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Vicky Kalogera

Publications and source records attributed to Vicky Kalogera.

At least 19 recordsLinked to original sources
arXiv2026

The Mass-Ratio Distribution of the Low-Mass Binary Black Hole Subpopulation: A Natural Outcome of Isolated Binary Evolution

The observed merging binary black hole population is increasingly consistent with being composed of a mixture of subpopulations, each likely the result of different formation mechanisms. In particular, the low-mass subpopulation, with primary black hole masses below $\approxeq 15\,M_{\odot}$, has been attributed to mergers formed through isolated binary evolution. We use the binary population synthesis code POSYDON to study the mass ratio $(q)$ distribution of binary black hole mergers from isolated binary evolution and compare to the observed low-mass subpopulation. We explore variations in supernova remnant prescriptions, common-envelope efficiency, and black hole accretion efficiency. We find that our models have a preference for asymmetric $q$, most with a broad peak near $q\approxeq0.5-0.7$, and a near-equal-mass component whose relative strength varies across models. This resultant $q$ distribution is consistent with the $q$ distribution of the low-mass subpopulation observed with gravitational waves. We find that for the majority of models these features arise from physically distinct formation subchannels: the asymmetric peak reflects contributions from common-envelope and stable mass-transfer systems, while the near-equal-mass component traces double-core common envelope and contact systems. We conclude that the features in the $q$ distribution of the observed low-mass subpopulation emerge naturally from isolated binary evolution across a range of model assumptions. As the gravitational-wave catalog continues to grow, the relative strength of these $q$ features will provide an increasingly powerful diagnostic of isolated binary evolution and its formation subchannels.

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arXiv2026

The Cross-Survey Decade: A Call to Action

By 2027, three flagship wide-field surveys will be operating simultaneously from ground and space, observing overlapping sky and representing more than $6 billion in US and European public investment. Together they will produce overlapping petabyte-scale datasets across thousands of square degrees. This is a different class of challenge: the observations are no longer the bottleneck; realizing their joint scientific return now depends on shared computational infrastructure and coordination. Decades of community studies show that combining these datasets does more than improve precision. For science ranging from weak lensing to transient discovery and Galactic-plane astronomy, joint processing and analysis can unlock capabilities no single survey provides alone. Yet the required infrastructure -- joint pixel-level processing, cross-calibration and validation, interoperable data access, and the people to build and sustain it -- falls outside any single mission or institution's mandate. We issue a call to action for cross-survey science infrastructure, built around four pillars: (1) joint pixel-level processing and validation; (2) an AI-ready data substrate for scientific foundation models; (3) standardized, interoperable data access across surveys, democratizing participation in astrophysical discovery; and (4) dedicated personnel and career pathways. We outline concrete steps for policymakers, agencies, observatories, universities, the research community, and philanthropy, and argue that the moment to act is now, while foundational technical choices can still be aligned at a fraction of the cost of reconciling them later.

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arXiv2026

High-mass binary black hole mergers from detailed binary evolution models

Gravitational-wave observations reveal a population of binary black hole (BBH) mergers with primary masses above ${\sim}40\,\mathrm{M}_\odot$, extending into and potentially beyond the pair-instability mass gap, with a possibly flat mass-ratio and broader \chi_\mathrm{eff} distribution. We investigate whether super-Eddington accretion during stable mass transfer in isolated binary evolution can produce BBH mergers consistent with these properties across primary BH mass, mass-ratio, and \chi_\mathrm{eff} distributions. Using POSYDON, we simulate BBH merger populations with primary BH masses above ${\sim}40\,\mathrm{M}_\odot$, under three BH accretion efficiencies: Eddington-limited, GRRMHD-informed, and fully conservative. We additionally vary the natal kick strength, including strong kicks at high BH masses. We find that super-Eddington accretion does not suppress BBH mergers in the high-mass regime. Fully-conservative accretion leads to an increase of BBH mergers in POSYDON with a strong kick-independent peak at $\chi_\mathrm{eff}=0.6$ and a sharp mass-ratio peak at $q\sim0.5$, whereas observations favor $\chi_\mathrm{eff}=0.0$ and a flatter mass-ratio distribution. The GRRMHD-informed and Eddington-limited accretion are compatible with the observed primary BH mass and mass ratio distribution, but require natal kicks to populate negative \chi_\mathrm{eff}. A joint analysis of the primary BH mass, mass ratio, and \chi_\mathrm{eff} distributions provides strong constraints on binary evolution physics, and disfavor fully-conservative BH accretion as the dominant formation mechanism for high-mass BBH mergers. The Eddington-limited and GRRMHD-informed prescriptions with modest kicks can explain part of the high-mass population, but an additional formation channel is still needed to account for the high fraction of negative \chi_\mathrm{eff} systems and high secondary BH spins.

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arXiv2026

A Four-dimensional Model-agnostic Probe into the Astrophysical Origins of Binary Black Hole Subpopulations

There is strong evidence of multiple binary black hole~(BBH) subpopulations in the cumulative gravitational wave catalog by the LIGO-Virgo-KAGRA collaboration that likely originate through distinct evolutionary channels. The astrophysical interpretation of this complex underlying population is subject to theoretical uncertainties in treatments of binary stellar evolution, core collapse, and host environments. Due to a lack of robust predictions and the sheer diversity of plausible features, strongly modelled population analyses often lead to prior-driven conclusions. On the other hand, flexible alternatives are often difficult to scale in higher dimensions, which can lead to a loss of critical information on astrophysically meaningful correlations. In this \textit{Letter}, we present the first data-driven reconstruction of the joint four-dimensional distribution of BBH primary masses, mass ratios, effective inspiral and effective precessing spin parameters, which yields novel model-agnostic constraints on the astrophysical origins of BBH subpopulations. We characterize four distinct subpopulations spanning different ranges of BBH masses and report new correlations in these specific mass ranges that are beyond the reach of strongly modeled parametrizations and lower-dimensional data-driven frameworks. Our results unveil novel insights into the abundances of specific subchannels of isolated binary evolution, dynamical assembly, and hierarchical mergers across various mass ranges in the astrophysical BBH population.

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arXiv2026

Double Neutron Star Delay Times Across Cosmic Metallicities: The Role of Helium Star Progenitors

Metallicity can play a significant role in massive binary evolution through its impact on the opacity within stellar interiors and wind-driven mass loss. In this work, we investigate how the double neutron star (DNS) delay time distribution (DTD) is shaped by the metallicity-dependent evolution of the helium star$-$NS progenitor system. Drawing from insights rooted in single and binary star physics, we argue that at a given metallicity, the stellar radius during the helium main-sequence sets a lower limit on the size of the DNS orbit at birth. We then perform population synthesis with the detailed binary evolution code POSYDON to illustrate the resulting DTD across a range of metallicities. Our results indicate that, independent of the common envelope efficiency and reasonable natal kicks, the majority of DNS mergers across metallicities occur typically no earlier than $\simeq 40\,\rm{Myr}$ after star formation and peaks strongly between $80-250\,\rm{Myr}$. Roughly $15\%$ of DNSs merge within 80 Myr, which may explain $r$-process enrichment in environments with brief star formation histories, while $\gtrsim 20\%$ merge on delay times $>1$Gyr, providing an explanation for short gamma-ray bursts in old, metal-poor galaxies. The shape of the DTD can be complex, with a metallicity-dependent split in the dominant formation channel imprinting a characteristic double-peaked structure. Although ideally oriented natal kicks can produce very short merging DNS, we find that the required kick magnitudes are inconsistent with observations. Our work has implications for assessing the contribution of DNS mergers to $r$-process enrichment and gamma-ray bursts/kilonovae transients across cosmic time.

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arXiv2026

Irregularly Sampled Time Series Interpolation for Binary Evolution Simulations Using Dynamic Time Warping

Binary stellar evolution simulations are computationally expensive. Stellar population synthesis relies on these detailed evolution models at a fundamental level. Producing thousands of such models requires hundreds of CPU hours, but stellar track interpolation provides one approach to significantly reduce this computational cost. Although single-star track interpolation is straightforward, stellar interactions in binary systems introduce significant complexity to binary evolution, making traditional single-track interpolation methods inapplicable. Binary tracks present fundamentally different challenges compared to single stars, which possess relatively straightforward evolutionary phases identifiable through distinct physical properties. Binary systems are complicated by mutual interactions that can dramatically alter evolutionary trajectories and introduce discontinuities difficult to capture through standard interpolation. In this work, we introduce a novel approach for track alignment and iterative track averaging based on Dynamic Time Warping to address misalignments between neighboring tracks. Our method computes a single shared warping path across all physical parameters simultaneously, placing them on a consistent temporal grid that preserves the causal relationships between parameters. We demonstrate that this joint-alignment strategy maintains key physical relationships such as the Stefan-Boltzmann law in the interpolated tracks. Our comprehensive evaluation across multiple binary configurations demonstrates that proper temporal alignment is crucial for track interpolation methods. The proposed method consistently outperforms existing approaches and enables the efficient generation of more accurate binary population samples for astrophysical studies.

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arXiv2026

Learning the Stellar Structure Equations via Self-supervised Physics-Informed Neural Networks

Stellar astrophysics relies critically on accurate descriptions of the physical conditions inside stars. Traditional solvers such as \texttt{MESA} (Modules for Experiments in Stellar Astrophysics), which employ adaptive finite-difference methods, can become computationally expensive and challenging to scale for large stellar population synthesis ($>10^9$ stars). In this work, we present an self-supervised physics-informed neural network (PINN) framework that provides a mesh-free and fully differentiable approach to solving the stellar structure equations under hydrostatic and thermal equilibrium. The model takes as input the stellar boundary conditions (at the center and surface) together with the chemical composition, and learns continuous radial profiles for mass $M_r(r)$, pressure $P(r)$, density $\rho(r)$, temperature $T(r)$, and luminosity $L_r(r)$ by enforcing the governing structure equations through physics-based loss terms. To incorporate realistic microphysics, we introduce auxiliary neural networks that approximate the equation of state and opacity tables as smooth, differentiable functions of the local thermodynamic state. These surrogates replace traditional tabulated inputs and enable end-to-end training. Once trained for a given star, the model produces continuous solutions across the entire radial domain without requiring discretization or interpolation. Validation against benchmark \texttt{MESA} models across a range of stellar masses yields a Mean Relative Absolute Error of $3.06\%$ and an average $R^2$ score of $99.98\%$. To our knowledge, this is the first demonstration that the stellar structure equations can be solved in a fully self-supervised and data-free fashion employing PINNs. This work establishes a foundation for scalable, physics-informed emulation of stellar interiors and opens the door to future extensions toward time-dependent stellar evolution.

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arXiv2026

On the Astrophysical Origin of Binary Black Hole Subpopulations: A Tale of Three Channels?

There is increasing evidence for multiple binary black hole~(BBH) subpopulations in the cumulative gravitational wave catalog by the LIGO-Virgo-KAGRA Collaboration. The astrophysical interpretation of this complex underlying population is subject to theoretical uncertainties in treatments of binary stellar evolution, core collapse, and host environments. In this \textit{Letter}, using parametrized mixture models, we show that the BBH detection sample comprises three astrophysical subpopulations that are likely dominated by specific formation channels. In particular, we show that the $10M_{\odot}$ peak and the $35M_{\odot}$ feature in the BBH mass spectrum correspond to distinct mass-ratio, spin alignment, spin precession, and redshift evolution properties. We show that mass-based transitions reported in the distribution of BBH parameters naturally emerge from our inferred distributions without explicit modeling. Our results are consistent with the current observed population arising from specific relative abundances of isolated binary evolution, dynamical formation in globular clusters, and higher-generation BBH mergers. Under this interpretation, we constrain the relative underlying fraction of these channels to be $79.0^{+11.5}_{-10.9}\%$, $14.5^{+11.6}_{-8.0}\%$, and, $2.5^{+5.5}_{-1.8}\%$, respectively, and find these relative fractions to be evolving over cosmic time with more than $1\sigma$ confidence. Our interpretation relies on simple theoretical predictions that are mostly robust against uncertainties in BBH formation, with more definite conclusions expected in the near future.

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arXiv2026

Disk-Regulated Mass Transfer Between Rotating Non-Degenerate Stars: Insights from Be and sdOB Binaries

Mass transfer between non-degenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically-motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk-star coupling model can produce more massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk-star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.

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arXiv2026

A case for Case A: detailed look at binary black hole formation through stable mass transfer

In isolated binary evolution, binary black hole (BBH) mergers are generally formed through stable mass transfer (SMT) or common envelope evolution. In recent years, the SMT channel has received significant attention due to detailed binary models showing increased mass transfer stability compared to previous studies. In this work, we perform a full zero-age-main-sequence to compact object merger analysis using detailed binary models at eight metallicities between $10^{-4}Z_\odot$ and $2Z_\odot$ to self-consistently model the population properties of BBH mergers in the SMT channel, determined their progenitor initial conditional, and investigate the binary physics governing their formation and metallicity dependence. We use the population synthesis code POSYDON to determine the population of BBH mergers from SMT. Using its extended grids of MESA binary models, we determine the essential physics in the formation of BBH mergers. SMT produces BBH mergers predominantly from systems with $P_{ZAMS}\leq10$ days. In these systems, both the initial mass transfer between two stars and the subsequent interaction between the remaining star and the first-born BH take place while the respective donor star is on the main-sequence (Case A). We find a limited contribution from wider Case B/C systems. Without a natal kick, the SMT channel does not produce BBH mergers above $Z>0.2Z_\odot$ due to orbital widening from stellar wind mass loss. The primary BH mass distribution shows a strong dependence on metallicity, while the mass ratio prefers unity independent of metallicity due to mass ratio reversal. Additionally, the $\chi_{eff}$ distributions contain peaks at $\chi_{eff}=0$ and ~0.15 of which the former disappears at high metallicities. A mass-scaled natal kick leave this sub-population unchanged but introduce a low-mass, unequal mass ratio sub-population that merges due to their high eccentricity.

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arXiv2025

Tides in Massive Binaries: Numerical Solutions and Semi-Analytical Comparisons

We present a systematic comparison between the tidal secular evolution timescales predicted by the direct numerical method and those given by the commonly used semi-analytic prescriptions implemented in 1-D hydrostatic binary evolution codes. Our study focuses on binary systems with intermediate- to high-mass primaries ($M_1 = 5$-$50\,M_\odot$), companion masses between $1.4\,M_\odot$ and $10\,M_\odot$, and orbital periods ranging from 0.5 to 50 days. Before mass transfer, both approaches predict synchronization and orbital decay timescales that agree within $\sim$2 orders of magnitude and typically exceed the stellar main sequence lifetime, implying negligible tidal impact on secular orbital evolution. However, the implied dissipation channels differ, and the differences become more pronounced once mass transfer begins. To test the theoretical predictions against observations, we apply both approaches to the well-characterized PSR J0045--7319 system, which has an orbital decay timescale of 0.5 Myr. The numerical solution reveals strong resonances with internal gravity waves, bringing the predicted orbital period change rate close to the observed value. In contrast, the semi-analytic prescriptions predict orbital decay timescales longer than the Hubble time. These results suggest that for population studies, modestly calibrated parameterized equations may suffice, but for individual systems, reliable interpretation requires direct numerical approaches.

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arXiv2025

Characterizing Binary Black Hole Subpopulations in GWTC-4 with Binned Gaussian Processes: On the Origins of the $35M_{\odot}$ Peak

Understanding the astrophysical origins of binary black holes requires accurate and flexible modeling of multi-dimensional population properties. In this \textit{Letter}, using a data-driven framework based on binned Gaussian processes, we characterize the joint distribution of BBH primary masses, mass ratios, and effective inspiral spins. We identify three distinct subpopulations in the GWTC-4 sample of observations and investigate their astrophysical origins. We find that only one of the three subpopulations exhibits the $35M_{\odot}$ peak, which is characterized by a strong preference for equal mass systems and isotropic spin orientations. Our inferred distributions are consistent with a predominantly dynamical origin of this feature. By comparing with theoretical simulations, we further show that the subpopulation that exhibits the $35M_{\sun}$ peak can exclusively comprise dynamically assembled systems in globular clusters, specifically if black hole birth spins are in the range~$(0.1-0.2)$, whereas the other two subpopulations require substantial contributions from alternative formation channels. We constrain the \textit{lower bound} on the merger rate of BBHs in globular clusters to be $0.69^{+0.23}_{-0.33} \rm{Gpc}^{-3}\rm{yr}^{-1}$, which is consistent with most theoretical predictions(that can range from $0.2-57\rm{Gpc}^{-3}\rm{yr}^{-1}$ depending on modeling assumptions). We conclude that dynamical formation in globular clusters remains a strong candidate for the origin of this excess near $30-40M_{\odot}$ and that more data and targeted parametric models are necessary to rigorously establish this interpretation.

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arXiv2025

Reexamining Evidence of a Pair-Instability Mass Gap in the Binary Black Hole Population

The fourth gravitational wave transient catalog~(GWTC-4) has enabled empirical probes of the theorized pair-instability gap in the higher end of the binary black hole~(BBH) mass-spectrum. In this letter, using flexibly parametrized models, we show that at present there is no evidence of a sharp drop-off in the spectrum of black hole masses near $~40-50M_{\odot}$. We simultaneously characterize the transition in the distribution of BBH mass-ratios, effective aligned and effective precessing spins using our flexible models. From the transitions in our inferred spin and mass-ratio distributions, we find that the high-mass broad-spin sub-population has a significant fraction~($52^{+18}_{-23}\%$) of systems with mass ratios in the range $0.6-1$. This suggests that alternatives to the hypothesis of 2G+1G hierarchical systems dominating BBH formation above $\sim 40-50 M_{\odot}$ are more consistent with the GWTC-4 detection sample. By comparing with the predictions of star cluster simulations, we further show that contributions from (2G+2G) systems are not abundant enough to alleviate this discrepancy. We also demonstrate the effects of strong model assumptions on this inference, which can lead to biased astrophysical interpretation from restricted priors. We note that our results do not exclude that a high-mass gap may be identified as our sample size increases. We constrain the lower bound on the location of a possible PISN cutoff still allowed within measurement uncertainties to be $(57^{+17}_{-10}M_{\odot})$ and discuss its implications on the S factor of $^{12}\mathrm{C}(\alpha, \gamma)^{16}O$ at 300 kev.

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arXiv2025

Modelling the Future of Gaia Neutron Star-Main Sequence Binaries: From Eccentric Orbits to Millisecond Pulsar-White Dwarfs

We model the evolution of 21 Gaia neutron star (NS)-main-sequence binaries (orbital period $P_{\mathrm{orb}}\sim200$--$1000$ days, eccentricity $e\gtrsim0.2$) using binary evolution with \texttt{MESA}. We examine eccentric mass transfer and models assuming prior circularization. All systems end as NS-white dwarf (WD) binaries, but transfer modes yield distinct outcomes. Under eccentric transfer, binaries are driven to higher $e$, forming orbits with $e\gtrsim0.6$ and $P_{\rm orb}\sim1000$-$4000$ days. Periastron bursts are brief ($\lesssim10^6$ yr), transfer only a few $\times10^{-2}$ M$_\odot$, and produce mildly recycled pulsars ($P_{\mathrm{spin}}\gtrsim50$ ms) with low-mass He WDs. Artificially circularized transfer gives $P_{\rm orb}\sim200$-$2000$ days, lasts $\sim10^7$ yr, and allows NSs to accrete $\sim0.1$ M$_\odot$, forming fully recycled MSPs ($P_{\mathrm{spin}}\sim$ few-30 ms) with CO WDs. Allowing super-Eddington accretion up to $100\times$ the canonical rate makes even eccentric systems efficient MSP producers, though torque coupling remains uncertain. Using an adaptive, field-dependent magnetic-field decay timescale, we find MSPs stay radio-active over Gyr spans. Gaia systems undergoing stable mass transfer remain wide and fail to match the Galactic MSP-WD population, where most, nearly circular systems have $P_{\rm orb}\lesssim100$ days. Binaries with different mass ratios and initial configurations -- likely leading to unstable mass transfer -- are needed to reproduce the observed MSP-WD distribution.

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arXiv2025

Evolutionary Links: From Gaia Neutron Star Binaries to Pulsar White Dwarf Endpoints

The discovery of wide, eccentric Gaia neutron stars (NSs) in binaries with still evolving (likely main sequence) companions offers a new probe of mass transfer and pulsar recycling beyond the compact-binary regime. We model their origins and fates using population synthesis with POSYDON and detailed binary evolution with MESA, contrasting two limiting prescriptions at Roche-lobe overflow (RLOF): enforced circularization versus explicitly eccentric mass transfer. Our MESA setups include updated treatments of eccentric, non-conservative transfer, magnetic-braking torques for cool stars, and neutron-star spin evolution with accretion and dipole spindown. Under optimistic assumptions, isolated evolution yields Gaia-like systems at only <1.5% relative rates of NS-evolving companion binaries, yet absolute numbers remain consistent with detections for continuous star formation. Synthetic populations indicate that many canonical millisecond pulsar-white dwarf (WD) binaries arise from unstable mass transfer and common envelope recycling, whereas Gaia systems typically avoid common envelope and only undergo stable mass transfer. In the case of capping accretion onto the NS at the Eddington rate, circular RLOF keeps the donor's mass-loss rate hovering around the Eddington limit and sustained over long timescales. Eccentric mass transfer instead produces briefer bursty signatures where the donor's mass-loss rate can climb up to a thousand times higher than in the circular case. The eccentric channel then leaves wide, eccentric NS-helium WD binaries with only mild recycling, whereas the circular channel yields long-lived transfer, circular NS-WD binaries (helium or carbon-oxygen core), and fully recycled millisecond pulsars.

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arXiv2025

Forming Double Neutron Stars using Detailed Binary Evolution Models with POSYDON: Comparison to the Galactic Systems

With over two dozen detections in the Milky Way, double neutron stars (DNSs) provide a unique window into massive binary evolution. We use the POSYDON binary population synthesis code to model DNS populations and compare them to the observed Galactic sample. By tracing their origins to underlying single and binary star physics, we place constraints on the detailed evolutionary stages leading to DNS formation. Our study reveals a bifurcation within the well-known common envelope (CE) formation channel for DNSs, which naturally explains an observed split in the orbital periods of the Galactic systems. The two subchannels are defined by whether the donor star has a helium core (Case B mass transfer) or a carbon-oxygen core (Case C) at the onset of the CE, with only the helium core systems eventually merging due to gravitational wave-modulated orbital decay. We find that across different treatments of the CE phase, the formation of DNSs through both subchannels requires either a generous core definition of $\simeq$ 30% H-fraction or a high CE ejection efficiency of $\alpha_{\rm CE}\gtrsim1.2$. By testing different supernova kick velocity models, we find that galactic DNSs are best reproduced using a prescription that favors low velocity kicks ($\lesssim 50 \, \rm km/s$), in agreement with previous studies. Furthermore, our models indicate that merging DNSs are born from a stripped progenitor with a median pre-supernova envelope mass $\sim$ 0.2$M_{\odot}$. Our results highlight the value of detailed evolutionary models for improving our understanding of exotic binary star formation.

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arXiv2025

Hiding Out at the Low End: No Gap and a Peak in the Black-Hole Mass Spectrum

In recent years, the existence of a gap in the mass spectrum of compact objects formed from stellar collapse, between the heaviest neutron stars and the lightest black holes, has been a matter of significant debate. The presence or absence of a mass gap has implications for the supernova mechanism, as well as being a fundamental property of the compact object mass function. In X-ray binaries containing black holes a gap is observed, but it is not known whether this is representative of a true gap in the mass function or due to selection effects or systematic biases in mass estimation. Binary black hole mergers detected from gravitational waves in the GWTC-3 transient catalog furnish a large sample of several tens of low-mass black holes with a well-understood selection function. Here we analyze the \nevts{} GWTC-3 merger events (along with GW230529\_181500) with at least one black hole ($3 \, M_\odot < m_1$) and chirp masses below those of a $20\,M_\odot$--$20\,M_\odot$ merger ($\mathcal{M} < 17.41 M_{\odot}$) to uncover the structure of the low-mass black hole mass function. Using flexible parameterized models for the mass function, we find, similar to existing studies, a sharp peak in the mass function at $m \simeq (8-10 M_{\odot})$. We observe a steady decline in the merger rate to lower masses, but by less than an order of magnitude in total, and find that the first percentile of black hole masses in our most flexible model is $m_{1\%} =3.13^{+0.18}_{-0.04}$. In other words, this sample of low-mass black holes is not consistent with the existence of a mass gap.

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arXiv2025

Structure and Skewness of the Effective Inspiral Spin Distribution of Binary Black Hole Mergers

The detection of gravitational waves has brought to light a population of binary black holes that merge within a Hubble time. Multiple formation channels can contribute to this population, making it difficult to definitively associate particular population features with underlying stellar physics. Black hole spins are considered an important discriminator between various channels, but they are less well-measured than masses, making conclusive astrophysical statements using spins difficult thus far. In this paper, we consider the distribution of the effective inspiral spin $\chi_{\rm eff}$ -- a quantity much better measured than individual component spins. We show that non-Gaussian features like skewness, asymmetry about zero, and multimodality can naturally arise in the $\chi_{\rm eff}$ distribution when multiple channels contribute to the population. Searching for such features, we find signs of skewness and asymmetry already in the current catalogs, but no statistically significant signs of bimodality. These features provide robust evidence for the presence of a subpopulation with spins preferentially aligned to the binary's orbital angular momentum; and we conservatively estimate the fraction of this subpopulation to be at least $12 \% - 17\%$ (at $90\%$ credibility). Our models do not find an excess of non-spinning systems and instead find that at least $\sim 20 \%$ of the binaries have some degree of negative $\chi_{\rm eff}$. The data also suggest that, if preferentially aligned mergers form a significant fraction of the population, they must have small spins.

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