 1% photometric accuracy in optical, or ~ 10% in UV bands or soft X-ray. It may be possible to identify stellar transits in the Pan-STARRS and LSST optical and the eROSITA X-ray surveys. Such observations could be used to constrain black hole mass, spin, inclination and accretion rate. Transit rates and durations could give valuable information on the circumnuclear stellar clusters as well. Transit lightcurves could be used to image accretion disks with unprecedented resolution, allowing to resolve the SMBH silhouette in distant AGNs.

abs pdf

Oct 17, 2012

1210.4159 (/preprints)
2012-10-17, 12:27 [edit]


[1210.2339] Post-Newtonian prediction for the (2,2) mode of the gravitational wave emitted by compact binaries

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

Date: 8 Oct 2012

Abstract: We present our 3.5PN computation of the (2,2) mode of the gravitational wave amplitude emitted by compact binaries, on quasi-circular orbits and in the absence of spins. We use the multipolar post-Newtonian wave generation formalism, extending and building on previous works which computed the 3PN order. This calculation required the extension of the multipolar post-Minkowskian algorithm, as well as the computation of the mass source quadrupole at 3.5PN order. Our result will allow more accurate comparisons to numerical relativity, and is a first step towards the computation of the full 3.5PN waveform amplitude, which would improve the estimation of the source parameters by gravitational wave detectors.

abs pdf

Oct 15, 2012

1210.2339 (/preprints)
2012-10-15, 22:58 [edit]


[1210.1891] Impact of Higher-order Modes on the Detection of Binary Black Hole Coalescences

Authors: Larne Pekowsky, James Healy, Deirdre Shoemaker, Pablo Laguna

Date: 5 Oct 2012

Abstract: The inspiral and merger of black-hole binary systems are a promising source of gravitational waves. The most effective method to look for a signal with a well understood waveform, such as the binary black hole signal, is matched filtering against a library of model waveforms. Current model waveforms are comprised solely of the dominant radiation mode, the quadrupole mode, although it is known that there can be significant power in the higher-order modes. The binary black hole waveforms produced by numerical relativity are accurate through late inspiral, merger, and ringdown and include the higher-order modes. The available numerical-relativity waveforms span an increasing portion of the physical parameter space of unequal mass, spin and precession. In this paper, we investigate the degree to which gravitational-wave searches could be improved by the inclusion of higher modes in the model waveforms, for signals with a variety of initial mass ratios and generic spins. Our investigation studies how well the quadrupole-only waveform model matches the signal as a function of the inclination and orientation of the source and how the modes contribute to the distance reach into the Universe of Advanced LIGO for a fixed set of internal source parameters. The mismatch between signals and quadrupole-only waveform can be large, dropping below 0.97 for up to 65% of the source-sky for the non-precessing cases we studied, and over a larger area in one precessing case. There is a corresponding 30% increase in detection volume that could be achieved by adding higher modes to the search; however, this is mitigated by the fact that the mismatch is largest for signals which radiate the least energy and to which the search is therefore least sensitive. Likewise, the mismatch is largest in directions from the source along which the least energy is radiated.

abs pdf

Oct 15, 2012

1210.1891 (/preprints)
2012-10-15, 22:57 [edit]


[1210.2569] Impact of the second order self-forces on the dephasing of the gravitational waves from quasi-circular extreme mass-ratio inspirals

Authors: Soichiro Isoyama, Ryuichi Fujita, Norichika Sago, Hideyuki Tagoshi, Takahiro Tanaka

Date: 9 Oct 2012

Abstract: The accurate calculation of long-term phase evolution of gravitational wave (GW) forms from extreme (intermediate) mass ratio inspirals (E(I)MRIs) is an inevitable step to extract information from this system. Achieving this goal, it is believed that we need to understand the gravitational self-forces. However, it is not quntatively demonstrated that the second order self-forces are necessary for this purpose. In this paper we revisit the problem to estimate the order of magnitude of the dephasing caused by the second order self-forces on a small body in a quasi-circular orbit around a Kerr black hole, based on the knowledge of the post-Newtonian (PN) approximation and invoking the energy balance argument. In particular, we focus on the averaged dissipative part of the self-force, since it gives the leading order contribution among the various components of them. To avoid the possibility that the energy flux of GWs becomes negative, we propose a new simple resummation called exponential resummation, which assures the positivity of the energy flux. In order to estimate the magnitude of the yet unknown second order self-forces, here we point out the scaling property in the absolute value of the PN coefficients of the energy flux. Using these new tools, we evaluate the expected magnitude of dephasing. Our analysis indicates that the dephasing due to the second order self-forces for quasi-circular E(I)MRIs may be well captured by the 3PN energy flux, once we obtain all the spin dependent terms, except for the case with an extremely large spin of the central Kerr black.

abs pdf

Oct 15, 2012

1210.2569 (/preprints)
2012-10-15, 22:56 [edit]


[1210.3007] Suitability of hybrid gravitational waveforms for unequal-mass binaries

Authors: Ilana MacDonald, Abdul H. Mroue, Harald P. Pfeiffer, Michael Boyle, Lawrence E. Kidder, Mark A. Scheel, Bela Szilagyi, Nicholas W. Taylor

Date: 10 Oct 2012

Abstract: This article studies sufficient accuracy criteria of hybrid post-Newtonian (PN) and numerical relativity (NR) waveforms for parameter estimation of strong binary black-hole sources in second- generation ground-based gravitational-wave detectors. We investigate equal-mass non-spinning binaries with a new 33-orbit NR waveform, as well as unequal-mass binaries with mass ratios 2, 3, 4 and 6. For equal masses, the 33-orbit NR waveform allows us to recover previous results and to extend the analysis toward matching at lower frequencies. For unequal masses, the errors between different PN approximants increase with mass ratio. Thus, at 3.5PN, hybrids for higher-mass-ratio systems would require NR waveforms with many more gravitational-wave (GW) cycles to guarantee no adverse impact on parameter estimation. Furthermore, we investigate the potential improvement in hybrid waveforms that can be expected from 4th order post-Newtonian waveforms, and find that knowledge of this 4th post-Newtonian order would significantly improve the accuracy of hybrid waveforms.

abs pdf

Oct 15, 2012

1210.3007 (/preprints)
2012-10-15, 22:56 [edit]


[1210.2829] Forecast constraints on cosmic strings from future CMB, pulsar timing and gravitational wave direct detection experiments

Authors: Sachiko Kuroyanagi, Koichi Miyamoto, Toyokazu Sekiguchi, Keitaro Takahashi, Joseph Silk

Date: 10 Oct 2012

Abstract: We study future observational constraints on cosmic string parameters from various types of next-generation experiments: direct detection of gravitational waves (GWs), pulsar timing array, and the cosmic microwave background (CMB). We consider both GW burst and stochastic GW background searches by ground- and space-based interferometers as well as GW background detection in pulsar timing experiments. We also consider cosmic string contributions to the CMB temperature and polarization anisotropies. These different types of observations offer independent probes of cosmic strings and may enable us to investigate cosmic string properties if the signature is detected. In this paper, we evaluate the power of future experiments to constrain cosmic string parameters, such as the string tension Gmu, the initial loop size alpha, and the reconnection probability p, by performing Fisher information matrix calculations. We find that combining the information from the different types of observations breaks parameter degeneracies and provides more stringent constraints on the parameters. We also find future space-borne interferometers independently provide a highly precise determination of the parameters.

abs pdf

Oct 15, 2012

1210.2829 (/preprints)
2012-10-15, 22:55 [edit]


[1210.2745] The classical mechanics of non-conservative systems

Authors: Chad R. Galley

Date: 9 Oct 2012

Abstract: Hamilton's principle of stationary action lies at the foundation of theoretical physics and is applied in many other disciplines from pure mathematics to economics. Despite its utility, Hamilton's principle has a subtle pitfall that often goes unnoticed in physics: it is formulated as a boundary value problem in time but is used to derive equations of motion that are solved with initial data. This subtlety can have undesirable effects. I present a formulation of Hamilton's principle that is compatible with initial value problems. Remarkably, this leads to a natural formulation for the Lagrangian and Hamiltonian dynamics of generic non-conservative systems, thereby filling a long-standing gap in classical mechanics. Thus dissipative effects, for example, can be studied with new tools that may have application in a variety of disciplines. The new formalism is demonstrated by two examples of non-conservative systems: an object moving in a fluid with viscous drag forces and a harmonic oscillator coupled to a dissipative environment.

abs pdf

Oct 15, 2012

1210.2745 (/preprints)
2012-10-15, 22:55 [edit]


[1210.2834] Gravitational radiation reaction along general orbits in the effective one-body formalism

Authors: Donato Bini, Thibault Damour

Date: 10 Oct 2012

Abstract: We derive the gravitational radiation-reaction force modifying the Effective One Body (EOB) description of the conservative dynamics of binary systems. Our result is applicable to general orbits (elliptic or hyperbolic) and keeps terms of fractional second post-Newtonian order (but does not include tail effects). Our derivation of radiation-reaction is based on a new way of requiring energy and angular momentum balance. We give several applications of our results, notably the value of the (minimal) "Schott" contribution to the energy, the radial component of the radiation-reaction force, and the radiative contribution to the angle of scattering during hyperbolic encounters. We present also new results about the conservative relativistic dynamics of hyperbolic motions.

abs pdf

Oct 15, 2012

1210.2834 (/preprints)
2012-10-15, 22:55 [edit]


[1210.3095] Outlook for detection of GW inspirals by GRB-triggered searches in the advanced detector era

Authors: Alexander Dietz, Nickolas Fotopoulos, Leo Singer, Curt Cutler

Date: 11 Oct 2012

Abstract: Short GRBs are believed to originate from the coalescence of two NSs or a NS and a BH. If this scenario is correct, short GRBs will be accompanied by the emission of strong GWs, detectable by GW observatories such as LIGO and Virgo. As compared with blind, all-sky, all-time GW searches, externally triggered searches for GW counterparts to short GRBs have the advantages of both significantly reduced detection threshold due to known time and sky location and preferentially higher GW amplitude because of face-on orientation. Based on the distribution of SNR in candidate CBC events in the most recent joint LIGO--Virgo data, we find an effective sensitive volume for GRB-triggered searches that ~2 times greater than for blind searches. For NSNS systems, a jet angle of 20 degrees, and an effective gamma-ray satellite field-of-view of 10% of the sky, this doubles the number of NSNS--short GRB associations to 4% of all NSNS detections. We also investigate the power of tests for statistical excesses in lists of sub-threshold events, and show that these are unlikely to reveal a sub-threshold population until finding individual sources is already routine.

abs pdf

Oct 15, 2012

1210.3095 (/preprints)
2012-10-15, 22:54 [edit]


[1210.2778] Observing the Galaxy's massive black hole with gravitational wave bursts

Authors: C. P. L. Berry, J. R. Gair

Date: 9 Oct 2012

Abstract: An extreme-mass-ratio burst (EMRB) is a gravitational wave signal emitted when a compact object passes through periapsis on a highly eccentric orbit about a much more massive object, in our case a stellar mass object about a 10ˆ6 M_sol black hole. EMRBs are a relatively unexplored means of probing the spacetime of massive black holes (MBHs). We conduct an investigation of the properties of EMRBs and how they could allow us to constrain the parameters, such as spin, of the Galaxy's MBH. We find that if an EMRB event occurs in the Galaxy, it should be detectable for periapse distances r_p < 65 r_g for a \mu = 10 M_sol orbiting object, where r_g = GM/cˆ2 is the gravitational radius. The signal-to-noise ratio scales as \rho ~ -2.7 log(r_p/r_g) + log(\mu/M_sol) + 4.9. For periapses r_p < 10 r_g, EMRBs can be informative, and provide good constraints on both the MBH's mass and spin. Closer orbits provide better constraints, with the best giving accuracies of better than one part in 10ˆ4 for both the mass and spin parameter.

abs pdf

Oct 15, 2012

1210.2778 (/preprints)
2012-10-15, 22:54 [edit]


[1210.3374] Consequences of Strong Compression in Tidal Disruption Events

Authors: Nicholas Stone, Re&#x27;em Sari, Abraham Loeb

Date: 11 Oct 2012

Abstract: The tidal disruption of a star by a supermassive black hole (SMBH) is a highly energetic event with consequences dependent on the degree to which the star plunges inside the SMBH's tidal sphere. We introduce a new analytic model for tidal disruption events (TDEs) to analyze the dependence of these events on beta, the ratio of the tidal radius to the orbital pericenter. We find, contrary to most previous work, that the spread in debris energy for a TDE is largely constant for all beta. This result has important consequences for optical transient searches targeting TDEs, which we discuss. We quantify leading-order general relativistic corrections to this spread in energy and find that they are small. We also examine the role of stellar spin, and find that a combination of spin-orbit misalignment, rapid rotation, and high beta may increase the spread in debris energy. Finally, we quantify for the first time the gravitational wave emission due to the strong compression of a star in a high-beta TDE. Although this signal is unlikely to be detectable for disruptions of main sequence stars, the tidal disruption of a white dwarf by an intermediate mass black hole can produce a strong signal visible to Advanced LIGO at tens of megaparsecs.

abs pdf

Oct 15, 2012

1210.3374 (/preprints)
2012-10-15, 22:53 [edit]


[1210.3489] Using Swarm Intelligence To Accelerate Pulsar Timing Analysis

Authors: Stephen R. Taylor, Jonathan R. Gair, L. Lentati

Date: 12 Oct 2012

Abstract: We provide brief notes on a particle swarm-optimisation approach to constraining the properties of a stochastic gravitational-wave background in the first International Pulsar Timing Array data-challenge. The technique employs many computational-agents which explore parameter space, remembering their most optimal positions and also sharing this information with all other agents. It is this sharing of information which accelerates the convergence of all agents to the global best-fit location in a very short number of iterations. Error estimates can also be provided by fitting a multivariate Gaussian to the recorded fitness of all visited points.

abs pdf

Oct 15, 2012

1210.3489 (/preprints)
2012-10-15, 22:53 [edit]


[1210.3402] Constraining the High-Density Behavior of Nuclear Symmetry Energy with the Tidal Polarizability of Neutron Stars

Authors: F. J. Fattoyev, J. Carvajal, W. G. Newton, Bao-An Li

Date: 12 Oct 2012

Abstract: Using a set of model equations of state satisfying the latest constraints from both terrestrial nuclear experiments and astrophysical observations as well as state-of-the-art nuclear many-body calculations of the pure neutron matter equation of state, the tidal polarizability of canonical neutron stars in coalescing binaries is found to be a very sensitive probe of the high-density behavior of nuclear symmetry energy which is among the most uncertain properties of dense neutron-rich nucleonic matter. Moreover, it changes less than $\pm 10%$ by varying various properties of symmetric nuclear matter and symmetry energy around the saturation density within their respective ranges of remaining uncertainty.

abs pdf

Oct 15, 2012

1210.3402 (/preprints)
2012-10-15, 22:53 [edit]


[1210.3770] Can we see pulsars around Sgr A*? - The latest searches with the Effelsberg telescope

Authors: R. P. Eatough, M. Kramer, B. Klein, R. Karuppusamy, D. J. Champion, P. C. C. Freire, N. Wex, K. Liu

Date: 14 Oct 2012

Abstract: Radio pulsars in relativistic binary systems are unique tools to study the curved space-time around massive compact objects. The discovery of a pulsar closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr A*, would provide a superb test-bed for gravitational physics. To date, the absence of any radio pulsar discoveries within a few arc minutes of Sgr A* has been explained by one principal factor: extreme scattering of radio waves caused by inhomogeneities in the ionized component of the interstellar medium in the central 100 pc around Sgr A*. Scattering, which causes temporal broadening of pulses, can only be mitigated by observing at higher frequencies. Here we describe recent searches of the Galactic centre region performed at a frequency of 18.95 GHz with the Effelsberg radio telescope.

abs pdf

Oct 15, 2012

1210.3770 (/preprints)
2012-10-15, 22:33 [edit]


[1210.4143] Next-to-next-to-leading order spin-orbit effects in the equations of motion of compact binary systems

Authors: Sylvain Marsat, Alejandro Bohe, Guillaume Faye, Luc Blanchet

Date: 15 Oct 2012

Abstract: We compute next-to-next-to-leading order spin contributions to the post-Newtonian equations of motion for binaries of compact objects, such as black holes or neutron stars. For maximally spinning black holes, those contributions are of third-and-a-half post-Newtonian (3.5PN) order, improving our knowledge of the equations of motion, already known for non-spinning objects up to this order. Building on previous work, we represent the rotation of the two bodies using a pole-dipole matter stress-energy tensor, and iterate Einstein's field equations for a set of potentials parametrizing the metric in harmonic coordinates. Checks of the result include the existence of a conserved energy, the approximate global Lorentz invariance of the equations of motion in harmonic coordinates, and the recovery of the motion of a spinning object on a Kerr background in the test-mass limit. We verified the existence of a contact transformation, together with a redefinition of the spin variables that makes our result equivalent to a previously published reduced Hamiltonian, obtained from the Arnowitt-Deser-Misner (ADM) formalism.

abs pdf

Oct 15, 2012

1210.4143 (/preprints)
2012-10-15, 22:33 [edit]


[1210.3854] Does a 'stochastic' background of gravitational waves exist in the pulsar timing band?

Authors: V. Ravi, J. S. B. Wyithe, G. Hobbs, R. M. Shannon, R. N. Manchester, D. R. B. Yardley, M. J. Keith

Date: 14 Oct 2012

Abstract: We investigate the effects of gravitational waves (GWs) from a simulated population of binary super-massive black holes (SMBHs) on pulsar timing array datasets. We construct a distribution describing the binary SMBH population from an existing semi-analytic galaxy formation model. Using realizations of the binary SMBH population generated from this distribution, we simulate pulsar timing datasets with GW-induced variations. We find that the statistics of these variations do not correspond to an isotropic, stochastic GW background. The "Hellings & Downs" correlations between simulated datasets for different pulsars are recovered on average, though the scatter of the correlation estimates is greater than expected for an isotropic, stochastic GW background. These results are attributable to the fact that just a few GW sources dominate the GW-induced variations in every Fourier frequency bin of a 5-year dataset. Current constraints on the amplitude of the GW signal from binary SMBHs will be biased. Individual binary systems are likely to be detectable in 5-year pulsar timing array datasets where the noise is dominated by GW-induced variations. Searches for GWs in pulsar timing array data therefore need to account for the effects of individual sources of GWs.

abs pdf

Oct 15, 2012

1210.3854 (/preprints)
2012-10-15, 22:32 [edit]


[1210.1294] The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole

Authors: L. Meyer, A. M. Ghez, R. Schoedel, S. Yelda, A. Boehle, J. R. Lu, T. Do, M. R. Morris, E. E. Becklin, K. Matthews

Date: 4 Oct 2012

Abstract: Stars with short orbital periods at the center of our galaxy offer a powerful and unique probe of a supermassive black hole. Over the past 17 years, the W. M. Keck Observatory has been used to image the Galactic center at the highest angular resolution possible today. By adding to this data set and advancing methodologies, we have detected S0-102, a star orbiting our galaxy's supermassive black hole with a period of just 11.5 years. S0-102 doubles the number of stars with full phase coverage and periods less than 20 years. It thereby provides the opportunity with future measurements to resolve degeneracies in the parameters describing the central gravitational potential and to test Einstein's theory of General Relativity in an unexplored regime.

abs pdf

Oct 04, 2012

1210.1294 (/preprints)
2012-10-04, 19:07 [edit]


[1210.1333] Finite, Intense Accretion Bursts from Tidal Disruption of Stars on Bound Orbits

Authors: Kimitake Hayasaki, Nicholas Stone, Abraham Loeb

Date: 4 Oct 2012

Abstract: We study accretion processes for tidally disrupted stars approaching supermassive black holes on bound orbits, by performing three dimensional Smoothed Particle Hydrodynamics simulations with a pseudo-Newtonian potential. We find that there is a critical value of the orbital eccentricity below which all the stellar debris remains bound to the black hole. For high but sub-critical eccentricities, all the stellar mass is accreted onto the black hole in a finite time, causing a significant deviation from the canonical $tˆ{-5/3}$ mass fallback rate. When a star is on a moderately eccentric orbit and its pericenter distance is deeply inside the tidal disruption radius, there can be several orbit crossings of the debris streams due to relativistic precession. This dissipates orbital energy in shocks, allowing for rapid circularization of the debris streams and formation of an accretion disk. The resultant accretion rate greatly exceeds the Eddington rate and differs strongly from the canonical rate of $tˆ{-5/3}$. By contrast, there is little dissipation due to orbital crossings for the equivalent simulation with a purely Newtonian potential. This shows that general relativistic precession is crucial for accretion disk formation via circularization of stellar debris from stars on moderately eccentric orbits.

abs pdf

Oct 04, 2012

1210.1333 (/preprints)
2012-10-04, 18:36 [edit]


[1210.1214] The runaway instability in general-relativistic accretion disks

Authors: O. Korobkin (1), E. Abdikamalov (2), N. Stergioulas (3), E. Schnetter (4), B. Zink (5), S. Rosswog (1), C. Ott (2) ((1) Stockholm University, (2) Caltech, (3) Thessaloniki, (4) Perimeter Institute, (5) University of T&#xfc;bingen)

Date: 3 Oct 2012

Abstract: When an accretion disk falls prey to the runaway instability, a large portion of its mass is devoured by the black hole within a few dynamical times. Despite decades of effort, it is still unclear under what conditions such an instability can occur. The technically most advanced relativistic simulations to date were unable to find a clear sign for the onset of the instability. In this work, we present three-dimensional relativistic hydrodynamics simulations of accretion disks around black holes in dynamical spacetime. We focus on the configurations that are expected to be particularly prone to the development of this instability. We demonstrate, for the first time, that the fully self-consistent general relativistic evolution does indeed produce a runaway instability.

abs pdf

Oct 04, 2012

1210.1214 (/preprints)
2012-10-04, 18:36 [edit]


[1210.0577] Two-step greedy algorithm for reduced order quadratures

Authors: Harbir Antil, Scott E. Field, Frank Herrmann, Ricardo H. Nochetto, Manuel Tiglio

Date: 1 Oct 2012

Abstract: We present an algorithm to generate application-specific, global reduced order quadratures (ROQ) for multiple fast evaluations of weighted inner products between parameterized functions. If a reduced basis (RB) or any other projection-based model reduction technique is applied, the dimensionality of integrands is reduced dramatically; however, the cost of evaluating the reduced integrals still scales as the size of the original problem. In contrast, using discrete empirical interpolation (DEIM) points as ROQ nodes leads to a computational cost which depends linearly on the dimension of the reduced space. Generation of a reduced basis via a greedy procedure requires a training set, which for products of functions can be very large. Since this direct approach can be impractical in many applications, we propose instead a two-step greedy targeted towards approximation of such products. We present numerical experiments demonstrating the accuracy and the efficiency of the two-step approach. The presented ROQ are expected to display very fast convergence whenever there is regularity with respect to parameter variation. We find that for the particular application here considered, one driven by gravitational wave physics, the two-step approach speeds up the offline computations to build the ROQ by more than two orders of magnitude. Furthermore, the resulting ROQ rule converges exponentially with the number of nodes, and a factor of ~50 savings, without loss of accuracy, is observed in evaluations of inner products when ROQ are used as a downsampling strategy for equidistant samples using the trapezoidal rule. While the primary focus of this paper is on quadrature rules for inner products of parameterized functions, our method can be easily adapted to integrations of single parameterized functions, and some examples of this type are also considered.

abs pdf

Oct 04, 2012

1210.0577 (/preprints)
2012-10-04, 18:35 [edit]


[1210.0901] Two stellar-mass black holes in the globular cluster M22

Authors: Jay Strader (Michigan State), Laura Chomiuk (NRAO/Michigan State), Thomas Maccarone (Southampton), James Miller-Jones (ICRAR-Curtin), Anil Seth (Utah)

Date: 2 Oct 2012

Abstract: Hundreds of stellar-mass black holes likely form in a typical globular star cluster, with all but one predicted to be ejected through dynamical interactions. Some observational support for this idea is provided by the lack of X-ray-emitting binary stars comprising one black hole and one other star ("black-hole/X-ray binaries") in Milky Way globular clusters, even though many neutron-star/X-ray binaries are known. Although a few black holes have been seen in globular clusters around other galaxies, the masses of these cannot be determined, and some may be intermediate-mass black holes that form through exotic mechanisms. Here we report the presence of two flat-spectrum radio sources in the Milky Way globular cluster M22, and we argue that these objects are black holes of stellar mass (each ~ 10-20 times more massive than the Sun) that are accreting matter. We find a high ratio of radio-to-X-ray flux for these black holes, consistent with the larger predicted masses of black holes in globular clusters compared to those outside. The identification of two black holes in one cluster shows that the ejection of black holes is not as efficient as predicted by most models, and we argue that M22 may contain a total population of ~ 5-100 black holes. The large core radius of M22 could arise from heating produced by the black holes.

abs pdf

Oct 04, 2012

1210.0901 (/preprints)
2012-10-04, 09:29 [edit]


[1210.0764] Third post-Newtonian spin-orbit effect in the gravitational radiation flux of compact binaries

Authors: Luc Blanchet, Alessandra Buonanno, Guillaume Faye

Date: 2 Oct 2012

Abstract: Gravitational waves contain tail effects that are due to the backscattering of linear waves in the curved space-time geometry around the source. The knowledge as well as the accuracy of the two-body inspiraling post-Newtonian (PN) dynamics and of the gravitational-wave signal has been recently improved, notably by computing the spin-orbit (SO) terms induced by tail effects in the gravitational-wave energy flux at the 3PN order. Here we sketch this derivation, which yields the phasing formula including SO tail effects through the same 3PN order. Those results can be employed to improve the accuracy of analytical templates aimed at describing the whole process of inspiral, merger, and ringdown.

abs pdf

Oct 03, 2012

1210.0764 (/preprints)
2012-10-03, 09:47 [edit]


[1210.0893] Systematic biases in parameter estimation of binary black-hole mergers

Authors: Tyson B. Littenberg, John G. Baker, Alessandra Buonanno, Bernard J. Kelly

Date: 2 Oct 2012

Abstract: Parameter estimation of binary-black-hole merger events in gravitational-wave data relies on matched-filtering techniques, which, in turn, depend on accurate model waveforms. Here we characterize the systematic biases introduced in measuring astrophysical parameters of binary black holes by applying the currently most accurate effective-one-body templates to simulated data containing non-spinning numerical-relativity waveforms. For advanced ground-based detectors, we find that the systematic biases are well within the statistical error for realistic signal-to-noise ratio (SNR). These biases grow to be comparable to the statistical errors at high ground-based-instrument SNRs (SNR=50), but never dominate the error budget. At the much larger signal-to-noise ratios expected for space-based detectors, these biases will become large compared to the statistical errors, but for astrophysical black hole mass estimates the absolute biases (of at most a few percent) are still fairly small.

abs pdf

Oct 03, 2012

1210.0893 (/preprints)
2012-10-03, 09:47 [edit]


[1210.0584] Accelerating pulsar timing data analysis

Authors: Rutger van Haasteren (AEI Hannover)

Date: 1 Oct 2012

Abstract: The analysis of pulsar timing data, especially in pulsar timing array (PTA) projects, has encountered practical difficulties: evaluating the likelihood and/or correlation-based statistics can become prohibitively computationally expensive for large datasets. In situations where a stochastic signal of interest has a power spectral density that dominates the noise in a limited bandwidth of the total frequency domain (e.g. the isotropic background of gravitational waves), a linear transformation exists that transforms the timing residuals to a basis in which virtually all the information about the stochastic signal of interest is contained in a small fraction of basis vectors. By only considering such a small subset of these "generalised residuals", the dimensionality of the data analysis problem is greatly reduced, which can cause a large speedup in the evaluation of the likelihood: the ABC-method (Acceleration By Compression). The compression fidelity, calculable with crude estimates of the signal and noise, can be used to determine how far a dataset can be compressed without significant loss of information. Both direct tests on the likelihood, and Bayesian analysis of mock data, show that the signal can be recovered as well as with an analysis of uncompressed data. In the analysis of IPTA Mock Data Challenge datasets, speedups of a factor of three orders of magnitude are demonstrated. For realistic PTA datasets the acceleration may become greater than six orders of magnitude due to the low signal to noise ratio.

abs pdf

Oct 03, 2012

1210.0584 (/preprints)
2012-10-03, 09:46 [edit]


[1210.0483] A Metric for Testing the Nature of Black Holes

Authors: Tim Johannsen (Waterloo, Perimeter, Arizona)

Date: 1 Oct 2012

Abstract: In general relativity, astrophysical black holes are uniquely described by the Kerr metric. Observational tests of the Kerr nature of these compact objects and, hence, of general relativity, require a metric that encompasses a broader class of black holes as possible alternatives to the usual Kerr black holes. Several such Kerr-like metrics have been constructed to date, which depend on a set of free parameters and which reduce smoothly to the Kerr metric if all deviations vanish. Many of these metrics, however, are valid only for small values of the spin or small perturbations of the Kerr metric or contain regions of space where they are unphysical hampering their ability to properly model the accretions flows of black holes. In this paper, I describe a Kerr-like black hole metric that is regular everywhere outside of the event horizon for black holes with arbitrary spins even for large deviations from the Kerr metric. This metric, therefore, provides an ideal framework for tests of the nature of black holes with observations of the emission from their accretion flows, and I give several examples of such tests across the electromagnetic spectrum with current and near-future instruments.

abs pdf

Oct 01, 2012

1210.0483 (/preprints)
2012-10-01, 18:16 [edit]


[1210.0224] PSR J1141-6545: a powerful laboratory of GR and tensor-scalar theories of gravity

Authors: J. P. W. Verbiest, N. D. R. Bhat, M. Bailes

Date: 30 Sep 2012

Abstract: Pulsars in close binary systems have provided some of the most stringent tests of strong-field gravity to date. The pulsar--white-dwarf binary system J1141-6545 is specifically interesting due to its gravitational asymmetry which makes it one of the most powerful probes of tensor-scalar theories of gravity. We give an overview of current gravitational tests provided by the J1141-6545 binary system and comment on how anomalous accelerations, geodetic precession and timing instabilities may be prevented from limiting future tests of gravity to come from this system.

abs pdf

Oct 01, 2012

1210.0224 (/preprints)
2012-10-01, 18:15 [edit]


[1210.0201] The Effective Field Theory of Dark Energy

Authors: Giulia Gubitosi, Federico Piazza, Filippo Vernizzi

Date: 30 Sep 2012

Abstract: We propose a universal description of dark energy and modified gravity that includes all single-field models. By extending a formalism previously applied to inflation, we consider the metric universally coupled to matter fields and we write in terms of it the most general unitary gauge action consistent with the residual unbroken symmetries of spatial diffeomorphisms. Our action is particularly suited for cosmological perturbation theory: the background evolution depends on only three operators. All other operators start at least at quadratic order in the perturbations and their effects can be studied independently and systematically. In particular, we focus on the properties of a few operators which appear in non-minimally coupled scalar-tensor gravity and galileon theories. In this context, we study the mixing between gravity and the scalar degree of freedom. We assess the quantum and classical stability, derive the speed of sound of fluctuations and the renormalization of the Newton constant. The scalar can always be de-mixed from gravity at quadratic order in the perturbations, but not necessarily through a conformal rescaling of the metric. We show how to express covariant field-operators in our formalism and give several explicit examples of dark energy and modified gravity models in our language. Finally, we discuss the relation with the covariant EFT methods recently appeared in the literature.

abs pdf

Oct 01, 2012

1210.0201 (/preprints)
2012-10-01, 18:15 [edit]


[1210.0443] Ultrarelativistic black hole formation

Authors: William E. East, Frans Pretorius

Date: 1 Oct 2012

Abstract: We study the ultrarelativistic head-on collision of equal mass particles, modeled as self-gravitating fluid spheres, by numerically solving the coupled Einstein-hydrodynamic equations. We focus on cases well within the kinetic energy dominated regime, where between 88-92% ($\gamma=8$ to 12) of the initial net energy of the spacetime resides in the translation kinetic energy of the particles. We find that for sufficiently large boosts, black hole formation occurs. Moreover, near yet above the threshold of black hole formation, the collision initially leads to the formation of two distinct apparent horizons that subsequently merge. We argue that this can be understood in terms of a focusing effect, where one boosted particle acts as a gravitational lens on the other and vice versa, and that this is further responsible for the threshold being lower (by a factor of a few) compared to simple hoop conjecture estimates. Cases slightly below threshold result in complete disruption of the model particles. The gravitational radiation emitted when black holes form reaches luminosities of 0.014 $cˆ5/G$, carrying $16\pm2%$ of the total energy.

abs pdf

Oct 01, 2012

1210.0443 (/preprints)
2012-10-01, 18:15 [edit]


[1210.0156] Probing the strong gravity regime with eLISA: Progress on EMRIs

Authors: Carlos F. Sopuerta (ICE (CSIC-IEEC))

Date: 29 Sep 2012

Abstract: The capture of a stellar-mass compact object by a supermassive black hole and the subsequent inspiral (driven by gravitational radiation emission) constitute one of the most important sources of gravitational waves for space-based observatories like eLISA/NGO. In this article we describe their potential as high-precision tools that can be used to perform tests of the geometry of black holes and also of the strong field regime of gravity.

abs pdf

Oct 01, 2012

1210.0156 (/preprints)
2012-10-01, 18:15 [edit]


[1209.6428] Applying Bayesian Inference to the first International Pulsar Timing Array data challenge

Authors: Neil J. Cornish

Date: 28 Sep 2012

Abstract: This is a very brief summary of the techniques I used to analyze the IPTA challenge 1 data sets. I tried many things, and more failed than succeeded, but in the end I found two approaches that appear to work based on tests done using the open data sets. One approach works directly with the time domain data, and the other works with a specially constructed Fourier transform of the data. The raw data was run through TEMPO2 to produce reduced timing residuals for the analysis. Standard Markov Chain Monte Carlo techniques were used to produce samples from the posterior distribution function for the model parameters. The model parameters include the gravitational wave amplitude and spectral slope, and the white noise amplitude for each pulsar in the array. While red timing noise was only included in Dataset 3, I found that it was necessary to include effective red noise in all the analyses to account for some of the spurious effects introduced by the TEMPO2 timing fit. This added an additional amplitude and slope parameter for each pulsar, so my overall model for the 36 pulsars residuals has 110 parameters. As an alternative to using an effective red noise model, I also tried to simultaneously re-fit the timing model model while looking for the gravitational wave signal, but for reasons that are not yet clear, this approach was not very successful. I comment briefly on ways in which the algorithms could be improved. My best estimates for the gravitational wave amplitudes in the three closed (blind) data sets are: (1) $A=(7.3\pm 1.0)\times 10ˆ{-15}$; (2) $A=(5.7\pm 0.6)\times 10ˆ{-14}$; and (3) $A=(4.6\pm 1.3)\times 10ˆ{-15}$.

abs pdf

Oct 01, 2012

1209.6428 (/preprints)
2012-10-01, 13:35 [edit]


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