arXiv subjects

Isobel Romero-Shaw

Publications and source records attributed to Isobel Romero-Shaw.

13 recordsLinked to original sources

arXiv2026

The Shape of Eccentricity: Rapid Classification of Eccentric Binaries with the Wavelet Scattering Transform

The gravitational-wave (GW) detections reported by the LIGO-Virgo-KAGRA (LVK) collaboration have so far been consistent with quasi-circular compact binary coalescences (CBCs). Nevertheless, a small fraction of binaries driven to merge through dynamical interactions in dense stellar environments or in field triples may retain measurable orbital eccentricity when entering the sensitive frequency band of LVK detectors. Confident measurement of eccentricity in the LVK band would provide strong evidence for such dynamically driven mergers; however, eccentric gravitational-waveform models are computationally expensive, and performing production-level inference on all detected signals is not an efficient use of resources when eccentric signals are expected to be rare. An intermediate step between detection and analysis, in which the signal is assessed for the potential presence of eccentricity, could provide quick recommendations for which signals should undergo full eccentric inference. We apply the wavelet scattering transform (WST) to a large set of synthetic waveforms in realistic noise and assess its discriminatory power using simple linear and shallow neural-network classifiers. We find that the WST representation enables effective discrimination between eccentric and quasi-circular binaries and provides a compact multi-scale representation of GW signals. Our approach achieves ~64% percent detection accuracy at a false alarm rate of 10%, with an AUC of 0.844 and an average precision of 0.876. We also examine the ability of our classifiers to distinguish eccentricity from spin-induced precession and find robust performance across a range of spin-precession magnitudes.

gr-qc↗

arXiv2026

Distinct spin properties and astrophysical origin of low mass binary black holes in gravitational wave data

We analyze the effective-spin distribution of binary black hole mergers in GWTC-5.0 as a function of primary black hole mass using hierarchical Bayesian inference. We model the population as a mixture of two spin components separated by a transition mass scale inferred directly from the data. We find strong evidence for a transition at $\tilde{m} = 15.2^{+4.3}_{-3.6}\, M_\odot$. Mock-catalog analyses show that such a transition is unlikely to arise from finite-sample fluctuations of a mass-independent $χ_{\rm eff}$ population and the posterior predictive distributions of $χ_{\rm eff}$ inferred below and above the transition are clearly distinct. Below the transition mass, the effective-spin distribution is narrow, peaks at a small positive value $χ_{\rm eff}>0$, but also shows significant support for negative $χ_{\rm eff}$. Above the transition, the distribution is broader and its peak shifts to values consistent with $χ_{\rm eff}\simeq0$, making its support at both positive and negative $χ_{\rm eff}$ roughly similar. These findings suggest that the dominant merger population concentrated around $10\,M_{\odot}$ is statistically distinct from the rest and that it arises from a different formation channel. We show that this low-mass population is broadly consistent with formation from massive stellar multiples in the field: it may either arise from isolated binary star evolution but only if black hole natal kicks below $\tilde{m}$ are generally very large ($\gtrsim100\,\rm km/s$) or be caused by the dynamical evolution of hierarchical triples. In contrast, isolated binary evolution with standard fallback kick models cannot reproduce the support for negative $χ_{\rm eff}$.

astro-ph.HE↗

arXiv2026

Transitions in the Mass-ratio and Spin Properties of Binary Black Holes in GWTC-5

We analyze the mass-ratio and effective-spin ($χ_{\rm eff}$) distributions of binary black hole mergers in the latest gravitational-wave catalog, GWTC-5, as a function of primary mass. Using hierarchical Bayesian inference with flexible Gaussian-process population models, we identify four distinct mass regions separated by sharp transitions in both mass-ratio and spin properties. Below $\sim15~M_{\odot}$, the population strongly favors equal-mass binaries and exhibits a narrow $χ_{\rm eff}$ distribution peaked at positive values. In the range $18$-$30\,M_{\odot}$, the mass-ratio distribution becomes substantially flatter, while the $χ_{\rm eff}$ distribution broadens, shifts to a peak consistent with zero, and shows tentative--but not statistically required--evidence for positive skewness. The region associated with the feature near $\simeq35~M_{\odot}$ returns to a narrow $χ_{\rm eff}$ distribution consistent with symmetry at zero and strongly favors equal-mass binaries. Above $\simeq 45~M_{\odot}$, both the mass-ratio and $χ_{\rm eff}$ distributions broaden significantly. The inferred support of the spin distribution converges toward the range expected for binaries containing remnants of previous black hole mergers, making the highest-mass region fully consistent with a star cluster population of hierarchical mergers. The close correspondence between transitions in mass ratio and effective spin suggests that different primary-mass ranges trace distinct formation channels, with isolated binary or triple evolution likely dominating the lower-mass population and dynamical assembly becoming increasingly important at higher masses.

astro-ph.HE↗

arXiv2026

Gravitational-wave constraints on the pair-instability mass gap and nuclear burning in massive stars

Pair-instability should prevent the direct formation of black holes above about $50M_\odot$ creating a pair-instability mass gap. Yet gravitational-wave observations have detected black holes in this mass range. These systems can be explained with uncertainties in massive-star evolution, or hierarchical mergers in stellar clusters, which are expected to produce large spins with isotropic orientations. Here we present evidence for the pair-instability mass gap in the LIGO--Virgo--KAGRA fourth transient catalog, with a lower edge at $44.3^{+5.9}_{-3.5}\,M_\odot$. We also obtain a measurement of the ${}^{12}\mathrm{C}(α,γ){}^{16}\mathrm{O}$ reaction rate, yielding an $S$-factor of $268^{+195}_{-116}\,\mathrm{keV\,b}$, a parameter critical for modeling helium burning and stellar evolution. The data reveal two populations: a low-spin group with no black holes above the gap, and a high-spin, isotropic group that extends across the full mass range and occupies the gap, consistent with hierarchical mergers. These findings are consistent with pair-instability playing a role in shaping the black hole mass spectrum, point to a connection between gravitational wave astronomy and nuclear astrophysics, and highlight dense stellar clusters as key environments in the growth of black holes.

astro-ph.HE↗

arXiv2026

Astrophysical Implications of Eccentricity in Gravitational Waves from Neutron Star-Black Hole Binaries

The gravitational-wave signal from the neutron star-black hole (NSBH) merger GW200105 is consistent with this binary having significant orbital eccentricity close to merger. This raises the question of how eccentric NSBHs form. Compact binaries that evolve in isolation radiate away any orbital eccentricity long before their gravitational-wave signal enters the sensitive frequency range of the LIGO-Virgo-KAGRA detector network. Meanwhile, dynamical environments -- which can be conducive to mergers on eccentric orbits -- produce very few NSBHs. Here, we focus on a formation channel that efficiently produces NSBHs with both misaligned spins and significant eccentricity close to merger: isolated field triples. We estimate the minimum measurable eccentricity of NSBHs at 10 Hz orbit-averaged gravitational-wave frequency, e_min,10, finding that for GW200105, GW200115, and GW230529-like systems, e_min,10 is O(0.01). For a GW190814-like unequal-mass binary with significant higher-order mode content, e_min,10 = 0.003; this is an order of magnitude lower than when higher modes are not present. For dominant-mode signals from binaries with $m_2=1.5$~M$_\odot$ and total masses from $3\,{\rm M_\odot} \leq M \leq50\,\rm M_\odot$, we find $0.008 \leq$ e_min,10 $\leq 0.022$. The relationship between $M$ and e_min,10 is linear when the binaries are non-spinning. When the binaries are maximally spin-precessing, e_min,10 decreases as mass ratio becomes more unequal. We estimate the sensitivity of a quasi-circular aligned-spin templated search to NSBH mergers from field triples, finding that we recover only 46% of systems that would have been detected with a search containing the full physics of the injected population. Finally, we show that if $\sim\nicefrac{1}{3}$ of present NSBH detections are measurably eccentric, then $\geq40\%$ are consistent with an isolated field triple origin.

astro-ph.HE↗

arXiv2026

Distinguishing the Origin of Eccentric Black Hole Mergers with Gravitational-wave Spin Measurements

It remains an open question whether the binary black hole mergers observed with gravitational-wave detectors originate from the evolution of isolated massive binary stars or were dynamically driven by perturbations from the environment. Recent evidence for non-zero orbital eccentricity in a handful of events is seen as support for a non-negligible fraction of the population experiencing external driving of the merger. However, it is unclear from which formation channel eccentric binary black-hole mergers would originate: dense star clusters, hierarchical field triples, active galactic nuclei, or wide binaries in the Galaxy could all be culprits. Here, we investigate whether the spin properties of eccentric mergers could be used to break this degeneracy. Using the fact that different formation channels are predicted to either produce eccentric mergers with mutually aligned or randomly oriented black-hole spins, we investigate how many confident detections would be needed in order for the two models to be statistically distinguishable. If a few percent of binary black hole mergers retain measurable eccentricity in the bandwidth of ground-based detectors, we report a $\sim9\,\%$ chance that we could confidently distinguish both models (Bayes factor $\ln\mathcal{B}>3$) after the fifth observing run of the LIGO-Virgo-KAGRA detector network, $\sim63\,\%$ for LIGO A#, and $\sim98\,\%$ for the Einstein Telescope and Cosmic Explorer.

astro-ph.HE↗

arXiv2025

GW200208_222617 as an eccentric black-hole binary merger: properties and astrophysical implications

Detecting orbital eccentricity in a stellar-mass black-hole merger would point to a non-isolated formation channel. Eccentric binaries can form in dense stellar environments such as globular clusters or active galactic nuclei, or from triple stellar systems in the Galactic field. However, confidently measuring eccentricity is challenging -- short signals from high-mass eccentric mergers can mimic spin-induced precession, making the two effects hard to disentangle. This degeneracy weakens considerably for longer-duration signals. Here, GW200208_222617 provides a rare opportunity. Originating from a relatively low-mass binary with source-frame chirp mass $\sim20$ M$_\odot$, its gravitational-wave signal spanned $\sim14$ orbital cycles in band, with no indication of data quality issues. Previous analyses for quasi-circular binaries found no evidence for spin precession, and multiple subsequent studies found the data to favour an eccentric merger despite notable technical differences. All in all, we believe GW200208_222617 is the black-hole merger event from GWTC-3 with the least ambiguous detection of eccentricity. We present a critical discussion of properties and astrophysical interpretation of GW200208_222617 as an eccentric black-hole merger using models of field triples, globular clusters, and active galactic nuclei. We find that if GW200208_222617 was indeed eccentric, its origin is consistent with a field triple or globular cluster. Formation in the inner regions of an active galactic nucleus is disfavoured. The outer regions of such a disk remain a viable origin for GW200208_222617; we demonstrate how future detections of eccentric mergers formed in such environments could be powerful tools for constraining the disk geometry.

astro-ph.HE↗

arXiv2025

Inferring the pair-instability mass gap from gravitational wave data

We use hierarchical Bayesian inference with non-parametric Gaussian process models to investigate the effective inspiral spin parameter, $χ_{\rm eff}$, as a function of primary black hole mass in the third gravitational-wave transient catalog (GWTC-3). Our analysis reveals a transition in the population at a primary mass of $46^{+7}_{-5}\,M_\odot$. Beyond this mass, the $χ_{\rm eff}$ distribution broadens, becomes consistent with being symmetric around zero, and has a median of $-0.03^{+0.36}_{-0.59}$ (90\% credibility). These results are consistent with the presence of a pair-instability mass gap that is repopulated by black holes that are the remnant of a previous merger, formed in dense star clusters. However, asymmetric distributions skewed toward positive $χ_{\rm eff}$ are not excluded by current data. Below the inferred transition mass, we constrain the fraction of second-generation black holes to be $\lesssim 10\%$. These results provide model-independent support for a high-mass and high-spin population of black holes in the data, consistent with earlier work using parametric models. Imminent gravitational-wave data releases will be essential to sharpen constraints on spin symmetry and clarify the origin of the black holes.

astro-ph.HE↗

arXiv2025

Rapid stellar and binary population synthesis with COMPAS: methods paper II

The COMPAS public rapid binary population synthesis code has undergone a number of key improvements since the original COMPAS methods paper (Team COMPAS: Riley et al., 2022) was published. These include more sophisticated and robust treatments of binary interactions: mass transfer physics, common-envelope events, tides and gravitational-wave radiation reaction; and updated prescriptions for stellar evolution, winds and supernovae. The code structure and outputs have also been updated, with a focus on improving resolution without sacrificing computational speed. This paper describes the substantive changes in the code between the previous methods paper and COMPAS v03.22.01.

astro-ph.SR↗

arXiv2024

Gravitational-Wave Data Analysis with High-Precision Numerical Relativity Simulations of Boson Star mergers

Gravitational-wave signals detected to date are commonly interpreted under the paradigm that they originate from pairs of black holes or neutron stars. Here, we explore the alternative scenario of boson-star signals being present in the data stream. We perform accurate and long ($\sim 20$ orbits) numerical simulations of boson-star binaries and inject the resulting strain into LIGO noise. Our Bayesian inference reveals that some boson-star signals. are degenerate with current approximants, albeit with biased parameters, while others exhibit smoking-gun signatures leaving behind conspicuous residuals.

gr-qc↗

arXiv2024

Residual eccentricity as a systematic uncertainty on the formation channels of binary black holes

Resolving the formation channel(s) of merging binary black holes is a key goal in gravitational-wave astronomy. The orbital eccentricity is believed to be a precious tracer of the underlying formation pathway, but is largely dissipated during the usually long inspiral between black hole formation and merger. Most gravitational-wave sources are thus expected to enter the sensitivity windows of current detectors on configurations that are compatible with quasi-circular orbits. In this paper, we investigate the impact of "negligible" residual eccentricity -- lower than currently detectable by LIGO/Virgo -- on our ability to infer the formation history of binary black holes, focusing in particular on their spin orientations. We trace the evolution of both observed and synthetic gravitational-wave events backward in time, while resampling their residual eccentricities to values that are below the detectability threshold. Eccentricities in-band as low as $\sim 10^{-4}$ can lead to significant biases when reconstructing the spin directions, especially in the case of loud, highly precessing systems. Residual eccentricity thus act like a systematic uncertainty for our astrophysical inference. As a mitigation strategy, one can marginalize the posterior distribution over the residual eccentricity using astrophysical predictions.

astro-ph.HE↗

arXiv2023

General-relativistic precession in a black-hole binary

The general-relativistic phenomenon of spin-induced orbital precession has not yet been observed in strong-field gravity. Gravitational-wave observations of binary black holes (BBHs) are prime candidates, since we expect the astrophysical binary population to contain precessing binaries. Imprints of precession have been investigated in several signals, but no definitive identification of orbital precession has been reported in any one of the 84 BBH observations to date by the Advanced LIGO and Virgo detectors. Here we report the measurement of strong-field precession in the LIGO-Virgo-Kagra (LVK) gravitational-wave signal GW200129. The binary's orbit precesses at a rate ten orders of magnitude faster than previous weak-field measurements from binary pulsars. We also find that the primary black hole is likely highly spinning. According to current binary population estimates a GW200129-like signal is extremely unlikely, and therefore presents a direct challenge to many current binary formation models.

gr-qc↗

arXiv2020

A Cryogenic Silicon Interferometer for Gravitational-wave Detection

The detection of gravitational waves from compact binary mergers by LIGO has opened the era of gravitational wave astronomy, revealing a previously hidden side of the cosmos. To maximize the reach of the existing LIGO observatory facilities, we have designed a new instrument that will have 5 times the range of Advanced LIGO, or greater than 100 times the event rate. Observations with this new instrument will make possible dramatic steps toward understanding the physics of the nearby universe, as well as observing the universe out to cosmological distances by the detection of binary black hole coalescences. This article presents the instrument design and a quantitative analysis of the anticipated noise floor.

astro-ph.IM↗