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Paul D. Lasky

Publications and source records attributed to Paul D. Lasky.

At least 19 recordsLinked to original sources
arXiv2026

Licence to Bin: Accurate and Scalable Inference for Binary Neutron Stars in Next-Generation Gravitational-Wave Detectors

Next-generation gravitational-wave observatories will observe binary neutron-star mergers with much higher signal-to-noise ratios, over much longer durations and across broader frequency bands than current detectors. These long-duration signals present a major computational challenge for Bayesian parameter estimation. Reduced-order quadrature is a promising approach for accelerating inference, but in this regime its standard construction encounters severe memory and accuracy limitations. We present a practical reduced-order quadrature construction for long binary neutron-star signals with time-dependent detector response and full effects of the observatories' free-spectral range. Our approach combines improved adaptive frequency sampling, disk-backed streaming, and subbanded reduced-order quadrature construction, enabling efficient and accurate reduced-order models for signals that were previously intractable. We demonstrate for the first time reduced-order Bayesian inference on an approximately 2 h binary neutron-star signal extending down to 5 Hz and with signal-to-noise ratio 2090. We show the resulting reduced-order quadrature remains sufficiently accurate for practical inference. The full analysis is carried out in about 48 h using 128 CPU cores. We also find that, when time-dependent detector-response effects are included, a single Cosmic Explorer detector can localize such a signal to a 90% credible sky area of approximately $41~\mathrm{deg}^2$, with important implications for multimessenger astronomy and cosmology. These results demonstrate that reduced-order methods can make next-generation binary neutron-star inference computationally feasible.

astro-ph.HE↗
arXiv2026

Reconstructing Core-Collapse Supernova Gravitational-Wave Signals with Transdimensional Bayesian Inference

Core-collapse supernovae (CCSNe) are promising future sources of gravitational waves for current and next-generation observatories. Reconstructing CCSN gravitational-wave signals is challenging as they contain stochastic elements, have multiple complex features, and cover a wide frequency band. The stochasticity of the signal in particular motivates the need for morphology-independent reconstruction techniques which, once observed, will enable us to infer properties of the newly born proto-neutron star, the rotation, and the unknown CCSN explosion mechanism. In this work, we investigate the reconstruction of gravitational-wave signals from CCSNe using the transdimensional Bayesian inference framework tBilby. We demonstrate the method using simulated signals in synthetic Advanced LIGO detector noise at a range of signal-to-noise ratios. We reconstruct the signals using two types of wavelets: sine Gaussians and chirplets. We calculate overlaps between injected and reconstructed signals of up to 85%. We find that even when reconstruction overlap values are low, enough of the time-frequency structure of the dominant mode is captured to still make statements about the size of the evolving proto-neutron star. These capabilities establish tBilby as a powerful tool for gravitational-wave astronomy with burst sources.

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arXiv2026

Revealing Four Subpopulations of Binary Black-Hole Mergers with the Fifth Gravitational-Wave Transient Catalog

Gravitational-wave data are beginning to reveal a structured landscape of black-hole masses and spins, suggesting multiple formation processes are now being resolved observationally. We analyze data from LIGO--Virgo--KAGRA's (LVK's) fifth Gravitational-Wave Transient Catalog and find that the population is naturally described by four distinct subpopulations. The dominant component, contributing $\simeq70\%$ of the astrophysical merger rate, is characterised by a low-mass population centred near $10M_\odot$ and is separated from heavier systems by a depletion near $14M_\odot$. This component may be associated with black holes formed from failed supernovae. Above this depletion, we find two intermediate-mass components: an unequal-mass branch pairing the lower-mass, $\simeq10M_\odot$ black hole with a heavier black hole, perhaps associated with isolated-binary/stable-mass-transfer formation, and a nearly equal-mass branch peaking near $30$--$35M_\odot$ whose low spins and mass distribution favour first-generation systems possibly born in dense stellar environments. A fourth, percent-level component extends to higher masses and is characterized by a broad mass-ratio distribution and large spin magnitudes, consistent with a hierarchical-merger population. Our four-component model is overwhelmingly preferred over a standard LVK population model by a natural-log Bayes factor of $\ln(\mathrm{BF}) = 19.2$. Our work observationally unveils a new subpopulation of black-hole mergers utilising a new hybrid data-driven and parametric-model discovery method, bringing us one step closer to understanding stellar-mass black-hole archaeology.

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arXiv2026

Fortifying gravitational-wave population inference with normalizing flows

As the LIGO-Virgo-KAGRA collaboration's (LVK's) gravitational-wave transient catalog grows, we are learning a wealth of information from the population properties of binary black hole mergers. Events in the catalog are represented with posterior samples describing the astrophysical parameters for each event. Population studies combine these samples to measure the distribution of astrophysical parameters such as black hole masses and spins. However, the posterior-sample representation of each event is only approximate. We construct a mock population with masses drawn from an astrophysically-motivated distribution with sharp features. Using this, we demonstrate that when $\gtrsim 300$ events are combined, even with each event's posterior represented by $1 \times 10^4 {-} 2 \times 10^4$ samples, the numerical error can become large enough that the resulting population inference is unreliable. We consider two solutions. In the short term, we show that nested samples (already produced by LVK analyses) can be used to more accurately describe each event in population studies. But this will only grant a temporary reprieve until the nested-sample representation becomes inadequate. In the longer term, we propose to represent each event with a normalizing flow. In order to represent each event with sufficient accuracy, each normalizing flow can be used to generate an arbitrarily large number of new posterior samples with a significantly reduced computational cost relative to traditional sampling methods. When compared to nested sampling, our normalizing flows produce posterior draws with a median of $\approx 80\%$ fewer likelihood evaluations per sample, while also providing greater opportunity for parallelization. We believe refinement of normalizing flow architectures and training techniques in future works could further reduce this per-sample cost significantly.

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arXiv2026

Joint inference for gravitational-wave signal and noise glitch: Method and application

Non-Gaussian noise transients ("glitches") in gravitational-wave observatories degrade our ability to accurately perform astrophysical inference. We present the analysis pipeline bilby_glitch, which allows for simultaneous Bayesian inference of gravitational-wave signals and glitches. Our framework is modular and built on top of the popular bilby framework, facilitating future extensions with additional glitch and signal models. We integrate transdimensional bilby into our framework and discuss three glitch models: a physically-motivated slow scattering model, and flexible sine-Gaussian and chirplet models. Using a combination of simulated and real data, we demonstrate that bilby_glitch produces reliable results. We then reanalyse two gravitational-wave events - GW191109 and GW200129 - which show signs of interesting black-hole spins, but which may also be affected by data-quality issues. Our results for GW191109 are consistent with previous analysis. For GW200129, we recover results consistent with Payne et al., where the evidence of spin-precession is much weaker when using the waveform approximant NRSur7dq4 in combination with wavelet-based glitch modeling. Furthermore, we show the astrophysical conclusion of this event is dependent on the interplay between the waveform approximant and glitch model, since in contrast to NRSur7dq4 we find that inference with the waveform approximant IMRPhenomXPHM shows strong evidence of spin-precession when used in combination with wavelet-based glitch modeling.

gr-qc↗
arXiv2026

Evidence for Three Subpopulations of Merging Binary Black Holes at Different Primary Masses

With the release of the fourth LIGO--Virgo--KAGRA gravitational-wave catalog (GWTC-4), we are starting to gain a detailed view of the population of merging binary black holes. The formation channels of these black holes is not clearly understood, but different formation mechanisms may lead to subpopulations with different properties visible in gravitational-wave data. Adopting a phenomenological approach, we find GWTC-4 data supports the presence of at least three subpopulations, each associated with a different range of black hole mass and with sharp transition boundaries between them. Each subpopulation is characterized by different distributions for either the mass ratios, the black-hole spin magnitudes or both. Subpopulation A with primary mass $m_1 \leq 27.7^{+4.1}_{-3.4} M_{\odot}$ ($90 \%$ credibility), is characterized by a nearly flat mass ratio distribution $q=m_2/m_1$, and by small spin magnitudes ($χ\leq 0.5^{+0.1}_{-0.1}$). Subpopulation B with $27.7^{+4.1}_{-3.4} M_{\odot} \leq m_1 \leq 40.2^{+4.7}_{-3.2} M_{\odot}$, has a much sharper preference for mass ratio $q \approx 1$. Subpopulation C, with $m_1 \geq 40.2^{+4.7}_{-3.2} M_{\odot}$, has support for large spin magnitudes, and tentative support for mass ratios $q\approx0.5$. We interpret these transitions as evidence for multiple subpopulations, each potentially associated with a different formation pathways. We suggest potential formation scenarios for each subpopulations, and suggest that Subpopulation B may be associated with chemically homogeneous evolution or population III stars. Our findings for Subpopulation C are largely consistent with recent claims of hierarchical mergers, but with some curious differences in properties.

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arXiv2026

A universal framework to identify eccentric binary mergers: GW200105 case study

Orbital eccentricity in gravitational-wave signals from merging compact object binaries is a powerful indicator of their formation channel. Several binary black hole mergers and a neutron star--black hole merger have been reported to exhibit signs of eccentricity, but which events are identified and the significance of the eccentricity differs between studies. Measurements of eccentricity can change depending on the choice of prior. The choice of prior is subtle: eccentricity is commonly measured at an arbitrary reference frequency, which varies from study to study. We use the candidate eccentric neutron star--black hole merger GW200105_162426 as a case study, employing a range of priors and reference frequencies, and find the results to be strongly prior-driven. We show that the varied results reported across different studies can be partially reconciled by accounting for the evolution of eccentricity with reference frequency. In order to make conclusive statements about eccentricity, we propose a detection statistic that does not depend on reference frequency, and which marginalises over astrophysically-motivated distributions in eccentricity. Using this detection statistic, we find reduced support for the eccentric hypothesis for GW200105_162426: we obtain a natural log Bayes factor ln B $\leq$ 0.9 comparing the eccentric, aligned-spin hypothesis to the quasi-circular, precessing hypothesis. Our results cast doubt on the eccentric interpretation of GW200105_162426 and underscore the importance of modelling the astrophysical distributions of eccentricity in nature.

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arXiv2026

Are all models wrong? Falsifying binary formation models in gravitational-wave astronomy

As the catalogue of gravitational-wave transients grows, several entries appear "exceptional" within the population. Tipping the scales with a total mass of $\approx 150 M_\odot$, GW190521 likely contained black holes in the pair-instability mass gap. The event GW190814, meanwhile, is unusual for its extreme mass ratio and the mass of its secondary component. A growing model-building industry has emerged to provide explanations for such exceptional events, and Bayesian model selection is frequently used to determine the most informative model. However, Bayesian methods can only take us so far. They provide no answer to the question: does our model provide an adequate explanation for the data? If none of the models we are testing provide an adequate explanation, then it is not enough to simply rank our existing models - we need new ones. In this paper, we introduce a method to answer this question with a frequentist $p$-value. We apply the method to different models that have been suggested to explain GW190521: hierarchical mergers in active galactic nuclei and globular clusters. We show that some (but not all) of these models provide adequate explanations for exceptionally massive events like GW190521.

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arXiv2026

A Gaussian process framework for testing general relativity with gravitational waves

Gravitational-wave astronomy provides a promising avenue for the discovery of new physics beyond general relativity as it probes extreme curvature and ultra-relativistic dynamics. However, in the absence of a compelling alternative to general relativity, it is difficult to carry out an analysis that allows for a wide range of deviations. To that end, we introduce a Gaussian process framework to search for deviations from general relativity in gravitational-wave signals from binary black hole mergers with minimal assumptions. We employ a kernel that enforces our prior beliefs that - if gravitational waveforms deviate from the predictions of general relativity - the deviation is likely to be localised in time near the merger with some characteristic frequency. We demonstrate this formalism with simulated data and apply it to events from Gravitational-Wave Transient Catalog 3. We find no evidence for a deviation from general relativity. We limit the fractional deviation in gravitational-wave strain to as low as 7% (90% credibility) of the strain of GW190701_203306.

gr-qc↗
arXiv2026

A comprehensive framework for phase-coherent mapping of the gravitational-wave sky with pulsar timing arrays

We present a practical implementation of a phase-coherent mapping technique for pulsar timing arrays that resolves the full complex polarisation state of the gravitational-wave sky as a function of direction and frequency. Unlike standard cross-correlation methods, this approach preserves the amplitude, phase, and polarisation of the signal in every sky pixel. The resulting maps constitute a compact, minimally processed summary of the data from which all subsequent analyses -- characterisation of a stochastic background, searches for anisotropy, and identification of individual sources -- can be derived within a single unified framework. Our implementation is fully compatible with established pulsar timing data analysis methods. We validate the framework through a series of realistic simulations with varying array configurations, noise properties, and signal types. We demonstrate robust recovery of source amplitudes and sky locations across different scenarios, and discuss the impact of polarisation leakage, noise, and direction-dependent array sensitivity on the recovery of astrophysical signals.

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arXiv2026

Sub-threshold post-merger gravitational waves can constrain the hot nuclear equation of state

We show how to coherently combine information from a population of sub-threshold, gravitational-wave binary neutron star post-merger remnants. Although no individual event in our synthetic population can be claimed as a confident detection, we show how to statistically determine the fraction of merger events that promptly collapse to form a black hole, compared to those for which a neutron star survives the merger for at least tens of milliseconds. This fraction, when combined with information about the neutron star mass distribution gleaned from the inspiral portion of the signals, provides an indirect measure of the neutron star maximum mass. Using conservative measures of the post-merger waveforms, we show that 50-70 events with binary neutron star inspiral measurements can be combined to give an $11-20\%$ fractional uncertainty on the maximum mass of rapidly rotating, hot neutron stars, which can potentially be turned into a $12-21\%$ fractional constraint on the Tolman-Oppenheimer-Volkoff mass. We discuss how this measure of the hot nuclear equation of state can be combined with information of cold neutron stars to see the effect of temperature on physics in the densest regions of the Universe by providing indirect evidence for first-order phase transitions in neutron star interiors.

gr-qc↗
arXiv2026

Is GW231123 a hierarchical merger?

The binary black hole merger GW231123 is both the most massive gravitational-wave event observed and has the highest component spins measured to date. The dimensionless spins of the more massive (primary) and less massive (secondary) black holes are measured to be $χ_1 = 0.90^{+0.10}_{-0.19}$ and $χ_2 = 0.80^{+0.20}_{-0.51}$ ($90\%$ credible intervals), respectively. Its large mass and extremal spins are challenging to explain through standard binary stellar physics, though a flurry of hypothetical scenarios have been proposed. Hierarchical assembly - i.e., mergers of black holes that are themselves formed from previous generations of mergers - is generally a promising way to explain massive and rapidly spinning black holes. Here, we investigate the possibility that GW231123 was assembled hierarchically in a dense star cluster as the merger of two second-generation black holes. Taking the inferred spin values at face value, we find that it is possible ($p\approx 5\%$) that a compact binary with component spins like GW231123 could form in a cluster from hierarchical assembly.

astro-ph.HE↗
arXiv2026

Realistic Equations of State Informing Neutron Star Post-Merger Gravitational-Wave Frequencies

Binary neutron star mergers are thought to produce hot, rapidly rotating neutron stars with masses that can far exceed their Tolman-Oppenheimer-Volkoff mass. The gravitational-wave emission from such remnants provides a unique opportunity to measure the nuclear equation of state at densities and temperatures not available to terrestrial experiments. Current detector design is informed by gravitational-wave signals from general relativistic hydrodynamics simulations of neutron star mergers, typically with hybrid thermal treatments for the equation of state, where a cold equation of state is modified by adding a thermal component. We use realistic equations of state based on the relativistic mean field model with consistent treatment of thermal effects to compute the distribution of expected peak gravitational-wave frequencies. Marginalising over equation of state and progenitor neutron star masses, we show the peak frequency of emission ranges from $\sim2.5$ to 4 kHz. The width of this distribution suggests the need for broadband observatories with kHz sensitivity, and calls into question some of the so-called post-merger optimised configurations. We show the proposed KAGRA high-frequency design is well-suited to measuring post-merger remnants when compared to the KAGRA broadband design.

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arXiv2026

Frequency- and phase-resolved polarimetry of millisecond pulsars and its application to timing

Pulsar timing is used for a variety of applications including tests of fundamental physics, probing the structure of neutron stars, and detecting nanohertz gravitational waves. Development of robust methods and generation of high-quality timing data is therefore of utmost importance. In this paper, we present a new technique for creating high-fidelity templates that can be used to measure the pulse times of arrival with significantly increased precision compared to existing methods. Our framework makes use of all available polarimetric information to generate frequency-dependent models of pulse-shape evolution of all four Stokes parameters. We apply this method to millisecond pulsars observed by the Parkes Pulsar Timing Array and show that it results in timing measurement uncertainties reduced up to $\sim$20-30%. We also present, for the first time, phase- and frequency-resolved polarimetric measurements of millisecond pulsars observed with the Parkes Murriyang ultra-widebandwith-low receiver. The data, plots and the code underlying this analysis are made publicly available.

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arXiv2026

Searching for gravitational waves from compact binary mergers powering long gamma-ray bursts during LIGO-Virgo-KAGRA's O3 run

Neutron star binary mergers are often associated with short gamma-ray bursts (GRBs), but the recent detection of kilonovae coincident with long GRBs suggest that some mergers may produce long GRBs. Motivated by these developments, we perform a search for binary neutron star and neutron star-black hole gravitational-wave signals coincident with long GRBs using data from the third LIGO--Virgo--KAGRA (LVK) observing run. We analyze LVK data coincident with long GRBs detected by Fermi's GRB Monitor and Swift's Burst Alert Telescope when at least two gravitational-wave observatories were running. We find no evidence of a coincident gravitational-wave signal and set limits on the luminosity distance to each of these long GRBs under the assumption that they were powered by binary mergers.

astro-ph.HE↗
arXiv2025

GW231123: extreme spins or microglitches?

The recently reported binary black hole merger, GW231123, has unusual properties that make it hard to explain astrophysically. Parameter estimation studies are consistent with maximally spinning black holes and the dimensionless spin of the more massive component is constrained to be $χ_1\gtrsim 0.8$. Analysis of data also revealed potential systematics that could not be fully replicated with simulated studies. We explore the possibility that these measurements are biased due to unmodeled non-Gaussian noise in the detectors, and that the actual black hole spins are more modest. We present evidence for a population of \textit{microglitches} in LIGO gravitational-wave strain data that can lead to biases in the parameter estimation of short-duration signals such as GW231123. Using simulated data of a massive event like GW231123, we demonstrate how microglitches can bias our measurements of black hole spins toward $χ\approx1$ with negligible posterior support for the true value of $χ\approx0.7$. We develop a noise model to account for microglitches and show that this model successfully reduces biases in the recovery of signal parameters. We characterize the microglitch population in real interferometer data surrounding GW231123 and find a single detector glitch duty cycle of $0.57_{-0.19}^{+0.21}$, which implies nearly a $100\%$ probability that at least one event through the fourth gravitational wave transient catalog coincides with microglitches in two detectors. We argue that further investigations are required before we can have a confident picture of the astrophysical properties of GW231123.

gr-qc↗
arXiv2025

Trends in the Population of Binary Black Holes Following the Fourth Gravitational-Wave Transient Catalog: a Data-Driven Analysis

Current population models of binary black hole distributions are difficult to interpret because standard population inferences hinge on modeling choices, which can mask or mimic real structure. The maximum population likelihood ``$\pistroke$ formalism'' provides a means to investigate and interpret features in the distribution of binary black holes using only data -- without specifying a population model. It tells us if features inferred from current population models are truly present in the data or if they arise from model misspecification. It also provides guidance for developing new models by highlighting previously unnoticed features. In this study, we utilize the $\pistroke$ formalism to examine the binary black hole population in the LIGO--Virgo--KAGRA (LVK) fourth Gravitational-Wave Transient Catalog (GWTC-4). Our analysis supports the existence of a gap around $45\,M_\odot$ in the secondary black hole mass distribution and identifies a widening in the distribution of the effective inspiral spin parameter $χ_\text{eff}$ near this mass as recently reported by Tong et al. (2025). Similar to earlier studies, we find support for an anti-correlation between $χ_\text{eff}$ and mass ratio. However, we argue that this may be a spurious correlation arising from misspecification of the joint distribution of black hole masses. Furthermore, we identify support for dimensionless black hole spin magnitudes at approximately $χ\approx 0.2$ and $χ\approx0.7$. The data support the existence of a correlation between the spin magnitudes $χ_1$ and $χ_2$, though subsequent study is required to determine if this feature is statistically significant. The accompanying data release includes $\pistroke$ samples, which can be used to compare theoretical predictions to LVK data and to assess assumptions in parameterised models.

astro-ph.HE↗
arXiv2025

Do both black holes spin in merging binaries? Evidence from GWTC-4 and astrophysical implications

Angular momentum transport in high-mass stars is commonly modeled by extrapolating the behavior of better-observed low-mass stars. According to the conventional picture, the cores of most black hole progenitors lose almost all of their angular momentum when their outer layers are ejected before core collapse. Accordingly, most black holes are expected to be born with dimensionless spin magnitudes of $χ\lesssim 0.01$, even if some black holes are born with non-negligible spin due to tidal interactions in a progenitor binary. One might therefore expect to find a large fraction of $χ\lesssim 0.01$ black holes in merging binary black hole (BBH) systems. We find that the conventional picture of angular momentum transport is in tension with data from LIGO--Virgo--KAGRA's fourth gravitational-wave transient catalog. We find no support for a sub-population of BBH systems with $χ\lesssim 0.01$. Neither do we find support for a sub-population with only one spinning black hole as expected for tidal spin-up scenarios. Instead, we find evidence for two subpopulations in which both black holes have non-negligible spin. Approximately 84% of BBH systems contain two black holes with modest spins $χ\approx 0.1$ and approximately 16% contain two black holes with large spins $χ\approx 0.8$. These estimates come from our best-fit model, which is favored with natural log Bayes factors $\ln B \gtrsim 3$ over models that require a sub-population of $χ\lesssim 0.01$ black holes, and models that do not contain multiple spin sub-populations. These results are difficult to reconcile with our current understanding of angular momentum transport.

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