arXiv subjects

Elizabeth Flanagan

Publications and source records attributed to Elizabeth Flanagan.

4 recordsLinked to original sources

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 $\chi_{\rm eff}$ population and the posterior predictive distributions of $\chi_{\rm eff}$ 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 $\chi_{\rm eff}>0$, but also shows significant support for negative $\chi_{\rm eff}$. Above the transition, the distribution is broader and its peak shifts to values consistent with $\chi_{\rm eff}\simeq0$, making its support at both positive and negative $\chi_{\rm eff}$ 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 $\chi_{\rm eff}$.

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 ($\chi_{\rm eff}$) 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 $\chi_{\rm eff}$ distribution peaked at positive values. In the range $18$-$30\,M_{\odot}$, the mass-ratio distribution becomes substantially flatter, while the $\chi_{\rm eff}$ 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 $\chi_{\rm eff}$ distribution consistent with symmetry at zero and strongly favors equal-mass binaries. Above $\simeq 45~M_{\odot}$, both the mass-ratio and $\chi_{\rm eff}$ 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

Second-Generation Mass Peak in the Gravitational-Wave Population as a Probe of Globular Clusters

Gravitational-wave observations have revealed an excess of binary black hole mergers with primary masses near $\sim 35\,M_\odot$. We show that if this feature originates from dynamical formation in dense stellar systems, and if the pair-instability supernova truncates the first-generation black hole mass spectrum, then second-generation mergers inevitably produce a second peak near $\sim 70\,M_\odot$. This structure reflects the suppression of first-generation black holes above a characteristic mass and the accumulation of merger remnants near twice that scale. Its location is robust, whereas its amplitude depends strongly on cluster initial conditions. Using a large suite of cluster population-synthesis models, we show that current gravitational-wave data already constrain the birth properties of globular clusters, irrespective of their overall contribution to the observed population. If clusters dominate mergers above the pair-instability scale, these constraints tighten further and imply a minimum first-generation merger rate of $\mathcal{R}(m_1 \leq 50\,M_\odot) \geq 0.099\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$ ($99\%$ confidence). We further show that a drop or gap in the secondary black hole mass spectrum is not a robust signature of a cluster origin for high-mass mergers within the pair-instability mass gap. A confirmed excess near $\sim 70\,M_\odot$ would support a dynamical origin of the $\sim 35\,M_\odot$ feature and provide independent evidence for a pair-instability mass gap with a lower edge at $\lesssim 50M_\odot$.

astro-ph.HE↗

arXiv2025

Gravitational-wave Observations Suggest Most Black Hole Mergers Form in Triples

The spin-orbit tilt angles $\theta_{1(2)}$ of merging stellar-mass black holes provide key insights into their astrophysical origin. Non-parametric population modelling of The LIGO, Virgo, and KAGRA Collaborations (2025a, arXiv:2508.18083) shows that the spin-orbit tilt distribution of mergers in the latest Gravitational-Wave Transient Catalog 4.0 exhibits a global peak at near-perpendicular directions $\cos\theta_{1(2)}\approx0$. Here, we recover this feature using hierarchical Bayesian inference with parametric models that are tailored to enhance the diagnostic power about astrophysical formation channels. We find that the spin distribution of the low-mass bulk of the binary black hole merger population ($m_1\lesssim 44.3^{+8.7}_{-4.6}\,\rm M_\odot$) can be well-modelled by a dominant Gaussian component that peaks at $\cos\theta_{1(2)}\approx0$, possibly mixed with a subdominant isotropic component. Models that include a component with spins preferentially aligned with the orbit are disfavoured by current data (with Bayes factors $|\Delta\ln\mathcal{B}|\approx1$ to $3$) and constrain its contribution to be likely small ($\xi\sim\mathcal{O}(1)\,\%$), although large contributions cannot yet ruled out with certainty. If these findings are reinforced by more detections, they would challenge any major contribution from the traditional isolated-binary formation scenario yielding closely aligned spins. Instead, the dominant component with near-perpendicular spins matches expectations from the evolution of isolated massive stellar triples in the galactic field, where the Lidov-Kozai effect naturally produces a unique overabundance of mergers with $\cos\theta_{1(2)}\approx0$.

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