arXiv subjects

R. O'Shaughnessy

Publications and source records attributed to R. O'Shaughnessy.

At least 19 recordsLinked to original sources

arXiv2026

Analysis of GWTC-3 with multiple quasicircular models

The interpretation of gravitational wave sources depends on the specific choice of model used to interpret signals. Previous analyses of GWTC-3 event candidates using SEOBNRv4PHM, IMRPhenomXPHM, and NRSur7dq4 have sometimes arrived at notably different conclusions about event properties. Subtle analysis settings and code differences can also produce notable differences. We revisit 60 publicly available events from the first three observing runs, released across GWTC-1, GWTC-2, GWTC-2.1, and GWTC-3, using a consistent analysis framework and three waveform models: two state-of-the-art time-domain models with higher-order modes (SEOBNRv5PHM and IMRPhenomTPHM) and one older frequency-domain model without higher-order modes (IMRPhenomPv2). The two state-of-the-art models agree well overall, although about 20\% of events have a visible difference in at least one one-dimensional marginal posterior. By contrast, the older model often arrives at qualitatively different conclusions for key events across the mass spectrum. We also identify a bug in published LVK GWTC-2.1 SEOBNRv4PHM results and in a later reprocessing of GW200105. At the adopted sampling rates, the model's Nyquist-frequency restriction excluded part of the allowed parameter space and artificially truncated the mass-ratio posterior away from equal mass for seven low-mass events. These results emphasize the need to use multiple state-of-the-art waveform models and carefully validated analysis settings to characterize uncertainty in gravitational-wave source properties.

gr-qc↗

arXiv2026

Physics-based phenomenological modeling of binary black hole hierarchical formation 2: Autodifferentiable functional inference of hierarchical compact-binary populations

The gravitational-wave (GW) census contains mass and spin structure consistent with contributions from black holes assembled through repeated mergers in dense environments. Connecting that structure to formation physics requires models that are both physically interpretable and tractable within population inference. We construct an autodifferentiable, physics-based phenomenological model in which each dense environment is represented by a coagulation response and a population of such environments produces an observable merger-rate density. Embedded in the gwkokab Poisson-likelihood framework, this model enables joint inference of natal-population and interaction parameters from the GW census. Applied to GWTC-5.0, the framework shows why simple pairwise coagulation models struggle to reproduce the observed high-mass, comparable-mass population and tests alternative interaction structures against the data, while retaining an explicitly modeled natal component.

gr-qc↗

arXiv2026

Physics-based phenomenological modeling of binary black hole hierarchical formation 1: Synthetic universes from globular cluster simulations for GWTC

We iteratively model the GWTC-5.0 binary-black-hole census with three components. First, a physically normalized population from Rapster globular-cluster simulations spans cluster mass, metallicity, formation redshift, compactness, and natal black-hole spin. Hierarchical mergers reproduce higher-mass events and their larger effective-spin dispersion, provided black holes are born with zero natal spin. Second, a phenomenological field (isolated-binary) channel supplies the low-mass, preferentially aligned population. Its inclusion flattens the cluster compactness likelihood, permitting ordinary dense globular-cluster birth radii without requiring nuclear-cluster-like conditions. Finally, residual tension at $15$--$30 M_\odot$ motivates an intermediate-mass isotropic component representing field remnants reprocessed in clusters. This is a hypothesis, not a detection: current numerical support precludes a reliable evidence comparison. Physical normalization converts the high-mass rate into $\hat{f}_{\rm GC}\simeq0.39\%$, with local rates $R_{\rm cl}\simeq9.1$ and $R_{\rm field}\simeq16.0$ Gpc$^{-3}$ yr$^{-1}$. The model predicts linked mass-spectrum breaks near $35$ and $70 M_\odot$, a $q\simeq0.5$ feature from first-plus-second-generation pairings, and a symmetric effective-spin distribution that broadens sharply above $45 M_\odot$. These correlated, mass-resolved predictions can be tested as the gravitational-wave census grows.

gr-qc↗

arXiv2025

Reconstructing the origin of black hole mergers using sparse astrophysical models

The astrophysical origin of binary black hole mergers discovered by LIGO and Virgo remains uncertain. Efforts to reconstruct the processes that lead to mergers typically rely on either astrophysical models with fixed parameters, or continuous analytical models that can be fit to observations. Given the complexity of astrophysical formation mechanisms, these methods typically cannot fully take into account model uncertainties, nor can they fully capture the underlying processes. Here, we present a merger population analysis that can take a discrete set of simulated model distributions as its input to interpret observations. The analysis can take into account multiple formation scenarios as fractional contributors to the total set of observations, and can naturally account for model uncertainties. We apply this technique to investigate the origin of black hole mergers observed by LIGO Virgo. Specifically, we consider a model of AGN assisted black hole merger distributions, exploring a range of AGN parameters along with several {SEVN} population synthesis models that vary in common envelope efficiency parameter ($α$) and metallicity ($Z$). We estimate the posterior distributions for AGN+SEVN models using $87$ BBH detections from the $O1--O3$ observation runs. The inferred total merger rate is $46.2 {Gpc}^{-3} {yr}^{-1}$, with the AGN sub-population contributing $21.2{Gpc}^{-3}{yr}^{-1}$ and the SEVN sub-population contributing $25.0 {Gpc}^{-3} {yr}^{-1}$.

astro-ph.HE↗

arXiv2025

Prospects for the formation of GW231123 from the AGN channel

The recent binary black hole (BBH) merger GW231123 consisted of the merger of two intermediate mass black holes (IMBH) which appear to have large spin magnitudes. Active galactic nuclei (AGN) are very promising environments for IMBH mergers and growth due to high escape velocities. Here we demonstrate how GW231123 can be produced in the AGN channel. Using the McFACTS code, we explore the impact of various choices of the black hole (BH) initial mass function (IMF) on predicted mass and spin magnitudes of BBH mergers from the AGN dynamical formation channel. By integrating the likelihood function for GW231123 with the detectable BBH population predicted from AGN using McFACTS, we demonstrate that GW231123 is consistent with a dynamical BBH merger from the AGN channel. We also postulate that the masses and spin magnitudes of GW231123 are most consistent with a merger of fourth and third generation BHs, for most choices of a segregated BH IMF and AGN lifetime.

gr-qc↗

arXiv2025

Narrowing RIFT: Focused simulation-based-inference for interpreting exceptional GW sources

The Rapid Iterative FiTting (RIFT) parameter inference algorithm provides a simulation-based inference approach to efficient, highly-parallelized parameter inference for GW sources. Previous editions of RIFT have conservatively optimized for robust inference about poorly constrained observations. In this paper, we summarize algorithm enhancements and operating point choices to enable inference for more exceptional compact binaries. Using the previously-reported RIFT/asimov interface to efficiently perform analyses on events with reproducible settings consistent with past work, we demonstrate that the latest version of RIFT can efficiently analyze events with multiple costly models including the effects of precession or eccentricity.

astro-ph.IM↗

arXiv2024

Efficient reanalysis of events from GWTC-3 with RIFT and asimov

Different waveform models can yield notably different conclusions about the properties of individual gravitational wave events. For instance, previous analyses using the SEOBNRv4PHM, IMRPhenomXPHM models, and NRSur7dq4 have led to varying results regarding event properties. This variability complicates the interpretation of the data and understanding of the astrophysical phenomena involved. There is an ongoing need to reassess candidate events with the best available interpretations and models. Current approaches lack efficiency or consistency, making it challenging to perform large-scale reanalyses with updated models or improved techniques. It is imperative that investigations into waveform systematics be reproducible. Frameworks like asimov can facilitate large-scale reanalyses with consistent settings and high-quality results, and can reliably show how different waveform models affect the interpretation of gravitational wave events. This, in combination with other provided tools, allow for reanalysis of several events from the GWTC-3 catalog. We include access to full analysis settings that facilitate public use of GWOSC data on the Open Science Grid, particularly those conducted with the IMRPhenomPv2, SEOBNRv4PHM, SEOBNRv5PHM, and NRSur7dq4 waveform models. Our parameter inference results find similar conclusions to previously published work: for several events, all models largely agree, but for a few exceptional events these models disagree substantially on the nature of the merging binary.

astro-ph.HE↗

arXiv2024

A novel emulator for fission event generators

A wide variety of emulators have been developed for nuclear physics, particularly for use in quantifying and propagating parametric uncertainties to observables. Most of these methods have been used to emulate structure observables, such as energies, masses, or separation energies, or reaction observables, such as cross sections. Rarely, if ever, have event generators for theory models been emulated. Here, we describe one such novel emulator for the fission fragment decay code, $\texttt{CGMF}$, which calculates the emission of prompt neutrons and $γ$ rays from fission fragments. The emulator described in this work uses a combination of a noisy emission model and mixture density network to model the neutron and $γ$-ray multiplicities and energies. In this manuscript, we display the power of this type of emulator for not only modeling average prompt fission observables but also correlations between fission fragment initial conditions and these observables, using both neutron-induced and spontaneous fission reactions.

nucl-th↗

arXiv2023

Eccentricity estimation for five binary black hole mergers with higher-order gravitational wave modes

The detection of orbital eccentricity for a binary black hole system via gravitational waves is a key signature to distinguish between the possible binary origins. The identification of eccentricity has been difficult so far due to the limited availability of eccentric gravitational waveforms over the full range of black hole masses and eccentricities. Here we evaluate the eccentricity of five black hole mergers detected by the LIGO and Virgo observatories for the first time using the TEOBResumSGeneral model. This model accounts for the full eccentricity range possible and incorporates higher-order gravitational wave modes critical to model emission from highly eccentric orbits. The binaries have been selected due to previous hints of eccentricity or due to their unusual mass and spin. While other studies found marginal evidence for eccentricity for some of these events, our analyses do not favor the incorporation of eccentricity compared to the quasi-circular case. While lacking the eccentric evidence of other analyses, we find our analyses marginally shifts the posterior in multiple parameters for several events when allowing eccentricity to be non-zero.

gr-qc↗

arXiv2023

Constraining inputs to realistic kilonova simulations through comparison to observed $r$-process abundances

Kilonovae, one source of electromagnetic emission associated with neutron star mergers, are powered by the decay of radioactive isotopes in the neutron-rich merger ejecta. Models for kilonova emission consistent with the electromagnetic counterpart to GW170817 predict characteristic abundance patterns, determined by the relative balance of different types of material in the outflow. Assuming the observed source is prototypical, this inferred abundance pattern in turn must match $r$-process abundances deduced by other means, such as what is observed in the solar system. We report on analysis comparing the input mass-weighted elemental compositions adopted in our radiative transfer simulations to the mass fractions of elements in the Sun, as a practical prototype for the potentially universal abundance signature from neutron-star mergers. We characterize the extent to which our parameter inference results depend on our assumed composition for the dynamical and wind ejecta and examine how the new results compare to previous work. We find that a dynamical ejecta composition calculated using the FRDM2012 nuclear mass and FRLDM fission models with extremely neutron-rich ejecta ($Y_{\rm{e}} = 0.035$) along with moderately neutron-rich ($Y_{\rm e} = 0.27$) wind ejecta composition yields a wind-to-dynamical mass ratio of $M_{\rm{w}}/M_{\rm{d}}$ = 0.47 which best matches the observed AT2017gfo kilonova light curves while also producing the best-matching abundance of neutron-capture elements in the solar system.

astro-ph.HE↗

arXiv2023

Expanding RIFT: Improving performance for GW parameter inference

The Rapid Iterative FiTting (RIFT) parameter inference algorithm provides a framework for efficient, highly-parallelized parameter inference for GW sources. In this paper, we summarize essential algorithm enhancements and operating point choices for the RIFT iterative algorithm, including choices used for analysis of LIGO/Virgo O3 observations. We also describe other extensions to the RIFT algorithm and software ecosystem. Some extensions increase RIFT's flexibility to produce outputs pertinent to GW astrophysics. Other extensions increase its computational efficiency or stability. Using many randomly-selected sources, we assess code robustness with two distinct code configurations, one designed to mimic settings as of LIGO O3 and another employing several performance enhancements. We illustrate RIFT's capabilities with analysis of selected events.

gr-qc↗

arXiv2023

Low-latency parameter inference enabled by a Gaussian likelihood approximation for RIFT

Rapid identification, characterization, and localization of gravitational waves from binary compact object mergers can enable well-informed follow-on multimessenger observations. In this work, we investigate a small modification to the RIFT parameter inference pipeline to enable extremely low-latency inference, tested here for nonprecessing sources.

gr-qc↗

arXiv2022

Black hole - neutron star mergers: The first mass gap and kilonovae

Observations of X-ray binaries indicate a dearth of compact objects in the mass range from $\sim 2-5 M_{\odot}$. The existence of this (first mass) gap has been used to discriminate between proposed engines behind core-collapse supernovae. From LIGO/Virgo observations of binary compact remnant masses, several candidate first mass gap objects (either neutron stars (NSs) or black holes (BHs)) were identified during the O3 science run. Motivated by these new observations, we study the formation of BH-NS mergers in the framework of isolated classical binary evolution, using population synthesis methods to evolve large populations of binary stars (Population I and II) across cosmic time. We present results on the NS to BH mass ratios ($q=M_{\rm NS}/M_{\rm BH}$) in merging systems, showing that although systems with a mass ratio as low as $q=0.02$ can exist, typically BH-NS systems form with moderate mass ratios $q=0.1-0.2$. If we adopt a delayed supernova engine, we conclude that $\sim 30\%$ of BH-NS mergers may host at least one compact object in the first mass gap (FMG$^\circ$). Even allowing for uncertainties in the processes behind compact object formation, we expect the fraction of BH-NS systems ejecting mass during the merger to be small (from $\sim 0.6-9\%$). In our reference model, we assume: (i) the formation of compact objects within the FMG, (ii) natal NS/BH kicks decreased by fallback, (iii) low BH spins due to Tayler-Spruit angular momentum transport in massive stars. We find that $\lesssim 1\%$ of BH-NS mergers will have any mass ejection and about the same percentage will produce kilonova bright enough to have a chance of being detected with a large (Subaru-class) $8$m telescope. Interestingly, all these mergers will have both a BH and an NS in the FMG.

astro-ph.HE↗

arXiv2022

Interpolating Detailed Simulations of Kilonovae: Adaptive Learning and Parameter Inference Applications

Detailed radiative transfer simulations of kilonovae are difficult to apply directly to observations; they only sparsely cover simulation parameters, such as the mass, velocity, morphology, and composition of the ejecta. On the other hand, semianalytic models for kilonovae can be evaluated continuously over model parameters, but neglect important physical details which are not incorporated in the simulations, thus introducing systematic bias. Starting with a grid of 2D anisotropic simulations of kilonova light curves covering a wide range of ejecta properties, we apply adaptive-learning techniques to iteratively choose new simulations and produce high-fidelity surrogate models for those simulations. These surrogate models allow for continuous evaluation across model parameters while retaining the microphysical details about the ejecta. Using a new code for multimessenger inference, we demonstrate how to use our interpolated models to infer kilonova parameters. Comparing to inferences using simplified analytic models, we recover different ejecta properties. We discuss the implications of this analysis which is qualitatively consistent with similar previous work using detailed ejecta opacity calculations and which illustrates systematic challenges for kilonova modeling. An associated data and code release provides our interpolated light-curve models, interpolation implementation which can be applied to reproduce our work or extend to new models, and our multimessenger parameter inference engine.

astro-ph.HE↗

arXiv2022

Eccentricity Estimate for Black Hole Mergers with Numerical Relativity Simulations

The origin of black hole mergers discovered by the LIGO and Virgo gravitational-wave observatories is currently unknown. GW190521 is the heaviest black hole merger detected so far. Its observed high mass and possible spin-induced orbital precession could arise from the binary having formed following a close encounter. An observational signature of close encounters is eccentric binary orbit; however, this feature is currently difficult to identify due to the lack of suitable gravitational waveforms. No eccentric merger has been previously found. Here we report 611 numerical relativity simulations covering the full eccentricity range and an estimation approach to probe the eccentricity of mergers. Our set of simulations corresponds to $\sim 10^5$ waveforms, comparable to the number used in gravitational wave searches, albeit with coarser mass-ratio and spin resolution. We applied our approach to GW190521 and found that it is the most consistent with a highly eccentric ($e=0.69^{+0.17}_{-0.22}$; 90% credible level) merger within our set of waveforms. This interpretation is supported over a non-eccentric merger with $>10$ Odds ratio if $\gtrsim10\%$ of GW190521-like mergers are highly eccentric. Detectable orbital eccentricity would be evidence against an isolated binary origin, which is otherwise difficult to rule out based on observed mass and spin.

astro-ph.HE↗

arXiv2021

LIGO--Virgo correlations between mass ratio and effective inspiral spin: testing the active galactic nuclei channel

Observations by LIGO--Virgo of binary black hole mergers suggest a possible anti-correlation between black hole mass ratio ($q=m_{2}/m_{1}$) and the effective inspiral spin parameter $χ_{\rm eff}$, the mass-weighted spin projection onto the binary orbital angular momentum (Callister et al. 2021). We show that such an anti-correlation can naturally occur for binary black holes assembled in active galactic nuclei (AGN) due to spherical and planar symmetry-breaking effects. We describe a phenomenological model in which: 1) heavier black holes live in the AGN disk and tend to spin up into alignment with the disk; 2) lighter black holes with random spin orientations live in the nuclear spheroid; 3) the AGN disk is dense enough to rapidly capture a fraction of the spheroid component. but small in radial extent to limit the number of bulk disk mergers; 4) migration within the disk is non-uniform, likely disrupted by feedback from migrators or disk turbulence; 5) dynamical encounters in the disk are common and preferentially disrupt binaries that are retrograde around their center of mass, particularly at stalling orbits, or traps. This model may explain trends in LIGO--Virgo data while offering falsifiable predictions. Comparisons of predictions in ($q,χ_{\rm eff}$) parameter space for the different channels may allow us to distinguish their fractional contributions to the observed merger rates.

astro-ph.HE↗

arXiv2021

A Broad Grid of 2D Kilonova Emission Models

Depending upon the properties of their compact remnants and the physics included in the models, simulations of neutron star mergers can produce a broad range of ejecta properties. The characteristics of this ejecta, in turn, define the kilonova emission. To explore the effect of ejecta properties, we present a grid of 2-component 2D axisymmetric kilonova simulations that vary mass, velocity, morphology, and composition. The masses and velocities of each component vary, respectively, from 0.001 to 0.1 M$_{\odot}$ and 0.05 to 0.3$c$, covering much of the range of results from the neutron star merger literature. The set of 900 models is constrained to have a toroidal low electron fraction ($Y_e$) ejecta with a robust r-process composition and either a spherical or lobed high-$Y_e$ ejecta with two possible compositions. We simulate these models with the Monte Carlo radiative transfer code SuperNu using a full suite of lanthanide and 4th row element opacities. We examine the trends of these models with parameter variation, show how it can be used with statistical tools, and compare the model light curves and spectra to those of AT2017gfo, the electromagnetic counterpart of GW170817.

astro-ph.HE↗

arXiv2020

Assessing and marginalizing over compact binary coalescence waveform systematics with RIFT

As Einstein's equations for binary compact object inspiral have only been approximately or intermittently solved by analytic or numerical methods, the models used to infer parameters of gravitational wave (GW) sources are subject to waveform modeling uncertainty. Using a simple scenario, we illustrate these differences, then introduce a very efficient technique to marginalize over waveform uncertainties, relative to a pre-specified sequence of waveform models. Being based on RIFT, a very efficient parameter inference engine, our technique can directly account for any available models, including very accurate but computationally costly waveforms. Our evidence and likelihood-based method works robustly on a point-by-point basis, enabling accurate marginalization for models with strongly disjoint posteriors while simultaneously increasing the reusability and efficiency of our intermediate calculations.

gr-qc↗