arXiv subjects

Richard O'Shaughnessy

Publications and source records attributed to Richard O'Shaughnessy.

At least 19 recordsLinked to original sources

arXiv2026

Constraining the Equation of State of Neutron Stars with third-generation Gravitational Wave detectors

We investigated the impact of the number of binary neutron star merger events and neutron star (NS) mass distributions on constraining the equation of state (EoS), tidal deformability and radius of NSs, as well as the nuclear parameters, using binary neutron star inspiral gravitational wave signals with third-generation detectors. We generate simulated gravitational wave signals and compute the Fisher information matrix for each event after the number of events and mass distribution models are given. The covariance of EoS parameters is obtained by holding the non-EoS waveform parameters fixed at their injected values and accumulating the Fisher matrix over all events. Finally, the posterior samples of EoS, tidal deformability, radius and nuclear parameters are generated based on this covariance matrix. We find that larger number of events lead to tighter constraints due to more observed events and data accumulation, as expected given the increased number of detections. Meanwhile, we note that the mass distribution plays a more important role in constraining the EoS. We compare a realistic bimodal Gaussian distribution, a uniform distribution and another uniform including sub-solar mass NSs. The results show that the uniform distribution yields tighter constraints than bimodal models because it includes more low-mass and massive NSs. This also implies that events near $1.4\,\Msun$ provide partly redundant information about the EoS. Additionally, including sub-solar mass NSs can further improve the constraints by significantly reducing the EoS uncertainties at sub-saturation densities and inner-crust, highlighting the importance of sub-saturation density EoS.

astro-ph.HE↗

arXiv2026

Detectability of bulk viscosity effects on post-merger gravitational wave signals from binary neutron star mergers

We investigate bulk viscosity (BV) effects on post-merger gravitational wave (PMGW) signals from binary neutron star (BNS) mergers and their detectability using realistic equation of state (EoS) posteriors constrained by recent progress in gravitational-wave (GW) and X-ray observations, as well as neutron skin thickness measurements. A linear fitting formula based on recent BNS simulations with BV is used to estimate the peak frequency shift of the PMGWs caused by BV effects. Subsequently, we evaluate this peak frequency shift for our set of EoS posterior samples. We use the Fisher information matrix and the dataset of simulated observable events from the ET and CE detector network in an optimistic scenario to estimate the measurement accuracy of the peak frequency. We find that BV effects on PMGWs may be detectable only for EoS models with large values of the symmetry energy slope ($L_{\rm sym}$), as favored by neutron-skin thickness measurements. Thus, the detection of BV effects on PMGWs itself can provide abundant information about the symmetry energy and its slope. However, BV effects on PMGWs are generally weak and may be observed only in some extreme and optimistic cases. Therefore, observations of PMGWs and their analyses may not be able to provide very accurate measurements of the symmetry energy through BV effects.

astro-ph.HE↗

arXiv2026

McFACTS. IV. Electromagnetic Counterparts to AGN-disk-embedded Binary Black Hole Mergers

The accretion disks of active galactic nuclei (AGNs) are promising environments for producing binary black hole (BBH) mergers, which have been detected via gravitational waves (GWs) with the LIGO-Virgo-KAGRA (LVK) GW detector network. BBH mergers embedded in AGN disks are unique among GW formation channels in their generic ability to produce electromagnetic (EM) counterparts, via interactions between the merger remnant and the surrounding disk gas (though these are not always observable). While such mergers represent valuable multimessenger sources, the lack of predictive statistical models in existing literature currently limits our ability to select possible EM counterparts with GW detections in archival data and in real time using time-domain surveys such as the Zwicky Transient Facility or LSST. Here we employ the Monte Carlo For AGN Channel Testing and Simulation code (McFACTS, https://www.github.com/mcfacts/mcfacts) to predict the bolometric luminosities of jets and shocks associated with LVK-detectable BBH merger remnants in AGN disks. McFACTS predicts the distribution of GW observables for an underlying black hole population and disk model. In this work we present a new capability that simultaneously generates the distribution of bolometric EM luminosities corresponding to these predicted GW detections. Our results are consistent with current observational surveys and indicate that (i) migration in dense, Sirko-Goodman-like disks preferentially produces EM counterparts from high-mass ($\mathscr{M} > 40M_{\odot}$), high-spin remnants across multiple merger generations and (ii) lower chirp mass mergers are more likely to contribute observable counterparts and with shorter emission breakout times in less dense, Thompson-Quataert-Murray-like disks.

astro-ph.HE↗

arXiv2026

Narrow Population Inference Enhanced by Analytical Likelihood Models

The growing catalog of gravitational-wave events has revealed substantial diversity in the properties of compact-binary mergers. However, commonly used population-inference methods based on discrete posterior samples can struggle to constrain narrow population features, resulting in biased or unstable estimates of population hyperparameters. We first demonstrate this limitation using a toy population model by comparing parameter recovery with a continuous likelihood model against discrete approximations constructed from $10^3$, $10^4$, and $10^5$ samples. We then perform the same comparison using synthetic eccentric and multisource populations introduced in previous studies. Although the continuous and discrete approaches yield broadly consistent results, the continuous approximation more accurately recovers the parameters of narrow simulated populations. In particular, while both methods produce similar mass distributions, appreciable differences arise for narrowly distributed parameters such as spin and eccentricity. Our results indicate that the continuous approach provides more reliable inference for spin and eccentricity, whose narrow population distributions can be inadequately represented by finite sample sets. Continuous likelihood models therefore offer a valuable tool for improving population inference and extracting more robust information about the formation and evolution of compact-binary systems.

astro-ph.HE↗

arXiv2026

Uncovering Hierarchical Sub-Population of Binary Black Holes

Enabled by improved instruments with increasing sensitivity, the ongoing gravitational wave census now contains 259 binary black holes, numerous enough to unveil trends, substructure, and subpopulations which may provide key clues to their underlying formation mechanisms. In this work, motivated by evidence for multiple formation channels including hierarchical formation, we build a natively multi component mixture model for the binary black hole population, in which each component has an independently recovered rate, mass, spin, and spin misalignment model. (The components share a common redshift distribution.) Using a model carefully tuned to avoid parameter degeneracies, a powerlaw model plus five successively higher mass gaussians, we recover overall merger rates versus mass and trends versus redshift which are consistent with previously published results. Too, we recover previously identified overall trends versus spin: preferential alignment and low spin at low mass; large spin and isotropic spins at high mass. Critically, however, our multi-component model disagrees with previously published results, finding all components except the lowest mass are consistent with isotropy. Too, our multi-component model has a roughly hierarchical spectrum of gaussian mass peaks, but without the expected correlations between spin and mass expected from naked hierarchical formation

astro-ph.HE↗

arXiv2026

Assessing the waveform systematics from parameter estimation to population inference with eccentricity

While masses and spins are routinely used to constrain compact binary formation channels, eccentricity provides an additional and potentially powerful diagnostic of binary origin, particularly for dynamically assembled systems. Recent advances in eccentric waveform modeling now make it possible to search for eccentric signatures in gravitational wave data; however, differences between waveform models can introduce systematic effects that may propagate into astrophysical population inference. In this work, we analyze 153 binary black holes, 2 binary neutron stars and 7 neutron star black hole binaries from the GWTC-4 catalog. We compare the source and population level inferences obtained with two eccentric waveform models, SEOBNRv5EHM and TEOBResumS-DALI, as well as with quasi circular waveform analyses. We find that the two eccentric models give broadly consistent source parameter estimates for most events, but some events exhibit subtle and coherent differences. These small, systematic offsets can accumulate in hierarchical population inference, leading to differences in inferred population properties, most notably in the redshift evolution and effective spin distribution. Because coherent event level biases can grow approximately as $\sqrt{N}$ for a catalog of N events, waveform systematics become increasingly important as gravitational wave catalogs expand. We also introduce a synthetic data framework that generates eccentric populations and corresponding RIFT posterior samples, enabling injection studies that test the recoverability of eccentric population properties.

astro-ph.HE↗

arXiv2026

Inference of Neutron Star Mass Distributions and the Dense Matter Equation of State from Multi-messenger Observations

We construct a combined model to incorporate neutron star (NS) mass measurements with electromagnetic mass-radius constraints and gravitational-wave observations using Bayesian inference. We use different mass distributions for three populations depending on the companion stars: double neutron stars, NS - white dwarfs, and low-mass X-ray binaries (LMXB). To observe the effects of different parametrizations, we use two equation of state (EoS) models: a piecewise polytrope and a fixed sound-speed model at high densities in combination with a low-density EoS. Our results show that the mass distributions of these NS populations are distinct and sensitive to the EoS prior choices. In addition, we show for the first time that using a uniform prior on the observable NS maximum mass, rather than a nuisance parameter in the unknown high-density EoS, shifts the posterior maximum mass to larger values. For polytropic EoSs, the maximum mass posterior changes from $M_\mathrm{max}=2.09_{-0.07}^{+0.18} M_\odot$ to $2.15_{-0.10}^{+0.19} M_\odot$ at 90% confidence level. This change in prior also impacts the shape of the mass distribution for NSs in LMXB, shifting the posterior for the population mean from $μ_\mathrm{lmxb} = 1.51_{-0.13}^{+0.13} M_\odot$ to $1.62_{-0.12}^{+0.15} M_\odot$ at 68% confidence level.

astro-ph.HE↗

arXiv2026

Limitations in constraining neutron star radii and nuclear properties from inspiral gravitational wave detections

We investigate the constraints on the neutron star equation of state (EoS) and nuclear properties achievable with third-generation gravitational wave detectors using the Fisher information matrix approach within the relativistic mean field (RMF) theory. Assuming an optimistic binary neutron star (BNS) merger rate, we generate simulated inspiral gravitational wave (GW) signals corresponding to one year of observation. From these simulated data, we compute the covariance matrix and posterior distributions for nuclear properties and EoS. Our results show that the EoS can be tightly constrained, particularly in the density range between one and four times nuclear saturation density. However, due to the scarcity of low-mass neutron stars in the GW sample, the EoS at sub-saturation densities remains poorly constrained. Thus, in turn, leads to weaker constraints on neutron star radii, as the radii are sensitive to the low-density EoS. Additionally, we present the expected correlations among nuclear parameters in general and plots of the inferred symmetry energy in particular, which represent degeneracies in their influence on the EoS and make them difficult to be constrained through GW observations alone. These highlights inherent limitations of inspiral GW signals in probing dense matter properties. Therefore, precise radius measurements, post-merger GW observations, and supplementary constraints from terrestrial nuclear experiments remain essential for a comprehensive understanding of dense matter.

astro-ph.HE↗

arXiv2026

Gravitational Wave Hyperbolic Catalog: Reanalyzing High-Mass Gravitational Wave Signals Using Hyperbolic Waveforms

Close hyperbolic encounters between black holes produce distinctive bursts of gravitational radiation with a time-frequency morphology that is qualitatively different from that of quasi-circular inspirals. Expected to arise in dense stellar environments through dynamical interactions, these encounters probe formation channels and mass ranges inaccessible to isolated binary evolution, making them a compelling target for current and next-generation detectors. In this work, we reanalyze \totalevents high-mass events from the LIGO-Virgo-KAGRA catalogs using the hyperbolic configuration of the~\dali~waveform model. We compare these with analyses using the quasi-circular, precessing configuration of the same model, computing Bayes factors to evaluate which description is favored by the data. We find that most events strongly to mildly favor the quasi-circular, precessing scenario, except for GW190521. For this event, we find that the signal is best fit by a dynamical capture waveform, with Bayes factor $\ln \mathcal{B}^{\rm hyp}_{\rm prec}=3.71^{+0.11}_{-0.11}$. We confirm this preference via further analyses with~\dali~in different configurations (quasi-circular, non-precessing; eccentric, non-precessing; and eccentric, precessing), as well as one using the quasi-circular, precessing numerical relativity surrogate model \nrsur. We also highlight the results we obtain for GW231123, another high-mass signal linked to evidence of strong precession, for which we find strong preference for the quasi-circular, precessing scenario, with $\ln \mathcal{B}^{\rm hyp}_{\rm prec}=-15.80^{+0.24}_{-0.24}$. The analysis of mock signals generated with the best fitting waveforms for GW190521 and GW231123 suggest that the former might belong to a region of parameter space where high-mass, bound, precessing signals can be hard to distinguish from dynamical captures in parameter estimation.

gr-qc↗

arXiv2026

Population Properties of Binary Black Holes with Eccentricity

The development of eccentric waveform models enables us to explore the growing catalog of gravitational-wave events with measurable eccentricity. This opens new opportunities to gain insight into the formation channels and evolutionary pathways of compact binary systems using eccentricity. However, most recent population analyses have been limited to quasi-circular binaries, primarily due to constraints in waveform modeling and sensitivity estimates. We are now entering an era where both of these limitations are being addressed, allowing for a more comprehensive investigation of eccentric binary populations. In this work, we perform a very first population analysis that simultaneously fits the mass, spin, redshift, and eccentricity distribution. Specifically, we use source-parameter estimation on 153 binary black holes in GWTC-4 catalog provided by the Rapid Iterative FiTting (RIFT) framework using the SEOBNRv5EHM waveform model. We extend the default O4a population model to include orbital eccentricity. We find that inferred population properties are broadly consistent with conclusions obtained in previous analyses assuming quasi-circular binaries. To assess sensitivity of our results to the most eccentric sources, we repeat our analysis excluding GW200129_065458. Consistent with our conclusions about each event and using Nonoverlapping Mixture eccentricity model, we bound the branching ratio for eccentric events to be below $0.051890$ and $0.022011$ at $90\%$ confidence with and without GW200129_065458 respectively. Using four different parametric population models for eccentricity, we argue that the rate of eccentric events is weakly constrained by observations and highly model-dependent.

astro-ph.HE↗

arXiv2026

Assessing the imprint of eccentricity in GW signatures using two independent waveform models

The gravitational wave signal from merging compact binaries encodes information about their orbital and intrinsic properties. Over the last few years, state-of-the-art waveform models have begun to incorporate the effects of orbital eccentricity into their estimated signal. Over a similar period, many groups have applied these waveforms to characterize whether the imprint of eccentricity is present and, if so, measure this time-evolving property (at a suitably-defined reference point). In this work, we present a comprehensive analysis of 162 confident sources identified in the O3 and O4a observing runs of the International Gravitational Wave Network (LIGO-Virgo-KAGRA). Using the RIFT parameter inference engine, we employ two independently implemented waveform models (SEOBNRv5EHM and TEOBResumS-Dali) which account for orbital eccentricity and the effects of aligned compact object spins. Using these two waveforms, we find consistent conclusions that disfavor the eccentric hypothesis. Unlike previous work, among binary black hole candidates, we find potential evidence for eccentricity in three events: GW200129, GW231001, and GW231123. For the latter two events, the evidence for eccentricity is ambiguous, with different degrees of support from different waveforms. Consistent with previous work, we find conclusions obtained about GW200129 can be sensitive to analysis settings, as expected, given the nonstationary noise present.

astro-ph.HE↗

arXiv2026

Parameter Estimation with Targeted Eccentric Numerical-Relativity Simulations for GW200208_22 and GW190620

We have analyzed LVK gravitational wave events that show some evidence of eccentricity from TEOBResumS modeling parameter estimations and have confronted them independently with full numerical generated waveforms from our bank of nearly two thousand simulations of binary black holes. We have used RIFT for Bayesian parameter estimation and found that GW200208_22 KDE estimates favor eccentricities $e_{20} = 0.198_{-0.180}^{+0.119}$ upon entering the LVK band at $\sim20$Hz within a $90\%$ confidence interval. Within this event analysis we employed 42 new targeted full numerical relativity simulations and we have thus found a top improved likelihood $\ln\mathcal{L}$ matching waveform, compared to model-based analysis, with an estimated eccentricity at 20Hz, $e_{20}=0.200$, thus reinforcing the eccentric hypothesis of the binary. We have also used our full bank of numerical waveforms on GW190620 finding that the KDE estimate favors eccentricities at 10 Hz in $e_{10}=0.190_{-0.186}^{+0.046}$. New specifically targeted simulations will be required to narrow these eccentricity ranges.

gr-qc↗

arXiv2026

Waveform Modelling for the Laser Interferometer Space Antenna

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome.

gr-qc↗

arXiv2026

Measuring Eccentricity and Addressing Waveform Systematics in GW231123

The gravitational-wave event GW231123_135430 is the heaviest binary black hole system observed by the LIGO--Virgo--KAGRA Collaboration to date, with the initial analysis indicating the individual black hole masses lie within or above the theorized pair-instability mass gap of roughly $60$--$130\,M_\odot$. The inference further suggests that both black holes possess high spins, measured to be $0.90^{+0.10}_{-0.19}$ and $0.80^{+0.20}_{-0.51}$. Therefore, the observation of this event suggests the formation of black holes from channels beyond the standard stellar collapse. However, different waveform models yield significantly different parameter estimates, possibly due to missing physics in the models used in inference. In this work, we carry out a reanalysis of GW231123 using a physically complete model, accounting for both spin precession and eccentricity. Our analysis shows that this event does not exhibit strong evidence for eccentricity and the exclusion of eccentricity has minimal impact on inference. Furthermore, for GW231123-like systems, even eccentricities as large as $0.15$ at $10$ Hz do not yield a confident nonzero eccentricity measurement. Through a zero-noise injection recovery study, we show that the observed discrepancies in the parameter estimates can be explained by disagreement in the waveform models at strong spin precession, with the degree of parameter bias in the zero-noise runs being comparable to that observed for the real signal. We also show that inference performed with an eccentric, aligned-spin waveform model can yield a confident nonzero eccentricity measurement due to the degeneracy between eccentricity and spin precession. Bayesian model selection, however, rules out this interpretation in favor of the eccentric, spin precessing hypothesis, which supports zero eccentricity -- a conclusion we confirm with additional zero-noise injection-recovery tests.

gr-qc↗

arXiv2026

Joint Electromagnetic and Gravitational Wave Inference of Binary Neutron Star Merger GW170817 Using Forward-Modeling Ejecta Predictions

We reassess the capacity for multimessenger inference of AT2017gfo/GW170817 using both kilonova and gravitational wave emission within the context of a recent simulation-based surrogate model for kilonova emission. Independent of the inclusion of gravitational wave observations, comparisons between observations that incorporate our kilonova model favor a narrow range of ejecta properties, even when allowing for a wide range of systematic uncertainties in our modeling approach. Conversely, we find that astrophysical conclusions about the neutron star itself, including its mass and radius, depend strongly on assumptions about how much material is ejected from the neutron star. Looking forward, our analysis highlights the importance of systematic uncertainty in general, the need for better modeling of neutron star merger mass ejection from first principles, and warns against uncontextualized applications of ejecta predictions using fits to numerical relativity simulations.

astro-ph.HE↗

arXiv2026

An Implementation to Identify the Properties of Multiple Population of Gravitational Wave Sources

The rapidly increasing sensitivity of gravitational wave detectors is enabling the detection of a growing number of compact binary mergers. These events are crucial for understanding the population properties of compact binaries. However, many previous studies rely on computationally expensive inference frameworks, limiting their scalability. In this work, we present GWKokab, a JAX-based framework that enables modular model building with independent rate for each subpopulation such as BBH, BNS, and NSBH binaries. It provides accelerated inference using the normalizing flow based sampler called flowMC and is also compatible with NumPyro samplers. To validate our framework, we generated two synthetic populations, one comprising spinning eccentric binaries and the other circular binaries using a multi-source model. We then recovered their injected parameters at significantly reduced computational cost and demonstrated that eccentricity distribution can be recovered even in spinning eccentric populations. We also reproduced results from two prior studies: one on non-spinning eccentric populations, and another on the BBH mass distribution using the third Gravitational Wave Transient Catalog (GWTC-4). We anticipate that GWKokab will not only reduce computational costs but also enable more detailed subpopulation analyses such as their mass, spin, eccentricity, and redshift distributions in gravitational wave events, offering deeper insights into compact binary formation and evolution.

gr-qc↗

arXiv2026

Analysis of GWTC-3 with fully precessing numerical relativity surrogate models

The third Gravitational-Wave Transient Catalog (GWTC-3) contains 90 binary coalescence candidates detected by the LIGO-Virgo-KAGRA Collaboration (LVK). We provide a re-analysis of binary black hole (BBH) events using a recently developed numerical relativity (NR) waveform surrogate model, NRSur7dq4, that includes all $\ell \leq 4$ spin-weighted spherical harmonic modes as well as the complete physical effects of precession. Properties of the remnant black holes' (BH's) mass, spin vector, and kick vector are found using an associated remnant surrogate model NRSur7dq4Remnant. Both NRSur7dq4 and NRSur7dq4Remnant models have errors comparable to numerical relativity simulations and allow for high-accuracy parameter estimates. We restrict our analysis to 47 BBH events that fall within the regime of validity of NRSur7dq4 (mass ratios greater than 1/6 and total masses greater than $60 M_{\odot}$). While for most of these events our results match the LVK analyses that were obtained using the semi-analytical models such as IMRPhenomXPHM and SEOBNRv4PHM, we find that for more than 20\% of events the NRSur7dq4 model recovers noticeably different measurements of black hole properties like the masses and spins, as well as extrinsic properties like the binary inclination and distance. For instance, GW150914_095045 exhibits noticeable differences in spin precession and spin magnitude measurements. Other notable findings include one event (GW191109_010717) that constrains the effective spin $χ_{eff}$ to be negative at a 99.3\% credible level and two events (GW191109_010717 and GW200129_065458) with well-constrained kick velocities. Furthermore, compared to the models used in the LVK analyses, NRSur7dq4 recovers a larger signal-to-noise ratio and/or Bayes factors for several events.

gr-qc↗

arXiv2026

GW200105: A detailed study of eccentricity in the neutron star-black hole binary

GW200105_162426 is the first neutron star-black hole merger to be confidently confirmed through either gravitational-wave or electromagnetic observations. Although initially analyzed after detection, the event has recently gained renewed attention following a study [Morras et al. arXiv:2503.15393] that employed a post-Newtonian inspiral-only waveform model and reported strong evidence for orbital eccentricity. In this work, we perform a detailed analysis of GW200105 using state-of-the-art effective-one-body waveform models. Importantly, we present the first study of this event utilizing a physically complete model that incorporates both orbital eccentricity and spin precession across the full inspiral, merger, and ringdown stages, along with higher-order gravitational wave modes. Our results support the presence of eccentricity in the signal, with zero eccentricity excluded from the 99% credible interval, but yielding a mass ratio closer to the original LIGO-Virgo-KAGRA analysis, differing from the findings of [Morras et al. arXiv:2503.15393]. Additionally, similar to a previous eccentric-only analysis [de Lluc Planas et al. Astrophys. J. 995, 47 (2025).], we observe a multimodal structure in the eccentricity posterior distribution. We conduct targeted investigations to understand the origin of this multimodality and complement our analysis with numerical relativity simulations to examine how the inclusion of eccentricity impacts the merger dynamics.

gr-qc↗