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Kyle Kremer

Publications and source records attributed to Kyle Kremer.

At least 19 recordsLinked to original sources
arXiv2026

Explaining the X-ray Precursor, Ultra-long Prompt Emission, and Week-long Decay of GRB250702B with a Jetted Micro-TDE

The longest detected gamma-ray burst, GRB250702B, exhibited seven hours of prompt $\gamma$-ray emission, preceded by a soft X-ray precursor ($\sim1$ day earlier) and followed by a weeks-long fading X-ray tail. Lacking an established progenitor for all three phases, we propose that this ultra-long GRB (ULGRB) is powered by a jetted micro-tidal disruption event (micro-TDE), in which a spinning stellar-mass black hole (BH) disrupts a Sun-like star and launches a relativistic jet via the Blandford-Znajek mechanism. Micro-TDE debris disks have hours-to-days viscous timescales, naturally explaining ULGRB durations. Using 3D hydrodynamic AREPO simulations of a $1\,M_\odot$ star disrupted by a $10\,M_\odot$ BH, we show that within $\sim1$ day the debris forms a quasi-steady envelope with a low-density polar funnel ($\rho\propto r^{-2}$, half-opening angle $\approx15^\circ$). Applying an analytic jet-stability framework to these profiles, we find that the $r^{-2}$ funnel keeps the jet below the kink-instability threshold, enabling stable propagation and breakout for jet powers, $L_{\rm jet}\gtrsim10^{47}$ erg s$^{-1}$. We attribute the X-ray precursor to pre-disk stream-fed accretion; the prompt GRB to a tightly beamed jet ($\theta_{\rm b}\lesssim1^\circ$, $L_{\gamma,\rm iso}\sim10^{51}$ erg s$^{-1}$) escaping the funnel, launched by a rapidly spinning BH ($a_\bullet\sim0.9$); and the weeks-long X-ray decline to disk-wind mass loss ($L_{\rm jet}\propto t^{-2}$) combined with jet widening ($\theta_{\rm b}\propto t$, initially steepening the decay to $L_{\rm X,iso}\propto L_{\rm jet}/\theta_{\rm b}^{2}\propto t^{-4}$). Our model reproduces the multi-phase evolution of GRB250702B and establishes jetted micro-TDEs as a physically motivated ULGRB engine.

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arXiv2026

A Framework for Linking Pre- and Post-Common Envelope Binary Properties with Star Clusters: The First Demonstration with a Massive White Dwarf+M Dwarf Binary in Alessi 12

Common envelope (CE) evolution is a critical phase in the lives of binary stars, producing close binaries that are progenitors of type Ia supernovae and gravitational wave sources. Despite its importance, CE evolution remains poorly understood, largely due to the scarcity of systems with constrained pre- and post-CE properties. Here, we present a star cluster-based framework for reconstructing the evolutionary histories of white dwarf+main-sequence (WD+MS) post-CE binaries, where cluster membership can provide an independent age constraint and/or rule out a merger origin for the WD. We demonstrate this method with Alessi12-PCE, the first such binary in an open cluster with precisely determined pre- and post-CE properties. We classify the companion as an M4V and measure a WD mass of $1.06 \pm 0.02 M_{\odot}$, making it the most massive WD+MS binary associated with a cluster. A 6.99-hour periodicity detected in a light curve is confirmed as the binary orbital period via radial velocity monitoring. Combined with the WD mass, WD cooling age, and Alessi 12 cluster age, stellar evolution models imply a $5.40 \pm 0.10 M_{\odot}$ WD progenitor that entered a CE on the asymptotic giant branch (AGB). CE evolution models where convection is the dominant physical mechanism that sets $\alpha_{\text{CE}}$ reproduce the observed orbital separation in exactly two scenarios: either a mid-AGB interaction with $\alpha_{\text{CE}}\approx0.99$, or a late-AGB interaction with $\alpha_{\text{CE}}\approx0.05$. Applicable to other post-CE binaries in star clusters, our new framework enables empirical constraints on CE physics inaccessible from field binaries alone.

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arXiv2026

Formation of rotating supergiants via stellar mergers in dense clusters: Implications for black hole natal spins

We investigate how massive stellar mergers in young star clusters imprint on black hole spin distributions and the broader implications for gravitational wave sources. The central hypothesis is that angular momentum transferred during stellar mergers substantially affects the spins of the merger products and resulting black holes, with some merger products evolving into collapsar-like objects that retain thick accretion disks that enable efficient spin up. This is in contrast to the more general expectation that black holes form with very small spins, having shed most of their envelope angular momentum via winds and expansion before core collapse. Using roughly 150 N-body models generated with the $\texttt{Cluster Monte Carlo}$ code, $\texttt{CMC}$, we analyze stellar mergers that lead to black hole formation, prioritizing ``significant'' events with mass ratio $q>0.1$. After identifying optimal candidates from our $\texttt{CMC}$ models, we explore detailed stellar structure and post-merger evolution implications with MESA stellar evolution models to capture angular momentum injection and pre-collapse profiles most relevant for the BH natal spin. In our current dataset representative of Milky Way-like globular clusters, up to roughly half of black holes are formed from such mergers, including up to roughly $10\%$ from significant mergers with $q>0.1$. Preliminary angular momentum estimates indicate substantial spin-up during the merger, and trends with mass ratio and stellar properties suggest strong correlations with the final black hole spin. In some cases, dimensionless spin parameters of $a\simeq 0.5$ or more are expected. This process has important implications for the dynamical formation and retention of gravitational wave sources in clusters.

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arXiv2026

Formation of Black Hole-White Dwarf X-ray Binaries in Globular Clusters

Globular clusters are host to significant populations of dynamically-active stellar remnants that connect to a variety of astrophysical sources. Using simulations performed with the Cluster Monte Carlo dynamics code, we study the formation of ultracompact binaries in which a stellar-mass black hole accretes material from a white dwarf companion in a sub-hour orbit. These binary systems are prime multimessenger targets, as they can be observed as both luminous X-ray sources, and as millihertz gravitational wave sources detectable by the Laser Interferometer Space Antenna (LISA). We find that black hole+giant collisions are the primary mechanism through which such systems form. We model the outcomes of these ``common envelope''-like events using the smoothed particle hydrodynamics code StarSmasher, and verify these collisions yield black hole+white dwarf binaries that enter Roche contact on sub-Gyr timescales via gravitational wave inspiral. We construct a mock catalog of local ultracompact X-ray sources and compare to candidate sources observed in globular clusters in the Milky Way (e.g., 47 Tuc X9) and external galaxies (e.g., RZ 2109 in NGC 4472). Finally, we compute the gravitational wave strain for these sources, and show that of order one source may be resolvable in the Milky Way by LISA, representing a potentially powerful tool for observing new black holes in globular clusters.

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arXiv2026

Red vs. Blue: How metallicity shapes black hole dynamics and mergers in dense star clusters

Dense star clusters are a well-established environment for the formation of gravitational wave sources through dynamical interactions. Recent LIGO-Virgo-KAGRA (LVK) events such as GW241011 and GW241110 provide some of the best evidence yet for a dynamical origin. However, their relatively low component masses are in tension with predictions from low-metallicity globular cluster models (which typically produce more massive black holes), hinting that these events may have originated in higher-metallicity environments. Here we present a new set of Monte Carlo star cluster simulations with refined coverage in metallicity, focusing specifically on clusters with [Fe/H] $\geq-1$, similar to the ''red'' globular cluster subpopulation observed in most galaxies. We show that metallicity has a significant effect on the mass function of black holes and black hole mergers, the total number of black hole mergers per cluster, black hole retention from natal kicks, the mass segregation time for black-hole-driven cluster dynamics, and the merger delay time distribution. We also show that high-metallicity cluster models produce low-mass hierarchical mergers consistent with the mass ratios and component masses of GW241011 and GW241110, motivating the importance of high-metallicity clusters in the astrophysical interpretation of future LVK catalogs.

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arXiv2026

Black hole mergers from dense star clusters with realistic binary populations

We present a suite of 24 full-lifetime simulations of dense star clusters with the Cluster Monte Carlo (CMC) code, featuring updated input physics and a realistic distribution of initial binary systems. The latter encompasses a mass-dependent binary fraction, period distribution, and eccentricity distribution based on observations of well-studied stellar populations in the Solar neighborhood and nearby star-forming regions. We predict the cosmic rate, masses, and spins of binary black hole (BBH) mergers formed through dynamical assembly, primordial binary evolution, and hierarchical mergers within dense clusters. As with previous model grids with fewer binaries, dynamically assembled first-generation (1G) mergers dominate the rate of cluster-derived mergers, and the total merger rate is consistent with that inferred from LIGO-Virgo-KAGRA observations as of GWTC-5.0. Our models naturally reproduce key features of the inferred BBH population, including the broken-power-law behavior of the primary BH mass spectrum for $m_1 \gtrsim 20 M_\odot$, the shallower (steeper) slope of the secondary mass spectrum relative to the primary for $m_2 \lesssim 10 M_\odot$ ($m_2 \gtrsim 30 M_\odot$), and the shape of the mass-ratio distribution in the low- and high-mass domains. We predict broad distributions of the spin parameters $\chi_{\mathrm{eff}}$ and $\chi_{\mathrm{p}}$, consistent with previous studies of dynamical assembly in clusters. The merger rate from primordial binary systems within clusters is a small fraction of the total; however, their merger products are frequently involved in subsequent hierarchical mergers, with the result that the hierarchical merger rate evolves more steeply than the 1G dynamical merger rate with redshift.

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arXiv2026

Eccentric Stellar-mass Binary Black Holes: Population, Detectability, and Waveform Analysis in the LISA and LIGO Era

Eccentric binary black holes (BBHs) formed through dynamical interactions can significantly contribute to gravitational wave (GW) detections. In this work, we present a simulated catalog of dynamically-formed, stellar-mass BBHs in the local universe, incorporating contributions from the Galactic field (flyby interactions), Galactic nucleus (eccentric Kozai-Lidov evolution), and globular clusters (N-body interactions). Our results predict a wide, highly eccentric BBH population in the Milky Way (MW), with source counts of $\sim 36, 13, 4.7, 2.3, 1.0$ (for $\mathrm{SNR} > 1, 3, 8, 20, 50$, respectively) during a 10-yr LISA observation. Extending this model to cosmological populations, we show that different dynamical channels can produce distinct eccentricity distributions in the LVK band and can contribute hundreds of additional low-SNR mHz sources. Specifically, our model yields a merger rate of $\Gamma \sim 9 \mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$ and $\sim 490$ extragalactic mHz BBHs with $\mathrm{SNR} > 1$. However, due to the lower mass and weaker GW signals of stellar-mass BBHs, this number declines sharply at higher detection thresholds (e.g., $\sim 1$ for $\mathrm{SNR} > 8$). We further highlight the impact of eccentric BBH signals on the LISA global fit, showing that their individual harmonics can be independently detected in the Milky Way, and may mimic circular binaries with systematically biased chirp masses. Lastly, we show that post-Newtonian waveforms converge reliably for eccentric BBHs with masses of $\lesssim 10^3 M_\odot$ in the mHz band. Overall, eccentric BBHs represent a prevalent and promising target for future space-based GW observatories. The simulated catalog and the LISA Eccentricity Astrophysics Package (LEAP) developed in this work are publicly available at https://github.com/zeyuanxuan/lisa-leap/.

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arXiv2026

The diverse outcomes of binary white dwarf mergers and connections to Galactic LISA sources

In the coming decade, the millihertz gravitational wave observatory LISA will provide the best constraints yet on the tens of thousands of close white dwarf binaries in the Milky Way, yielding unprecedented insights into the most abundant class of compact object binaries. Following inspiral via gravitational wave emission, interacting white dwarf binary pairs can lead to a multitude of outcomes, including AM Canum Venaticorum (AM CVn) binaries, R Coronae Borealis stars, young, rapidly-spinning single white dwarfs, (millisecond) magnetars, and a variety of explosive transients, most notably Type Ia supernovae. Current and future electromagnetic observations of these various outcomes coupled with the forthcoming flood of data from LISA place us on the precipice of a significant advance in our understanding of the long-term fate of white dwarf binaries. In this paper, we present a suite of mock catalogs of the Milky Way's white dwarf merger history, created using the population synthesis code $\texttt{COSMIC}$ combined with a metallicity-dependent star formation history from FIRE-2 galaxy simulations. We summarize the various merger outcomes expected (based upon varying white dwarf masses and chemical compositions) and explore ways the rates of these outcomes may vary with model uncertainties pertaining to binary evolution. We publicly release these merger catalogs as a tool for facilitating connections between gravitational wave science and white dwarf binary astrophysics.

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arXiv2026

An analytical approach to binary populations in globular clusters

Globular clusters (GCs) display much lower binary fractions than found among main-sequence stars in the solar neighborhood. The physical cause of this difference is debatable: does it reflect different star formation outcomes at low metallicity and/or high density, the dynamical processing of primordial binaries over cluster lifetimes, or a combination of the two? Starting from the assumption that the initial binary distribution in GCs is the same as the binary distribution observed in the solar neighborhood, we show with straightforward analytical calculations that the dynamical dissolution of "soft" primordial binaries can fully explain the main-sequence binary fractions in present-day GCs. We validate our estimates against a detailed N-body simulation with the Cluster Monte Carlo code. Adopting the view that the observed binary fraction in a given cluster constrains the location of the hard/soft boundary at birth, we infer that surviving Milky Way GCs had a similar distribution of birth radii to young massive clusters in the local universe. Our findings underscore the crucial role of stellar black holes (through "black hole burning") in sculpting GC binary populations and reinforce the need for realistic initial conditions in theoretical modeling of GC dynamics.

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arXiv2026

A cosmological framework for stellar collisions at high redshift in proto-globular clusters, nuclear star clusters, and Little Red Dots

Observations and cosmological simulations indicate that the early Universe hosted numerous compact, high-density stellar systems, where close encounters and physical collisions between stars were likely common. We develop a bottom-up framework for stellar dynamics in such environments, spanning systems with and without intermediate- and supermassive black holes, and covering regimes where stellar collisions may or may not dominate the evolution. This radially-resolved analytic model connects dense star clusters in their cosmological context to observable outcomes mediated by stellar collisions. Initial conditions and environmental properties are drawn from high-resolution cosmological simulations, enabling exploration across a broad region of parameter space. The analytic predictions are validated against Monte Carlo simulations, demonstrating good agreement across key regimes. We find that stellar collisions are ubiquitous in many high-redshift environments, with runaway sequences naturally leading to the formation of very massive stars at early times. Finally, we show that high rates of destructive collisions can rapidly build up extremely dense gaseous environments around massive black holes, potentially providing an analogue to the observed population of Little Red Dots.

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arXiv2026

A Path to an All-Sky Survey with Roman

A deep, space-based, all-sky near-infrared survey carried out with the Nancy Grace Roman Space Telescope would constitute a foundational astronomical infrastructure for decades to come. In this white paper, we present a concrete and feasible path to imaging the entire sky at $\sim0.1''$ resolution, beginning with high-impact fields in Cycle 1 and scaling to ultra-wide coverage within the nominal mission. This first-epoch survey will reach $\mathrm{H}\sim25.5$ AB mag (5$\sigma$) and maximize synergies with contemporaneous observatories, while preserving substantial time for other ambitious Roman programs. We outline representative scheduling scenarios and an example Cycle 1 program that triples early Roman-LSST overlap and delivers high-value community data products such as LSST forced photometry, joint \textit{Gaia}-Roman astrometry, and catalogs of Galactic substructure, stong lenses, and other rare systems. The Cycle 1 program will lay the foundation for an eventual all-sky survey, while also delivering high-impact early science. We invite broad community participation in shaping and carrying out both the initial program and the long-term vision of an all-sky Roman survey.

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arXiv2026

Machine Learning Methods for Stellar Collisions. I. Predicting Outcomes of SPH Simulations

Stellar collisions can occur frequently in dense cluster environments, and play a crucial role in producing exotic phenomena from blue stragglers in globular clusters to high-energy transients in galactic nuclei. Successive collisions and mergers of massive stars could also lead to the formation of massive black holes, serving as seeds for supermassive black hole in the early universe. While analytic fitting formulae exist for predicting collision outcomes, they do not generalize across different energy scales or stellar evolutionary phases. Smoothed particle hydrodynamics (SPH) simulations are often used to compute the outcomes of stellar collisions, but, even at low resolution, their computational cost makes running on-the-fly calculations during an $N$-body simulation quite challenging. Here we present a new grid of $27,720$ SPH calculations of main-sequence star collisions, spanning a wide range of masses, ages, relative velocities, and impact parameters. Using this grid, we train machine learning models to predict both collision outcomes (merger vs disruption, or flyby) and final remnant masses. We compare the performance of nearest neighbors, support vector machines, and neural networks, achieving classification balanced accuracy of $98.4\%$, and regression relative errors as low as $0.11\%$ and $0.15\%$ for the final stars $1$ and $2$, respectively. We make our trained models publicly available as part of the package collAIder, enabling rapid predictions of stellar collision outcomes in $N$-body models of dense star cluster dynamics.

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arXiv2026

Discovery of a double white dwarf in the Galactic globular cluster NGC 6397

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a large number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with an extremely low-mass He-core WD nature. This is further supported by the mass of 0.23 +/- 0.03 Msun obtained from fitting the star's spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 Msun for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50 deg. Some significant residuals in radial velocity remain with our best orbital solution and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.

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arXiv2026

Thick Disks, Thin Hopes: Suppressed Capture and Merger Rates in AGN

Multiple models have been suggested over the years to explain the structure and support of accretion disks around supermassive black holes, from the standard thin thermal-pressure-dominated $\alpha$-disk model to more recent models that describe geometrically thicker radiation or magnetic or turbulence-dominated disks. In any case, objects embedded in the disk (e.g. compact objects, stars, gas, dust) can undergo gravitational and hydrodynamic interactions with each other leading to interesting processes such as binary interaction/capture, gravitational wave merger events, dynamical friction, accretion, gap opening, etc. It has long been argued that disks of active galactic nuclei (AGN) can enhance the rates for many of these events; however, almost all of that analysis has assumed specific thin-disk models (with aspect ratios $H/R \lesssim 0.01$). We show here that the rates for processes such as these that are mediated by gravitational cross-sections has a very strong inverse dependence on the thickness $H/R$ (scaling as steeply as $(H/R)^{-8}$), and $H/R$ can vary in the outer disk (where these processes are often invoked) by factors $\gtrsim 1000$ depending on the assumed source of pressure support in the disk. This predicts rates that can be lower by tens of orders-of-magnitude in some models, demonstrating that it is critical to account for disk parameters such as aspect ratio and different sources of disk pressure when computing any meaningful predictions for these rates. For instance, if magnetic pressure is important in the outer disk, as suggested in recent work, capture rates would be suppressed by factors $\sim 10^{10}-10^{20}$ compared to previous studies where magnetic pressure was ignored.

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arXiv2025

Shadows of the Colossus: Hierarchical Black Hole Mergers in a 10-million-body Globular Cluster Simulation

The LIGO/Virgo/Kagra (LVK) Collaboration has detected numerous binary black hole mergers with properties that challenge standard binary evolution scenarios, such as component masses above the pair-instability gap and high spin magnitudes. Dense stellar environments such as globular clusters provide a natural channel for producing such systems through hierarchical mergers, where black hole remnants formed in earlier mergers are retained in the cluster and undergo successive mergers. However, gravitational-wave recoil kicks often eject merger remnants from typical globular clusters, which limits hierarchical growth. Massive clusters with deeper potential wells, such as those found in giant elliptical galaxies like M87, may overcome this barrier, but direct simulations of such massive globular clusters remains computationally challenging. In this study, we present a 10-million-body cluster simulation performed with the $\texttt{Cluster Monte Carlo}$ ($\texttt{CMC}$) code, referred to as $\texttt{colossus}$, which serves as a proxy for the most massive low-metallicity globular clusters observed in the local Universe. This simulation demonstrates that extended chains of hierarchical mergers can occur in massive globular clusters, producing black holes up to fifth generation with masses approaching $250\,M_\odot$, comparable to the most massive LVK events observed to date (e.g., GW231123). Combining the $\texttt{colossus}$ simulation with the previous $\texttt{CMC Cluster Catalog}$, we develop a framework to extrapolate binary black hole merger predictions for the thousands of globular clusters seen in the Virgo Supercluster.

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arXiv2025

Connecting Cores and Black Hole Dynamics Across Scales: From Globular Clusters to Massive Ellipticals

The centers of massive elliptical galaxies exhibit a wide range in density profiles, from central cusps to resolved cores with order kiloparsec sizes. The cored ellipticals have been linked to the presence of supermassive black hole binaries that excavate their hosts' central stellar populations through three-body encounters. This connection between cores and black holes similarly operates in globular clusters, which also exhibit a bimodality in cored and core-collapsed architectures, respectively rich and depleted in stellar black holes. We report new estimates of the total black hole mass in 25 Galactic globular clusters based on a suite of roughly 150 Monte Carlo $N$-body simulations that fit observed surface brightness and velocity dispersion profiles. We show that both globular clusters and massive elliptical galaxies individually exhibit strong correlations between total black hole mass ($M_\bullet$) and core radius ($r_c$), and that these individual relations share a common power-law exponent to within $1\sigma$ statistical precision: $M_\bullet \sim r_c^{1.3}$. The individual relations appear to be offset, suggesting swarms of stellar black holes scour globular cluster cores more efficiently than lone supermassive black holes scour the cores of massive ellipticals. Yet the shared basis of core scouring via black hole binaries hints at a unified $M_{\bullet}-r_c$ connection across over 10 orders of magnitude in $M_\bullet$. Our findings imply core radius measurements may offer a powerful observational constraint on black hole merger rates, from kilohertz sources detectable by LIGO/Virgo/KAGRA formed in globular clusters to millihertz and nanohertz sources formed in massive elliptical galaxies.

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arXiv2025

Observing compact Pop III star clusters and the presence of cosmic streaming

The formation of the Universe's first luminous stellar structures depends on the unique conditions at "Cosmic Dawn," which are set by the underlying cosmological model and early baryonic physics. Observations suggest that high-$z$ star clusters reached stellar surface densities above $10^5 M_\odot$ pc$^{-2}$, suggesting scenarios where models predict that the ability of stellar feedback to counter gravitational collapse is severely limited. We investigate the first star clusters in a suite of AREPO simulations, which explore the capacity for $\Lambda$CDM halos to maximally form high-density systems without feedback. We include the effects of the supersonic baryon-dark matter streaming velocity, an effect that impacts gas density and distribution in early minihalos. We show that early star clusters can reach high densities even in regions of strong supersonic streaming, provided feedback is weak. We analyze the interplay of the stream velocity and the dynamical processes of structure formation, finding that JWST has the opportunity to detect the brightest, most massive objects in our computational box. The detection of individual $z\geq12$ Pop III star clusters below $10^7M_\odot$ is challenging, although lensing could reveal these objects in rare configurations, especially if a top-heavy IMF is present. We find that accounting for baryonic clusters separately from dark matter halos complicates predictions for the faint-end of the high-$z$ UVLF, with competing effects from the stream velocity and low-mass clusters outside of halos. Finally, we explore clustering of star clusters as a promising probe of the stream velocity in these systems.

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arXiv2025

Beyond Hierarchical Mergers: Accretion-Driven Origins of Massive, Highly Spinning Black Holes in Dense Star Clusters

GW231123, the most massive binary black hole (BBH) merger detected by LIGO/Virgo/KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called "hierarchical merger" channel), most star clusters with masses $\lesssim 10^6\,M_\odot$ have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH--star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. Using $N$-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process and we find that accretion can drive BH masses up to at least $\sim200\,M_\odot$, with spins set by the details of the growth history. BHs up to about $150\,M_\odot$ can reach dimensionless spins $\chi \gtrsim 0.7$ via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down to $\chi \lesssim 0.4$. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to $\sim10\%$ of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems.

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