Searcharxiv⌕ Search

arXiv subjects

Sean T. McWilliams

Publications and source records attributed to Sean T. McWilliams.

At least 19 recordsLinked to original sources
arXiv2026

The Direct Wave is Not a Meaningful Test of Horizon Properties

Recently, a distinct non--quasinormal mode component of black hole binary merger radiation, named the direct wave, has been identified. The frequency and damping time of the direct wave have been associated with properties of the remnant horizon. This has led to direct-wave based analysis of GW250114, including a test of Hawking's area law. However, as we demonstrate here using numerical relativity strain data, the direct wave frequency is not correlated with the horizon frequency or surface gravity, other than an incidental crossing around $χ_f \approx 0.7$ corresponding closely to the remnant spin of GW250114. We show that while the instantaneous frequency of the direct wave is quasi-stable, the damping time shows significant evolution and therefore a single damped sinusoid model, containing a fixed damping time, is not appropriate. We further show that an evolving frequency model based on horizon properties also does not model the direct wave component for systems with large remnant spins. We demonstrate that testing Hawking's area law with a horizon frequency based on the direct wave interpretation will lead to apparent violations of Hawking's area law when no violation actually occurs. Our results therefore indicate that the direct wave is not a reliable probe of the remnant horizon's properties.

gr-qc↗
arXiv2026

Direct Waves in Black-Hole Binary Mergers: Insights from the Backwards One Body Model

The merger-ringdown radiation from a black hole binary merger is accurately modeled by a sum of linear quasinormal modes (QNMs). Recently, a non-QNM ``direct wave" component of the radiation, associated with prompt emission from a plunging perturber, has been identified. Motivated by the behavior of null geodesics perturbed from the remnant light ring, the Backwards One Body (BOB) approach has been shown to model the full merger-ringdown radiation to high accuracy, while using only a minimal number of parameters. In this work, using the Pöschl--Teller potential, we first show how the BOB amplitude evolution can be recovered from the QNM pole contributions. We then apply rational filters to isolate the non-QNM content in BOB and numerical relativity waveforms. We show that BOB naturally captures the direct wave component of the merger radiation, explaining its accuracy near the waveform peak. Finally, we use BOB to show that the direct wave frequency is largely uncorrelated with the horizon frequency, even for high spin remnants, and instead tracks the News frequency at the time of the peak News amplitude.

gr-qc↗
arXiv2025

Modeling Relative Peak Times of Gravitational Wave Harmonics

Accurate modeling of gravitational waves from binary black hole mergers is essential for extracting their rich physics. A key detail for understanding the physics of mergers is predicting the precise time when the amplitude of the gravitational wave strain peaks, which can differ significantly among the different harmonic modes. We propose two semi-analytical methods to predict these differences using the same three inputs from Numerical Relativity (NR): the remnant mass and spin and the instantaneous frequency of each mode at its peak amplitude. The first method uses the frequency evolution predicted by the Backwards-One-Body model, while the second models the motion of an equatorial timelike geodesic in the remnant black hole spacetime. We compare our models to the SXS waveform catalog for quasi-circular, non-precessing systems and find excellent agreement for $l = |m|$ modes up to $l=8$, with mean and median differences from NR below 1$M$ in nearly all cases across the parameter space. We compare our results to the differences predicted by leading Effective-One-Body and NR surrogate waveform models and find that in cases corresponding to the largest timing differences, our models can provide significant increases in accuracy.

gr-qc↗
arXiv2025

BOB the (Waveform) Builder: Optimizing Analytical Black-Hole Binary Merger Waveforms

The Backwards-One-Body (BOB) model provides a fully analytical and physically motivated description of the merger-ringdown gravitational radiation emanating from a black hole binary merger. We perform a comprehensive validation of BOB for the dominant $(2,2)$ mode of quasi-circular and non-precessing systems, assessing its accuracy against numerical relativity (NR) simulations, state-of-the-art waveform models, and a sum of quasinormal modes. We demonstrate that BOB most accurately describes the gravitational wave news, achieving accuracy comparable to highly-calibrated Effective-One-Body and NR surrogate models. Because BOB is minimally tuned to NR catalogs, it retains a high level of accuracy in regions of the parameter space sparsely covered by current NR catalogs. BOB yields an analytic link between the amplitude of the fundamental quasinormal mode and the peak amplitude of the News, which we verify to within the errors of a surrogate ringdown model. We identify a flavor of BOB that requires only the remnant mass and spin, yet matches the accuracy of models that fit a sum of many overtones. Lastly, we show that BOB accurately models both the mass and current quadrupole waves for superkick configurations, contrary to a claim in the literature, and explain why that study was not actually implementing BOB as it has been defined. Our findings establish BOB as a powerful tool for gravitational wave analysis, for providing independent tests of NR-calibrated models, and for better understanding the underlying physics of the merger. We provide a companion python package, gwBOB, allowing for the easy construction of various flavors of BOB and comparison to NR waveforms.

gr-qc↗
arXiv2025

Spinning Effective-to-Backwards One Body ($\texttt{SEBOB}$): combining Effective One-Body inspirals and Backwards One-Body merger-ringdowns for aligned spin black hole binaries

High-fidelity gravitational waveform models are essential for realizing the scientific potential of next-generation gravitational-wave observatories. While highly accurate, state-of-the-art models often rely on extensive phenomenological calibrations to numerical relativity (NR) simulations for the late-inspiral and merger phases, which can limit physical insight and extrapolation to regions where NR data is sparse. To address this, we introduce the Spinning Effective-to-Backwards One Body (SEBOB) formalism, a hybrid approach that combines the well-established Effective-One-Body (EOB) framework with the analytically-driven Backwards-One-Body (BOB) model, which describes the merger-ringdown from first principles as a perturbation of the final remnant black hole. We present two variants building on the state-of-the-art $\texttt{SEOBNRv5HM}$ model: $\texttt{seobnrv5_nrnqc_bob}$, which retains standard NR-calibrated non-quasi-circular (NQC) corrections and attaches a BOB-based merger-ringdown; and a more ambitious variant, $\texttt{seobnrv5_bob}$, which uses BOB to also inform the NQC corrections, thereby reducing reliance on NR fitting and enabling higher-order ($\mathcal{C}^2$) continuity by construction. Implemented in the open-source $\texttt{NRPy}$ framework for optimized C-code generation, the SEBOB model is transparent, extensible, and computationally efficient. By comparing our waveforms to a large catalog of NR simulations, we demonstrate that SEBOB yields accuracies comparable to the highly-calibrated $\texttt{SEOBNRv5HM}$ model, providing a viable pathway towards more physically motivated and robust waveform models

gr-qc↗
arXiv2024

Analytical and Numerical Methods for Circumbinary Disk Dynamics -- II: Inclined Disks

(Abridged) To gain insight into the dynamical influence of a supermassive black hole binary on a circumbinary accretion disk, we investigate the binary and viscous torque densities throughout such a disk, with emphasis on the final density distribution, particularly the size and stability of the central gap between the binary and the inner edge of the disk. We limit ourselves to the simplified case of a massless viscous thin accretion disk under the influence of the gravitational potential from a binary system whose orbital plane is inclined relative to the disk. The orbital plane could be inclined if it is not coeval with the disk, or the black holes have spin angular momentum misaligned with respect to the disk's orbital angular momentum, so that the binary can precess to an inclined orientation. We employ 2D Newtonian hydrodynamics simulations to examine the influence of two model parameters: the mass ratio of the binary and the inclination angle between the binary and the disk. We investigate their impact on the density and torque distribution. In our analytical approach, we consider the stability of epicycles induced by the perturbative effect of the asymmetric inclined binary gravitational potential on Keplerian circular orbits. Through our simulations, we observe that certain configurations never attain a quasi-steady state, where the density profile averaged over many orbits stabilizes. This instability occurs when the inclination is close to 45 degrees. Furthermore, we identify configurations where there is never a persistent balance between the dynamical and viscous torque densities, as well as cases where the location of this balance oscillates or exhibits other time-dependent behavior over viscous timescales. These findings have implications for understanding both the expected gravitational-wave signal and electromagnetic counterparts from supermassive black hole binaries.

astro-ph.HE↗
arXiv2024

Testing the Boundary-to-Bound Correspondence with Numerical Relativity

The Boundary-to-Bound (B2B) correspondence, which connects orbital and radiative observables between bound and unbound orbits, has recently been introduced and demonstrated in the perturbative regime. We produce a large number of numerical relativity simulations of bound and unbound encounters between two nonspinning equal mass black holes in order to test this correspondence in the non-perturbative regime. We focus on testing the radiated energy and angular momentum, as well as orbital parameters such as the period and periastron advance. We find that, across a wide range of eccentricities, the B2B relationships do not hold in the non-perturbative regime, thereby placing a clear limit on the applicability of these relationships. We also approximate the separatrix between bound and unbound relativistic encounters as a function of their initial energies and angular momenta.

gr-qc↗
arXiv2023

Analytical and Numerical Analysis of Circumbinary Disk Dynamics -- I: Coplanar Systems

We present an analytical and numerical study of a system composed of a stellar binary pair and a massless, locally isothermal viscous accretion disk that is coplanar to the binary orbital plane. Analytically, we study the effect of the binary's gravitational potential over short timescales through the study of stability for epicyclic orbits, and over long timescales by revisiting the concept of resonant torques. Numerically, we perform two-dimensional Newtonian numerical simulations of the disk-binary system over a range of binary mass ratios. We find that the results of our simulations are consistent with previous numerical studies. We additionally show, by comparison of the analytical and numerical results, that the circumbinary gap is maintained on the orbital timescale through the driving of epicyclic instabilities, and does not depend on resonant torquing, contrary to standard lore. While our results are applicable to any disk-binary system, we highlight the importance of this result in the search for electromagnetic and gravitational-wave signatures from supermassive black-hole binaries.

astro-ph.SR↗
arXiv2020

Sensitivity of present and future detectors across the black-hole binary gravitational wave spectrum

Black-holes are known to span at least 9 orders of magnitude in mass: from the stellar-mass objects observed by the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration and Virgo Collaboration, to supermassive black-holes like the one observed by the Event Horizon Telescope at the heart of M87. Regardless of the mass scale, all of these objects are expected to form binaries and eventually emit observable gravitational radiation, with more massive objects emitting at ever lower gravitational-wave frequencies. We present the tool, gwent, for modelling the sensitivities of current and future generations of gravitational wave detectors across the entire gravitational-wave spectrum of coalescing black-hole binaries. We provide methods to generate sensitivity curves for pulsar timing arrays (PTAs) using a novel realistic PTA sensitivity curve generator, space-based interferometers using adaptive models that can represent a wide range of proposed detector designs, and ground-based interferometers using realistic noise models that can reproduce current, second, and third generation designs, as well as novel variations of the essential design parameters. To model the signal from black-hole binaries at any mass scale, we use phenomenological waveforms capable of modelling the inspiral, merger, and ringdown for sources with varying mass ratios and spins. Using this adaptable framework, we produce signal-to-noise ratios for the combination of any modelled parameter, associated with either the detector or the source. By allowing variation across each detector and source parameter, we can pinpoint the most important factors to determining the optimal performance for particular instrument designs. The adaptability of our detector and signal models can easily be extended to new detector designs and other models of gravitational wave signals.

gr-qc↗
arXiv2019

Induced Spins from Scattering Experiments of Initially Nonspinning Black Holes

When two relativistically boosted, nonspinning black holes pass by one another on a scattering trajectory, we might expect the tidal interaction to spin up each black hole. We present the first exploration of this effect, appearing at fourth post-Newtonian order, with full numerical relativity calculations. The basic setup for the calculations involves two free parameters: the initial boost of each black hole and the initial angle between the velocity vectors and a line connecting the centers of the black holes, with zero angle corresponding to a head-on trajectory. To minimize gauge effects, we measure final spins only if the black holes reach a final separation of at least $20M$. Fixing the initial boost, we find that as the initial angle decreases toward the scattering/nonscattering limit, the spin-up grows nonlinearly. In addition, as initial boosts are increased from $0.42c$ to $0.78c$, the largest observed final dimensionless spin on each black hole increases nonlinearly from $0.02$ to $0.20$. Based on these results, we conclude that much higher spin-ups may be possible with larger boosts, although achieving this will require improved numerical techniques.

gr-qc↗
arXiv2019

Building A Field: The Future of Astronomy with Gravitational Waves, A State of The Profession Consideration for Astro2020

Harnessing the sheer discovery potential of gravitational wave astronomy will require bold, deliberate, and sustained efforts to train and develop the requisite workforce. The next decade requires a strategic plan to build -- from the ground up -- a robust, open, and well-connected gravitational wave astronomy community with deep participation from traditional astronomers, physicists, data scientists, and instrumentalists. This basic infrastructure is sorely needed as an enabling foundation for research. We outline a set of recommendations for funding agencies, universities, and professional societies to help build a thriving, diverse, and inclusive new field.

astro-ph.IM↗
arXiv2019

The Laser Interferometer Space Antenna: Unveiling the Millihertz Gravitational Wave Sky

The first terrestrial gravitational wave interferometers have dramatically underscored the scientific value of observing the Universe through an entirely different window, and of folding this new channel of information with traditional astronomical data for a multimessenger view. The Laser Interferometer Space Antenna (LISA) will broaden the reach of gravitational wave astronomy by conducting the first survey of the millihertz gravitational wave sky, detecting tens of thousands of individual astrophysical sources ranging from white-dwarf binaries in our own galaxy to mergers of massive black holes at redshifts extending beyond the epoch of reionization. These observations will inform - and transform - our understanding of the end state of stellar evolution, massive black hole birth, and the co-evolution of galaxies and black holes through cosmic time. LISA also has the potential to detect gravitational wave emission from elusive astrophysical sources such as intermediate-mass black holes as well as exotic cosmological sources such as inflationary fields and cosmic string cusps.

astro-ph.IM↗
arXiv2019

Space Based Gravitational Wave Astronomy Beyond LISA

The Laser Interferometer Space Antenna (LISA) will open three decades of gravitational wave (GW) spectrum between 0.1 and 100 mHz, the mHz band. This band is expected to be the richest part of the GW spectrum, in types of sources, numbers of sources, signal-to-noise ratios and discovery potential. When LISA opens the low-frequency window of the gravitational wave spectrum, around 2034, the surge of gravitational-wave astronomy will strongly compel a subsequent mission to further explore the frequency bands of the GW spectrum that can only be accessed from space. The 2020s is the time to start developing technology and studying mission concepts for a large-scale mission to be launched in the 2040s. The mission concept would then be proposed to Astro2030. Only space based missions can access the GW spectrum between 10 nHz and 1 Hz because of the Earths seismic noise. This white paper surveys the science in this band and mission concepts that could accomplish that science. The proposed small scale activity is a technology development program that would support a range of concepts and a mission concept study to choose a specific mission concept for Astro2030. In this white paper, we will refer to a generic GW mission beyond LISA as bLISA.

astro-ph.IM↗
arXiv2019

Analytical Black-Hole Binary Merger Waveforms

We present a highly accurate, fully analytical model for the late inspiral, merger, and ringdown of black-hole binaries with arbitrary mass ratios and spin vectors, including the contributions of harmonics beyond the fundamental mode. This model assumes only that nonlinear effects remain small throughout the entire coalescence, and is developed based on a physical understanding of the dynamics of late stage binary evolution, in particular on the tendency of the dynamical binary spacetime to behave like a linear perturbation of the static merger-remnant spacetime, even at times before the merger has occurred. We demonstrate that our model agrees with the most accurate numerical relativity results to within their own uncertainties throughout the merger-ringdown phase, and it does so for example cases spanning the full range of binary parameter space that is currently testable with numerical relativity. Furthermore, our model maintains accuracy back to the innermost stable circular orbit of the merger-remnant spacetime over much of the relevant parameter space, greatly decreasing the need to introduce phenomenological degrees of freedom to describe the late inspiral.

gr-qc↗
arXiv2019

Multimessenger science opportunities with mHz gravitational waves

LISA will open the mHz band of gravitational waves (GWs) to the astronomy community. The strong gravity which powers the variety of GW sources in this band is also crucial in a number of important astrophysical processes at the current frontiers of astronomy. These range from the beginning of structure formation in the early universe, through the origin and cosmic evolution of massive black holes in concert with their galactic environments, to the evolution of stellar remnant binaries in the Milky Way and in nearby galaxies. These processes and their associated populations also drive current and future observations across the electromagnetic (EM) spectrum. We review opportunities for science breakthroughs, involving either direct coincident EM+GW observations, or indirect multimessenger studies. We argue that for the US community to fully capitalize on the opportunities from the LISA mission, the US efforts should be accompanied by a coordinated and sustained program of multi-disciplinary science investment, following the GW data through to its impact on broad areas of astrophysics. Support for LISA-related multimessenger observers and theorists should be sized appropriately for a flagship observatory and may be coordinated through a dedicated mHz GW research center.

astro-ph.HE↗
arXiv2019

Astro2020 Decadal Science White Paper: The state of gravitational-wave astrophysics in 2020

While still in its infancy, the budding field of gravitational-wave astronomy has so far exceeded most expectations, and the achievements that have already been made bode well for the decade to come. While the discoveries made possible by LIGO have captured the imagination of experts and nonexperts alike, it is important when looking ahead to consider those discoveries in the context of the field as a whole. Just as radio, optical, and x-ray radiation probe different physical phenomena occurring on a range of length and energy scales, the future of gravitational-wave astrophysics depends on our ability to open up the entire spectrum. We will describe the scientific prospects for the field of gravitational-wave astronomy as a whole as we enter the coming decade, and we will place the specific contributions from a future space-based gravitational-wave observatory within this context.

astro-ph.HE↗
arXiv2019

What we can learn from multi-band observations of black hole binaries

The LIGO/Virgo gravitational-wave (GW) interferometers have to-date detected ten merging black hole (BH) binaries, some with masses considerably larger than had been anticipated. Stellar-mass BH binaries at the high end of the observed mass range (with "chirp mass" ${\cal M} \gtrsim 25 M_{\odot}$) should be detectable by a space-based GW observatory years before those binaries become visible to ground-based GW detectors. This white paper discusses some of the synergies that result when the same binaries are observed by instruments in space and on the ground. We consider intermediate-mass black hole binaries (with total mass $M \sim 10^2 -10^4 M_{\odot}$) as well as stellar-mass black hole binaries. We illustrate how combining space-based and ground-based data sets can break degeneracies and thereby improve our understanding of the binary's physical parameters. While early work focused on how space-based observatories can forecast precisely when some mergers will be observed on the ground, the reverse is also important: ground-based detections will allow us to "dig deeper" into archived, space-based data to confidently identify black hole inspirals whose signal-to-noise ratios were originally sub-threshold, increasing the number of binaries observed in both bands by a factor of $\sim 4 - 7$.

astro-ph.HE↗
arXiv2018

Improving performance of SEOBNRv3 by $\sim$300x

When a gravitational wave is detected by Advanced LIGO/Virgo, sophisticated parameter estimation (PE) pipelines spring into action. These pipelines leverage approximants to generate large numbers of theoretical gravitational waveform predictions to characterize the detected signal. One of the most accurate and physically comprehensive classes of approximants in wide use is the "Spinning Effective One Body--Numerical Relativity" (SEOBNR) family. Waveform generation with these approximants can be computationally expensive, which has limited their usefulness in multiple data analysis contexts. In prior work we improved the performance of the aligned-spin approximant SEOBNR version 2 (v2) by nearly 300x. In this work we focus on optimizing the full eight-dimensional, precessing approximant SEOBNR version 3 (v3). While several v2 optimizations were implemented during its development, v3 is far too slow for use in state-of-the-art source characterization efforts for long-inspiral detections. Completion of a PE run after such a detection could take centuries to complete using v3. Here we develop and implement a host of optimizations for v3, calling the optimized approximant v3_Opt. Our optimized approximant is about 340x faster than v3, and generates waveforms that are numerically indistinguishable.

gr-qc↗