Alessandro Nagar

Recoil velocities from equal-mass binary black-hole mergers: A systematic investigation of spin-orbit aligned configurations

The final evolution of a binary-black-hole system gives rise to a recoil velocity if an asymmetry is present in the emitted gravitational radiation. Measurements of this effect for nonspinning binaries with unequal masses have pointed out that kick velocities approximately 175 km/s can be reached for a mass ratio approximately 0.36. However, a larger recoil can be obtained for equal-mass binaries if the asymmetry is provided by the spins. Using two independent methods we show that the merger of such binaries yields velocities as large as approximately 440 km/s for black holes having unequal spins that are antialigned and parallel to the orbital angular momentum.

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

The gravitational wave signal from a binary neutron star inspiral contains information on the nuclear equation of state. This information is contained in a combination of the tidal polarizability parameters of the two neutron stars and is clearest in the late inspiral, just before merger. We use the recently defined tidal extension of the effective one-body formalism to construct a controlled analytical description of the frequency-domain phasing of neutron star inspirals up to merger. Exploiting this analytical description we find that the tidal polarizability parameters of neutron stars can be measured by the advanced LIGO-Virgo detector network from gravitational wave signals having a reasonable signal-to-noise ratio of

Error-analysis and comparison to analytical models of numerical waveforms produced by the NRAR Collaboration

\rho=16

Improved analytical description of inspiralling and coalescing black-hole binaries

. This measurability result seems to hold for all the nuclear equations of state leading to a maximum mass larger than

Accurate effective-one-body waveforms of inspiralling and coalescing black-hole binaries

1.97M_\odot

Analytic modelling of tidal effects in the relativistic inspiral of binary neutron stars

. We also propose a promising new way of extracting information on the nuclear equation of state from a coherent analysis of an ensemble of gravitational wave observations of separate binary merger events.

Accurate numerical simulations of inspiralling binary neutron stars and their comparison with effective-one-body analytical models

The Numerical–Relativity–Analytical–Relativity (NRAR) collaboration is a joint effort between members of the numerical relativity, analytical relativity and gravitational-wave data analysis communities. The goal of the NRAR collaboration is to produce numerical-relativity simulations of compact binaries and use them to develop accurate analytical templates for the LIGO/Virgo Collaboration to use in detecting gravitational-wave signals and extracting astrophysical information from them. We describe the results of the first stage of the NRAR project, which focused on producing an initial set of numerical waveforms from binary black holes with moderate mass ratios and spins, as well as one non-spinning binary configuration which has a mass ratio of 10. All of the numerical waveforms are analysed in a uniform and consistent manner, with numerical errors evaluated using an analysis code created by members of the NRAR collaboration. We compare previously-calibrated, non-precessing analytical waveforms, notably the effective-one-body (EOB) and phenomenological template families, to the newly-produced numerical waveforms. We find that when the binarys total mass is ~100–200M⊙, current EOB and phenomenological models of spinning, non-precessing binary waveforms have overlaps above 99% (for advanced LIGO) with all of the non-precessing-binary numerical waveforms with mass ratios ≤4, when maximizing over binary parameters. This implies that the loss of event rate due to modelling error is below 3%. Moreover, the non-spinning EOB waveforms previously calibrated to five non-spinning waveforms with mass ratio smaller than 6 have overlaps above 99.7% with the numerical waveform with a mass ratio of 10, without even maximizing on the binary parameters.

Faithful effective-one-body waveforms of equal-mass coalescing black-hole binaries

We present an analytical formalism, within the effective-one-body framework, which predicts gravitational-wave signals from inspiralling and coalescing black-hole binaries that agree, within numerical errors, with the results of the currently most accurate numerical-relativity simulations for several different mass ratios. In the equal-mass case, the gravitational-wave energy flux predicted by our formalism agrees, within numerical errors, with the most accurate numerical-relativity energy flux. We think that our formalism opens a realistic possibility of constructing a sufficiently accurate, large bank of gravitational-wave templates, as needed both for detection and data analysis of (nonspinning) coalescing binary black holes.

Modeling the Dynamics of Tidally Interacting Binary Neutron Stars up to the Merger

The effective-one-body (EOB) formalism contains several flexibility parameters, notablya5,vpole, and � aRR. We show here how to jointly constrain the values of these parameters by simultaneously best-fitting the EOB waveform to two, independent, numerical relativity (NR) simulations of inspiralling and/or coalescing binary black-hole systems: published Caltech-Cornell inspiral data (considered for gravitational wave frequenciesM! � 0:1) on one side, and newly computed coalescence data on the other side. The resulting, approximately unique, ‘‘best-fit’’ EOB waveform is then shown to exhibit excellent agreement with NR coalescence data for several mass ratios. The dephasing between this best-fit EOB waveform and published Caltech-Cornell inspiral data is found to vary between � 0:0014 and þ0:0008 radians over a time span of � 2464M up to gravitational wave frequency M! ¼ 0:1, and between þ0:0013 and � 0:0185 over a time span of 96M after M! ¼ 0:1 up to M! ¼ 0:1565. The dephasings between EOB and the new coalescence data are found to be smaller than: (i) � 0:025 radians over a time span of 730M (11 cycles) up to merger, in the equal-mass case, and (ii) � 0:05 radians over a time span of about 950M (17 cycles) up to merger in the 2:1 mass-ratio case. These new results corroborate the aptitude of the EOB formalism to provide accurate representations of general relativistic waveforms, which are needed by currently operating gravitational wave detectors.

Modeling the Complete Gravitational Wave Spectrum of Neutron Star Mergers.

To detect the gravitational-wave (GW) signal from binary neutron stars and extract information about the equation of state of matter at nuclear density, it is necessary to match the signal with a bank of accurate templates. We present the two longest (to date) general-relativistic simulations of equal-mass binary neutron stars with different compactnesses, C=0.12 and C=0.14, and compare them with a tidal extension of the effective-one-body (EOB) model. The typical numerical phasing errors over the ≃22   GW cycles are Δϕ≃±0.24   rad. By calibrating only one parameter (representing a higher-order amplification of tidal effects), the EOB model can reproduce, within the numerical error, the two numerical waveforms essentially up to the merger. By contrast, the third post-Newtonian Taylor-T4 approximant with leading-order tidal corrections dephases with respect to the numerical waveforms by several radians.