arXiv subjects

Debatri Chattopadhyay

Publications and source records attributed to Debatri Chattopadhyay.

At least 19 recordsLinked to original sources

arXiv2026

Neutron Star Mass across Binary Pulsar Subpopulations: Mass-Spin Correlation, Mass Distributions, and Moment of Inertia Effects

We present a hierarchical Bayesian analysis of the joint mass, spin, and orbital properties of $\sim 50$ Galactic binary radio pulsars with measured neutron star masses, classified by binary type into pulsar-white dwarf (PSR-WD) and double neutron star (DNS) systems. We find moderate evidence for an anti-correlation between neutron star mass and spin period in the pooled recycled population (correlation coefficient $ρ= -0.26$, with $96\%$ of the posterior probability at $ρ<0$; the $90\%$ credible interval excludes zero), robust to the treatment of candidate DNSs and to a radio-detectability selection correction. Although consistent with accretion-driven recycling, the correlation cannot statistically distinguish an accretion origin from a moment of inertia-driven spin-up mechanism, because the neutron star moment of inertia is nearly linear in mass over the observed range. The DNS systems alone instead lean to the positive side expected from the moment-of-inertia mechanism ($ρ=+0.13$), though with only ten systems this is not statistically conclusive. Mass shows no significant correlation with orbital period or inclination, and only a weak one with eccentricity. As a secondary result, neutron stars with helium white dwarf companions are marginally more massive than those with carbon-oxygen/oxygen-neon white dwarf companions ($Δ\simeq 0.06\,M_\odot$), consistent with more extensive accretion in the helium white dwarf channel. We confirm, in a hierarchical framework, the previously reported correlation between companion mass and orbital eccentricity in double neutron stars ($ρ=+0.82$). We interpret these results within a two-channel picture -- accretion-grown PSR-WD versus birth-mass-dominated DNS.

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.

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arXiv2026

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 $α_{\rm CE}\gtrsim1.2$. 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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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

Repopulating the pair-instability mass gap without sustained growth to massive IMBHs: the case of 47\,Tuc

We model the formation and retention of the most massive black hole (BH) in 47~Tuc using the semi-analytical code \texttt{cBHBd}, coupling cluster evolution with binary BH dynamics and computing merger-remnant masses, spins, and gravitational-wave recoil kicks via numerical-relativity surrogate prescriptions. We evolve 80\,000 cluster realisations spanning initial masses, densities, IMFs, and metallicities, in both a baseline scenario ($m_{\rm max} = 130\,\mathrm{M}_{\odot}$) and an extended-IMF scenario with ${\sim}\,50-110$ primordial BH seeds above the pair-instability gap ($M_{\rm BH} \sim 130-700\,\mathrm{M}_{\odot}$). Selecting models reproducing 47~Tuc's present-day mass and half-mass radius, we find hierarchical mergers alone yield a most massive retained BH of $M_{\rm BH} \sim 45-70\,\mathrm{M}_{\odot}$ with spin $χ_{\rm BH} \sim 0.65$, limited to ${\sim}\,1-3$ mergers, as second-generation remnants acquire spin $χ\sim 0.7$ that amplifies recoil kicks in subsequent generations. When primordial seeds are included, the retained-mass distribution becomes bimodal -- in ${\sim}\,90\%$ of realisations all seeds are ejected, but in ${\sim}\,10\%$ a massive seed ($M_{\rm BH} \gtrsim 450\,\mathrm{M}_{\odot}$) survives -- while the joint mass-spin distribution is trimodal; seeds surviving via stellar-mass BH mergers retain low spin ($χ\lesssim 0.3$), whereas seed-seed mergers produce high-mass, high-spin remnants ($χ\sim 0.65-0.7$), yielding 90th-percentile retained masses of ${\sim}\,500-1100\,\mathrm{M}_{\odot}$. Both scenarios are consistent with the $3σ$ dynamical upper limit of $578\,\mathrm{M}_{\odot}$. Our results favour a dark-remnant subsystem over a single massive IMBH and provide a spin-mass diagnostic testable with LIGO-Virgo-KAGRA, the Einstein Telescope, Cosmic Explorer, and LISA.

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arXiv2025

Modelling Millisecond Pulsar Populations in Globular Clusters with NBODY6++GPU

Millisecond pulsars (MSPs) are neutron stars with spin periods as short as a few milliseconds, formed through mass accretion from companion stars. In the dense environments of globular clusters (GCs), MSPs are likely to originate through dynamically assembled interacting binaries. Over 300 MSPs have been detected in GCs to date, more than half of the known Galactic MSP population. In this work, we model MSP populations in intermediate-mass GCs using the direct $N$-body code \textsc{NBODY6++GPU}. We update the code by implementing pulsar spin-down due to magnetic braking and spin-up through accretion, and use this framework to model the pulsar population in the globular cluster M71 to investigate the pulsar population within and the associated gravitational wave transients.

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arXiv2025

Possible binary neutron star merger history of the primary of GW230529

Black holes (BHs) with masses between $\sim 3-5M_{\odot}$, produced by a binary neutron star (BNS) merger, can further pair up with a neutron star or BH and merge again within a Hubble time. However, the astrophysical environments in which this can happen and the rate of such mergers are open questions in astrophysics. Gravitational waves may play an important role in answering these questions. In this context, we discuss the possibility that the primary of the recent LIGO-Virgo-KAGRA binary GW230529_181500 (GW230529, in short) is the product of a previous BNS merger. Invoking numerical relativity (NR)-based fitting formulas that map the binary constituents' masses and tidal deformabilities to the mass, spin, and kick velocity of the remnant BH, we investigate the potential parents of GW230529's primary. Our calculations using NR fits based on BNS simulations reveal that the remnant of a high-mass BNS merger similar to GW190425 is consistent with the primary of GW230529. This argument is further strengthened by the gravitational wave-based merger rate estimation of GW190425-like and GW230529-like populations. We show that around 18% (median) of the GW190425-like remnants could become the primary component in GW230529-like mergers. The dimensionless tidal deformability parameter of the heavier neutron star in the parent binary is constrained to $67^{+163}_{-61}$ at 90% credibility. Using estimates of the gravitational-wave kick imparted to the remnant, we also discuss the astrophysical environments in which these types of mergers can take place and the implications for their future observations.

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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

Binary population synthesis of the Galactic canonical pulsar population

Pulsars are rapidly rotating neutron stars that emit radiation across the electromagnetic spectrum, from radio to gamma-rays. We use the rapid binary population synthesis suite COMPAS to model the Galactic population of canonical pulsars. We account for both radio and gamma-ray selection effects, as well as the motion of pulsars in the Galactic potential due to natal kicks. We compare our models to the catalogues of pulsars detected in the radio, and those detected in gamma-rays by Fermi, and find broad agreement with both populations. We reproduce the observed ratio of radio-loud to radio-quiet gamma-ray pulsars. We further examine the possibility of low spin-down luminosity (Edot) pulsars emitting weak, unpulsed gamma-ray emission and attempt to match this with results from a recent gamma-ray stacking survey of these pulsars. We confirm the correlation between the latitude of a pulsar and its Edot arises due to natal kicks imparted to pulsars at birth, assuming that all pulsars are born in the Galactic disk.

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.

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arXiv2025

Properties of black hole-star binaries formed in $N$-body simulations of massive star clusters: implications for Gaia black holes

We investigate black hole-star binaries formed in $N$-body simulations of massive, dense star clusters. We simulate 32 clusters with varying initial masses ($10^{4}~\rm M_{\odot}$ to $10^{6}~\rm M_{\odot}$), densities ($1200~\rm M_{\odot}~pc^{-3}$ to $10^{5}~\rm M_{\odot}~pc^{-3}$), and metallicities $(Z = 0.01,~0.001,~0.0001)$. Our results reveal that star clusters produce a diverse range of BH-star binaries, with dynamical interactions leading to extreme systems characterised by large orbital separations and high black hole masses. Of the ejected BH-main sequence (BH-MS) binaries, $20\%$ form dynamically, while the rest originate from the primordial binary population initially present in the cluster. Ejected BH-MS binaries that are dynamically formed have more massive black holes, lower-mass stellar companions, and over half are in a hierarchical triple system. All unbound BH-giant star (BH-GS) binaries were ejected as BH-MS binaries and evolved into the BH-GS phase outside the cluster. Due to their lower-mass companions, most dynamically formed binaries do not evolve into BH-GS systems within a Hubble time. Consequently, only 2 of the 35 ejected BH-GS binaries are dynamically formed. We explore the formation pathways of Gaia-like systems, identifying two Gaia BH1-like binaries that formed through dynamical interactions, and two Gaia BH2-like systems with a primordial origin. We did not find any system resembling Gaia BH3, which may however be attributed to the limited sample size of our simulations.

astro-ph.GA↗

arXiv2025

Predictions of a simple parametric model of hierarchical black hole mergers

Hierarchical mergers of black holes are proposed as a mechanism to explain the observations of binary black holes with component masses between $\sim 50M_{\odot}\hbox{--}130M_{\odot}$ by LIGO/Virgo, often referred to as "upper mass gap". We study the efficiency with which hierarchical mergers can produce higher and higher masses using a simple model of the forward evolution of binary black hole populations in gravitationally bound systems like stellar clusters. The model relies on pairing probability and initial mass functions for the black hole population, along with numerical relativity fitting formulas for the mass, spin, and kick speed of the merger remnant. We carry out an extensive comparison of the predictions of our model with clusterBHBdynamics (cBHBd) model, a fast method for the evolution of star clusters and black holes therein. For this comparison, we consider three different pairing functions of black holes and consider simulations from high- and low-metallicity cluster environments from cBHBd. We find good agreements between our model and the cBHBd results when the pairing probability of binaries depends on both total mass and mass ratio. We also assess the efficiency of hierarchical mergers as a function of merger generation and derive the mass distribution of black holes using our model. We find that the multi-modal features in the observed binary black hole mass spectrum -- revealed by the non-parametric population models -- can be interpreted by invoking the hierarchical merger scenario in dense, metal-rich, stellar environments. Further, the two subdominant peaks in the GWTC-3 component mass spectrum are consistent with second and third-generation mergers in metal-rich, dense environments. With more binary black hole detections, our model could be used to infer the black hole initial mass function and pairing probability exponent.

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arXiv2024

The Impact of Astrophysical Priors on Parameter Inference for GW230529

We investigate the effects of prior selection on the inferred mass and spin parameters of the neutron star-black hole merger GW230529\_181500. Specifically, we explore models motivated by astrophysical considerations, including massive binary and pulsar evolution. We examine mass and spin distributions of neutron stars constrained by radio pulsar observations, alongside black hole spin observations from previous gravitational wave detections. We show that the inferred mass distribution highly depends upon the spin prior. Specifically, under the most restrictive, binary stellar evolution models, we obtain narrower distributions of masses with a black hole mass of $4.3^{+0.1}_{-0.1}\,M_{\odot}$and neutron star mass of $1.3^{+0.03}_{-0.03}\,M_{\odot}$ where, somewhat surprisingly, it is the prior on component spins which has the greatest impact on the inferred mass distributions. Re-weighting using neutron star mass and spin priors from observations of radio pulsars, with black hole spins from observations of gravitational waves, yields the black hole and the neutron star masses to be $3.8^{+0.5}_{-0.6} \,M_\odot$ and $1.4^{+0.2}_{-0.1} \,M_\odot$ respectively. The sequence of compact object formation -- whether the neutron star or the black hole formed first -- cannot be determined at the observed signal-to-noise ratio. However, there is no evidence that the black hole was tidally spun up.

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arXiv2024

Reconstructing the Genealogy of LIGO-Virgo Black Holes

We propose a Bayesian inference framework to predict the merger history of LIGO-Virgo binary black holes (BHs), whose binary components may have undergone hierarchical mergers in the past. The framework relies on numerical relativity predictions for the mass, spin, and kick velocity of the remnant BHs. This proposed framework computes the masses, spins, and kicks imparted to the remnant of the parent binaries, given the initial masses and spin magnitudes of the binary constituents. We validate our approach by performing an ``injection study'' based on a constructed sequence of hierarchically formed binaries. Noise is added to the final binary in the sequence, and the parameters of the `parent' and `grandparent' binaries in the merger chain are then reconstructed. This method is then applied to three GWTC-3 events: GW190521, GW200220_061928, and GW190426_190642. These events were selected because at least one of the binary companions lies in the putative pair-instability supernova mass gap, in which stellar processes alone cannot produce BHs. Hierarchical mergers offer a natural explanation for the formation of BHs in the pair-instability mass gap. We use the backward evolution framework to predict the parameters of the parents of the primary companion of these three binaries. For instance, the parent binary of GW190521 has masses $72_{-22}^{+32}M_{\odot}$ and $31_{-23}^{+24}M_{\odot}$ within the 90% credible interval. Astrophysical environments with escape speeds $\geq100{\rm \, km \, s^{-1}}$ are preferred sites to host these events. Our approach can be readily applied to future high-mass gravitational wave events to predict their formation history under the hierarchical merger assumption.

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arXiv2024

Everything everywhere all at once: A detailed study of GW230529

This study investigates the origins of GW230529, delving into its formation from massive stars within isolated binary systems. Utilizing population synthesis models, we present compelling evidence that the neutron star component forms second. However, the event's low signal-to-noise ratio introduces complexities in identifying the underlying physical mechanisms driving its formation. Augmenting our analysis with insights from numerical relativity, we estimate the final black hole mass and spin to be approximately $5.3 M_\odot$ and $0.53$, respectively. Furthermore, we employ the obtained posterior samples to calculate the ejecta mass and kilonova light curves resulting from r-process nucleosynthesis. We find the ejecta mass to range within $0-0.06 M_{\odot}$, contingent on the neutron star equation of state. The peak brightness of the kilonovae light curves indicates that targeted follow-up observations with a Rubin-like observatory may have detected this emission.

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arXiv2023

Black hole binary mergers in dense star clusters: the importance of primordial binaries

Dense stellar clusters are expected to house the ideal conditions for binary black hole (BBH) formation, both through binary stellar evolution and through dynamical encounters. We use theoretical arguments as well as $N$-body simulations to make predictions for the evolution of BBHs formed through stellar evolution inside clusters from the cluster birth (which we term $\textbf{primordial binaries}$), and for the sub-population of merging BBHs. We identify three key populations: (i) BBHs that form in the cluster, and merge before experiencing any $\textit{strong}$ dynamical interaction; (ii) binaries that are ejected from the cluster after only one dynamical interaction; and, (iii) BBHs that experience more than one strong interaction inside the cluster. We find that populations (i) and (ii) are the dominant source of all BBH mergers formed in clusters with escape velocity $v_{\mathrm{esc}}\leq 30$ $\mathrm{km\,s^{-1}}$. At higher escape velocities, dynamics are predicted to play a major role both for the formation and subsequent evolution of BBHs. Finally, we argue that for sub-Solar metallicity clusters with $v_{\mathrm{esc}}\lesssim100$ $\mathrm{km\,s^{-1}}$, the dominant form of interaction experienced by primordial BBHs (BBHs formed from primordial binaries) within the cluster is with other BBHs. The complexity of these binary-binary interactions will complicate the future evolution of the BBH and influence the total number of mergers produced.

astro-ph.GA↗

arXiv2023

Double black hole mergers in nuclear star clusters: eccentricities, spins, masses, and the growth of massive seeds

We investigate the formation of intermediate mass black holes (IMBHs) through hierarchical mergers of stellar origin black holes (BHs), as well as BH mergers formed dynamically in nuclear star clusters. Using a semi-analytical approach which incorporates probabilistic mass-function-dependent double BH (DBH) pairing, binary-single encounters, and a mass-ratio-dependent prescription for energy dissipation in hardening binaries, we find that IMBHs with masses of $O(10^2)$-$O(10^4)\rm M_\odot$ can be formed solely through hierarchical mergers in timescales of a few $100$ Myrs to a few Gyrs. Clusters with escape velocities $\gtrsim400$ km s$^{-1}$ inevitably form high-mass IMBHs. The spin distribution of IMBHs with masses $\gtrsim 10^3$ M$_\odot$ is strongly clustered at $χ\sim 0.15$; while for lower masses, it peaks at $χ\sim 0.7$. Eccentric mergers are more frequent for equal-mass binaries containing first-and/or second-generation BHs. Metal-rich, young, dense clusters can produce up to $20\%$ of their DBH mergers with eccentricity $\geq0.1$ at $10\,\rm Hz$, and $\sim2$-$9\%$ of all in-cluster mergers can form at $>10$ Hz. Nuclear star clusters are therefore promising environments for the formation of highly-eccentric DBH mergers, detectable with current gravitational-wave detectors. Clusters of extreme mass ($\sim10^8$ M$_\odot$) and density ($\sim10^8$ M$_\odot$pc$^{-3}$) can have about half of all of their DBH mergers with primary masses $\geq100$ M$_\odot$. The fraction of in-cluster mergers increases rapidly with increasing cluster escape velocity, being nearly unity for $v_{\rm esc}\gtrsim 200$ km s$^{-1}$. Cosmological merger rate of DBHs from nuclear clusters varies $\lessapprox0.01-1$ Gpc$^{-3}$yr$^{-1}$.

astro-ph.HE↗