Fast and faithful Effective One Body models for gravitational waves from generic compact binaries

The detection and analysis of gravitational waves (GWs) from compact binary systems relies on accurate modeling of the expected signals emitted by such sources. In this thesis we develop computationally efficient yet accurate models for coalescing binary black holes (BBHs) and binary neutron stars (BNSs), relying on the effective-one-body (EOB) framework as implemented in the TEOBResumS family of models. Building on its multipolar aligned-spin avatar, we improve TEOBResumS to include the description of spins precession via an efficient hybrid PN-EOB scheme, thus obtaining a new state-of-the-art inspiral-merger-ringdown model for BBHs and the first multipolar precessing model for coalescing BNSs. We validate our model in terms of NR faithfulness, finding that TEOBResumS agrees to more than 97% with NR results over a considerable portion of the parameter space. Its efficiency is demonstrated by directly employing the model in the parameter estimation (PE) of a handful of events detected by the LVK collaboration (GW150914, GW190412 and GW170817) without the need of surrogates or reduced models. Employing a flavor of TEOBResumS able to model the evolution of systems coalescing along non-circular trajectories, we then study the phenomenology of the GWs that are produced by systems merging along initially unbound orbits. After comparing our waveforms with a set of highly eccentric NR simulations, we analyze GW190521 under the hypothesis that it originated from a dynamical capture of two BHs. Our results suggest that GW190521 may be the the first detected GW signal to correspond to such a system. Finally, we refine the TEOBResumS description of matter effects: after critically assessing the importance of resonant tidal effects for quasi-circular and eccentric BNS mergers, we considerably improve the model performance by including high-order PN information and few NR-informed parameters

A Multipolar Effective One Body Model for Non-Spinning Black Hole Binaries

We introduce \TEOBiResumSM{}, a nonspinning inspiral-merger-ringdown waveform model built within the effective one body (EOB) framework that includes gravitational waveform modes beyond the dominant quadrupole $(\ell,|m|) = (2,2)$. The model incorporates: (i) an improved Pad\'e resummation of the factorized waveform amplitudes $\rho_{\ell m}^{\rm orb}$ entering the EOB-resummed waveform where the 3PN, mass-ratio dependent, terms are hybridized with test-mass limit terms up to 6PN relative order for most of the multipoles up to $\ell=6$ included; (ii) an improved determination of the effective 5PN function $a_6^c(\nu)$ entering the EOB interaction potential done using the most recent, error-controlled, nonspinning numerical relativity (NR) waveforms from the Simulating eXtreme Spacetimes (SXS) collaboration; and (iii) a NR-informed phenomenological description of the multipolar ringdown. Such representation stems from 19 NR waveforms with mass ratios up to $m_1/m_2=18$ as well as test-mass waveform data, although it does not incorporate mode-mixing effects. The NR-completed higher modes through merger and ringdown considered here are: $(\ell,|m|) = \lbrace (2,1), (3,3), (3,2),(3,1),(4,4), (4,3),(4,2), (4,1),(5,5)\rbrace$. For simplicity, the other subdominant modes, up to $\ell=8$, are approximated by the corresponding, purely analytical, factorized and resummed EOB waveform. To attempt an estimate of (some of) the underlying analytic uncertainties of the model, we also contrast the effect of the 6PN-hybrid Pad\'e-resummed $\rho_{\ell m}$'s with the standard $3^{+2}$PN, Taylor-expanded, ones used in previous EOB works. The maximum unfaithfulness $\bar{F}$ against the SXS waveforms including all NR-completed modes up to $\ell=m=5$ is always $\lesssim 2\%$ for binaries with total mass $M$ as $50 M_{\odot} \leq M \lesssim 200 M_{\odot}$.Comment: 24 pages, 18 figures. Improved figures and presentation. Submitted to Phys. Rev.

TEOBResumS: Analytic systematics in next-generation of effective-one-body gravitational waveform models for future observations

The success of analytic waveform modeling within the effective-one-body (EOB) approach relies on the precise understanding of the physical importance of each technical element included in the model. The urgency of constructing progressively more sophisticated and complete waveform models (e.g. including spin precession and eccentricity) partly defocused the research from a careful comprehension of each building block (e.g. Hamiltonian, radiation reaction, ringdown attachment). Here we go back to the spirit of the first EOB works. We focus first on nonspinning, quasi-circular, black hole binaries and analyze systematically the mutual synergy between numerical relativity (NR) informed functions and the high post-Newtonian corrections (up to 5PN) to the EOB potentials. Our main finding is that it is essential to correctly control the noncircular part of the dynamics during the late plunge up to merger. When this happens, either using NR-informed non-quasi-circular corrections to the waveform (and flux) or high-PN corrections in the radial EOB potentials $(D,Q)$, it is easy to obtain EOB/NR unfaithfulness $\sim 10^{-4}$ with the noise of either Advanced LIGO or 3G detectors. We then improve the {\tt TEOBResumS-GIOTTO} waveform model for quasi-circular, spin-aligned binaries black hole binaries. We obtain maximal EOB/NR unfaithfulness ${\bar{\cal F}}^{\rm max}_{\rm EOBNR}\sim 10^{-3}$ (with Advanced LIGO noise and in the total mass range $10-200M_\odot$) for the dominant $\ell=m=2$ mode all over the 534 spin-aligned configurations available through the Simulating eXtreme Spacetime catalog. The model performance, also including higher modes, is then explored using NR surrogate waveform models to validate {\tt TEOBResumS-GIOTTO} up to mass ratio $m_1/m_2=15$.Comment: 23 pages, 27 figures, submitted to Phys. Rev.

GW190521 as a dynamical capture of two nonspinning black holes

Gravitational waves from $\sim 90$ black holes binary systems have currently been detected by the LIGO and Virgo experiments, and their progenitors' properties inferred. This allowed the scientific community to draw conclusions on the formation channels of black holes in binaries, informing population models and -- at times -- defying our understanding of black hole astrophysics. The most challenging event detected so far is the short duration gravitational-wave transient GW190521. We analyze this signal under the hypothesis that it was generated by the merger of two nonspinning black holes on hyperbolic orbits. The best configuration matching the data corresponds to two black holes of source frame masses of $81^{+62}_{-25}M_\odot$ and $52^{+32}_{-32}M_\odot$ undergoing two encounters and then merging into an intermediate-mass black hole. We find that the hyperbolic merger hypothesis is favored with respect to a quasi-circular merger with precessing spins with Bayes' factors larger than 4300 to 1, although this number will be reduced by the currently uncertain prior odds. Our results suggest that GW190521 might be the first gravitational-wave detection from the dynamical capture of two stellar-mass nonspinning black holes.Comment: Version accepted for publicatio

Unveiling the merger structure of black hole binaries in generic planar orbits

The precise modeling of binary black hole coalescences in generic planar orbits is a crucial step to disentangle dynamical and isolated binary formation channels through gravitational-wave observations. The merger regime of such coalescences exhibits a significantly higher complexity compared to the quasicircular case, and cannot be readily described through standard parameterizations in terms of eccentricity and anomaly. In the spirit of the Effective One Body formalism, we build on the study of the test-mass limit, and show how gauge-invariant combinations of the binary energy and angular momentum, such as a dynamical "impact parameter" at merger, overcome this challenge. These variables reveal simple "quasi-universal" structures of the pivotal merger parameters, allowing to build an accurate analytical representation of generic (bounded and dynamically-bounded) orbital configurations. We demonstrate the validity of these analytical relations using 255 numerical simulations of bounded noncircular binaries with nonspinning progenitors from the RIT and SXS catalogs, together with a custom dataset of dynamical captures generated using the Einstein Toolkit, and test-mass data in bound orbits. Our modeling strategy lays the foundations of accurate and complete waveform models for systems in arbitrary orbits, bolstering observational explorations of dynamical formation scenarios and the discovery of new classes of gravitational wave sources.Comment: Main: 10 pages, 3 figures; w suppl. mater.: 19 pages, 5 figures, 2 table

Inferring eccentricity evolution from observations of coalescing binary black holes

The origin and formation of stellar-mass binary black holes remains an open question that can be addressed by precise measurements of the binary and orbital parameters from their gravitational-wave signal. Such binaries are expected to circularize due to the emission of gravitational waves as they approach merger. However, depending on their formation channel, some binaries could have a non-negligible eccentricity when entering the frequency band of current gravitational-wave detectors. In order to measure eccentricity in an observed gravitational-wave signal, accurate waveform models that describe binaries in eccentric orbits are necessary. In this work we demonstrate the efficacy of the improved TEOBResumS waveform model for eccentric coalescing binaries with aligned spins. We first validate the model against mock signals of aligned-spin binary black hole mergers and quantify the impact of eccentricity on the estimation of other intrinsic binary parameters. We then perform a fully Bayesian reanalysis of GW150914 with the eccentric waveform model. We find (i) that the model is reliable for aligned-spin binary black holes and (ii) that GW150914 is consistent with a non-eccentric merger although we cannot rule out small values of initial eccentricity at a reference frequency of $20$ Hz. Finally, we present a systematic method to measure the eccentricity and its evolution directly from the gravitational-wave posterior samples. Such an estimator is useful when comparing results from different analyses as the definition of eccentricity may differ between models. Our scheme can be applied even in the case of small eccentricities and can be adopted straightforwardly in post-processing to allow for direct comparison between models.Comment: 20 pages, 13 figures, version accepted by PR