arXiv subjects

Nicholas C. Stone

Publications and source records attributed to Nicholas C. Stone.

At least 19 recordsLinked to original sources

arXiv2026

Chaos and loss: reevaluating the loss-cone formalism in the context of chaotic diving orbits in flattened potentials

The region close to a massive black hole is one of the few truly spherical geometries in galactic dynamics. Stellar structures, which dominate from pc to kpc scales, are generally at least flattened (axisymmetric) if not more irregular and time evolving. Here we show that orbits that pass between these diverse geometries are commonly chaotic, especially those which dive - visiting their minimum possible periapse. We show that these chaotic diving orbits are common across a wide range of potentials, and can lead to large variation in angular momentum orbit to orbit. We characterise the transition from regular to chaotic orbits with a notional critical angular momentum, $j_d$. We then model the flux of orbits into the loss cone including the effects of these diving orbits combined with evolution through two-body scattering. Systems which cross into the chaotic region are quickly advected away, and thus there is a large net flux across the boundary. We formulate the problem as a probability density, rather than a number density, allowing us to find a true equilibrium state. We find that the rate of disruptions when including diving orbits is boosted by a multiplicative factor of $1+\ln(j_d/j_t)$, where $j_t$ is the angular momentum corresponding to a disruption. This boost can increase disruption rates by an order of magnitude compared to the classical loss cone formalism.

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arXiv2026

Ejected and Rejected: Strong Scatterings in Dense Star Clusters

Understanding the evolution of collisional stellar systems is key to predicting both the bulk evolution of dense star clusters and also the rates of tidal disruption events, extreme mass ratio inspirals (EMRIs), stellar ejections, and other loss cone phenomena. Although a majority of the evolution is dictated by weak, small-angle scatterings, rare close encounters resulting in strong scatterings can sometimes produce qualitatively different outcomes. To systematically determine the astrophysical importance of strong scatterings, we derive the exact differential rate distribution of velocity kicks in isotropic and spherically symmetric distribution functions representing globular and nuclear star clusters, while also deriving the distribution boundaries and approximate rate scalings. We then extend this formalism to derive approximate rates of energy and angular momentum kicks for objects in the nuclear star cluster, and then apply this framework to study (i) ejections and (ii) ``cancellation'' of extreme mass ratio inspirals. We find that ejection rates are complicated in some regimes due to the saturation of the strong kick distribution and the breakdown of the weak limit, likely enhancing the effect of loss cone shielding. We also determine that a sizable fraction of proto-EMRIs will likely be canceled by strong scatterings, highlighting the need for future work to determine to what degree strong scatterings impact the true EMRI rate.

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arXiv2026

Intermediate States in Chaotic Triple Evolution and Applications to Black Hole Merger Statistics

Three-body interactions exhibit phases of strong chaotic evolution as well as hierarchical motion where one body separates from a binary and follows a hyperbolic or elliptic trajectory around it. The binaries produced during phases of hierarchical motion may lead to gravitational wave (GW) inspirals, but this depends on the outcomes of the chaotic states. In this paper we re-derive the elliptic outcome distribution using equilibrium statistical mechanics and explore it together with the hyperbolic distribution. When comparing to N-body simulations, we find that we can reduce the elliptic outcome model to one free parameter instead of the previously used two and that the predicted disintegration probabilities agree except for very low angular momentum triples. We then use both outcome distributions along with a star cluster model to design a Monte Carlo algorithm for repeated binary-single scatterings within dense star systems. We explore star cluster masses of $[10^5 - 10^7] M_{\odot}$, with the goal of quantifying observably eccentric merger (OEM) GWs, visible to instruments such as LIGO and Virgo. Assuming an OEM detection sensitivity of $f_{\rm min}=10 {\rm Hz}, e_{\rm min} = 0.1$, we find the elliptic OEMs are about $\sim (32 - 63)\%$ of the total elliptic mergers and that the total cluster mass greatly impacts the fraction of ejected binaries. The OEM to total merger fraction (OEM fraction) is found to be $(2.6 - 4.4)\%$. Considering the detection sensitivity that GW interferometers have today $(f_{\rm min} \simeq 34.4 {\rm Hz})$ we obtain the OEM fraction in the $(1.6 - 3.1)\%$ range.

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arXiv2026

EMRIs in Kerr Spacetime: The Role of Inclination Diffusion

Extreme mass ratio inspirals (EMRIs) are expected to be a major source of gravitational waves (GWs) for the future {\it Laser Interferometer Space Antenna} ({\it LISA}). An EMRI occurs when a compact object, driven by the emission of gravitational radiation, slowly inspirals into a massive black hole (MBH). The classical formation channel involves repeated two-body interactions within dense nuclear star clusters. However, both EMRI event rates and the expected distribution of their orbital parameters remain highly uncertain. A major source of this uncertainty is the poorly constrained ratio between gradually inspiraling EMRIs and direct plunges, which do not produce detectable GWs. While most studies of astrophysical EMRI production focus on the Schwarzschild limit, previous work examining the Kerr metric found that the spin of the central MBH can significantly affect the EMRI-to-plunge ratio. This enhancement depends sensitively on orbital inclination, yet earlier studies held inclination fixed. In this work, we develop the mathematical framework describing inclination evolution driven by two-body encounters evolving along Kerr geodesics. By implementing 3D diffusion into Monte Carlo simulations, we show that initially prograde orbits may evolve into retrograde states, thereby converting potential EMRIs into plunges or vice versa. We estimate the EMRI-to-plunge ratio and find that substantial MBH spin enhances the EMRI rate by a factor of $\sim 2.5$. Additionally, even for a non-spinning MBH, modeling the orbits as relativistic geodesics rather than Newtonian trajectories increases the number of EMRIs by a similar factor of $\sim 2.5$. The distribution of Kerr EMRI inclinations has a substantial prograde bias that may aid in identifying the subset of loss cone EMRIs within large future EMRI samples.

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arXiv2026

Discovery of a second-generation planet candidate accreting onto a white dwarf

Many white dwarfs accrete the debris of disrupted planetary bodies, detected in the form of metal-enrichment in their atmospheres, dusty or gaseous circumstellar discs, and photometric transits from debris. The composition of accreted debris has been shown to be diverse yet overall closely resembles Solar System bodies. We report the discovery of a white dwarf accreting material that is unlike any Solar System object. The atmosphere of HS 0209+0832 is strongly enriched in trans-iron elements including zinc, copper, and niobium but depleted in the canonical rock-forming elements silicon and iron. The composition of the accreted object is consistent with a candidate second-generation planet, formed from the stellar material ejected during the giant phase. The key abundance signature which differentiates it from a first-generation planet is the high enrichment of s-process elements. Photospheric helium and the absence of terrestrial rock-forming elements implies the white dwarf is accreting from the escaped atmosphere of a photo-evaporating giant planet candidate. This is further supported by the detection of a sinusoidal photometric period of 4.399+/-0.026 days at an amplitude of 0.120+/-0.018 %, which we attribute to thermal emission phase variability from a planetary day-night cycle or a transiting cometary tail of an evaporating gas giant.

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arXiv2026

How Flat is a Plateau? Evolution of Late-Time TDE Disks

Late-time light curve plateaus in tidal disruption events (TDEs) are often approximated as flat and time-independent. This simplification is motivated by theoretical modeling of spreading late time TDE disks, which predicts slow light curve evolution. However, if time evolution can be detected, late-time light curves yield more information than previously accessible. In this work, we re-examine late-time TDE data to test how well the flat plateau assumption holds. We use Markov Chain Monte Carlo to estimate the maximum likelihood for a family of theory-agnostic models and apply the Akaike information criterion to find that roughly one third of our sample favors evolving plateaus, one third favors truly flat plateaus, and one third shows no statistically significant evidence for any plateau. Next, we refit the TDEs that exhibit statistically significant plateaus using a magnetically elevated $α$-disk model, motivated by the lack of clear thermal instability in late time TDE light curves. From these model-dependent fits, we obtain estimates for the supermassive black hole (SMBH) mass, the mass of the disrupted star, and $α$. Fitted $α$ values have a mean $α=10^{-1.5}$, with scatter of 1 dex, broadly consistent with results from magnetohydrodynamic simulations, albeit with some outliers. Finally, we estimate the timescales of disk precession in magnetically elevated TDE models. Theoretically, we find that disk precession times may be orders of magnitude shorter than in unmagnetized Shakura-Sunyaev disks, and grow in time as $T_{\rm prec}\propto t^{35/36}$; empirically, by using fitted $α$ parameters, we estimate that late time disks may experience $\sim$few-10 precession cycles.

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arXiv2026

The Delay Time Distribution of Tidal Disruption Events

Tidal disruption events (TDEs) can be observed when stars get too close to supermassive black holes and are torn apart and accreted. The delay time distribution of TDEs, or rate of TDEs as a function of time since a burst of star formation, can be used to determine what mechanisms influence the TDE rate. We compile a catalog of 41 TDE host galaxies with optical spectra, model the stellar populations with Bagpipes, and retrieve the age of the most recent burst of star formation to construct the delay time distribution of TDEs. TDEs occur more frequently in post-starburst galaxies than in other types of galaxies, though the mechanism causing this rate enhancement is unknown. We find that the TDE rate increases with post-burst age to reach a peak at ~1 Gyr relative to a control sample. We compare the observational TDE delay time distribution to theoretical models, which propose overdense stellar nuclei, radial anisotropies in stellar orbits, supermassive black hole binaries, and AGN disks as potential mechanisms that may enhance the TDE rate in post-starburst galaxies. Most models predict a TDE rate that declines with post-burst age, in contrast to our observational results, though some models are still feasible at certain ages (e.g., the black hole binary model matches at old burst ages and the stellar overdensity model matches at intermediate burst ages).

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arXiv2026

Multigroup Radiation Diffusion on a Moving Mesh: Implementation in RICH and Application to Tidal Disruption Events

Radiation-hydrodynamics (RHD) determines the bulk evolution and observable emission in a wide variety of high-energy astrophysical phenomena. Due to their complexity, RHD problems must usually be studied through numerical simulation. We have extended the publicly available RICH code, which previously solved the equations of RHD in the limit of grey flux-limited diffusion (FLD), to operate with a multigroup FLD solver. RICH is a semi-Lagrangian code that solves the equations of RHD on an unstructured moving mesh, and is the first multigroup RHD moving mesh code, making it uniquely applicable to problems with extreme dynamic range and dynamically important radiation forces. We validate our multigroup module against multiple analytic benchmarks, including a novel test of the RHD Doppler term. The computational efficiency of the code is aided by a novel scheme to accelerate convergence in optically thick cells by limiting the absorption coefficients. Finally, we apply multigroup \textsc{rich} in a pilot three dimensional study of a stellar tidal disruption event (TDE), using a $10^4 M_\odot$ intermediate-mass black hole. Our simulations self-consistently produce a bright early-time X-ray flash prior to peak optical/UV light, in qualitative agreement with post-processing of (grey) RICH simulations of supermassive black hole TDEs, as well as X-ray observations of the TDE AT 2022dsb.

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arXiv2025

Late-Time Evolution of Magnetized Disks in Tidal Disruption Events

In classic time-dependent 1D accretion disk models, the inner radiation pressure dominated regime is viscously unstable. However, late-time observations of accretion disks formed in tidal disruption events (TDEs) do not exhibit evidence of such instabilities. The common theoretical response is to modify the viscosity parametrization, but typically used viscosity parametrization are generally ad hoc. In this study, we take a different approach, and investigate a time-dependent 1D $α$-disk model in which the pressure is dominated by magnetic fields rather than photons. We compare the time evolution of thermally stable, strongly magnetized TDE disks to the simpler linear viscosity model. We find that the light curves of magnetized disks evolve as $L_{\rm UV}\propto t^{-5/6}$ for decades to centuries, and that this same evolution can be reproduced by the linear viscosity model for specific parameter choices. Additionally, we show that TDEs remain UV-bright for many years, suggesting we could possibly find fossil TDEs decades after their bursts. We estimate that ULTRASAT could detect hundreds of such events, providing an opportunity to study late-stage TDE physics and supermassive black hole (SMBH) properties. Finally, we explore the connection between TDE disks and quasi-periodic eruptions (QPEs) suggested by recent observations. One theoretical explanation involves TDE disks expanding to interact with extreme mass ratio inspirals (EMRIs), which produce X-ray flares as the EMRI passes through the disk. Our estimates indicate that magnetized TDE disks should exhibit QPEs earlier than those observed in AT2019qiz, suggesting that the QPEs may have begun before their first detection.

astro-ph.HE↗

arXiv2025

The unreasonable effectiveness of the $n Σv$ approximation

In kinetic theory, the classic $n Σv$ approach calculates the rate of particle interactions from local quantities: the number density of particles $n$, the cross-section $Σ$, and the average relative speed $v$. In stellar dynamics, this formula is often applied to problems in collisional (i.e. dense) environments such as globular and nuclear star clusters, where blue stragglers, tidal capture binaries, binary ionizations, and micro-tidal disruptions arise from rare close encounters. The local $n Σv$ approach implicitly assumes the ergodic hypothesis, which is not well motivated for the densest star systems in the Universe. In the centers of globular and nuclear star clusters, orbits close into 1D ellipses because of the degeneracy of the potential (either Keplerian or harmonic). We find that the interaction rate in perfectly Keplerian or harmonic potentials is determined by a global quantity -- the number of orbital intersections -- and that this rate can be far lower or higher than the ergodic $n Σv$ estimate. However, we find that in most astrophysical systems, deviations from a perfectly Keplerian or harmonic potential (due to e.g. granularity or extended mass) trigger sufficient orbital precession to recover the $n Σv$ interaction rate. Astrophysically relevant failures of the $n Σv$ approach only seem to occur for tightly bound stars orbiting intermediate-mass black holes, or for the high-mass end of collisional cascades in certain debris disks.

astro-ph.HE↗

arXiv2025

Disruptions of stars and binary systems on chaotic orbits in an axisymmetric Milky Way center

Non-spherical potentials allow a wide range of trajectories, both regular and chaotic, whose periapse distances can vary orbit to orbit. In particular chaotic trajectories can bring a system arbitrarily close to the central massive black hole leading to a disruption. In this paper, we work with an observationally benchmarked model of the innermost 200 pc of the Milky Way and show that low z-angular momentum trajectories are commonly chaotic. We compute the timescales and properties of close pericenter passages, and compare the implied collisionless disruption rate to the well-studied collisional rate from 2-body scatterings. We find that the relative collisionless rate can dominate by orders of magnitude. Our calculations are relevant for a wide range of disruption phenomena, including the production of hypervelocity stars (HVSs) and tidal disruption events (TDEs). Most of these disruptions involve stars come from the Nuclear Stellar Cluster, with a pericenter distribution that strongly favours shallow encounters, and a preference for high inclination interactions. The latter implies that unbound disrupted material - whether ejected stars or stellar debris - would be preferentially directed towards the galactic poles. Many of our conclusions apply generally to any galaxy with a non-spherical galactic centre potential and central massive black hole.

astro-ph.GA↗

arXiv2024

Counting the Unseen II: Tidal Disruption Event Rates in Nearby Galaxies with REPTiDE

Tidal disruption events (TDEs) are a class of transients that occur when a star is destroyed by the tides of a massive black hole (MBH). Their rates encode valuable MBH demographic information, but this can only be extracted if accurate TDE rate predictions are available for comparisons with observed rates. In this work, we present a new, observer-friendly Python package called REPTiDE, which implements a standard loss cone model for computing TDE rates given a stellar density distribution and an MBH mass. We apply this software to a representative sample of 91 nearby galaxies over a wide range of stellar masses with high-resolution nuclear density measurements from arXiv:2407.10911. We measure per-galaxy TDE rates ranging between 10$^{-7.7}$ and 10$^{-2.9}$ per year and find that the sample-averaged rates agree well with observations. We find a turnover in the TDE rate as a function of both galaxy stellar mass and black hole mass, with the peak rates being observed in galaxies at a galaxy mass of $10^{9.5}$ M$_\odot$ and a black hole mass of $10^{6.5}$ M$_\odot$. Despite the lower TDE rates inferred for intermediate-mass black holes, we find that they have gained a higher fraction of their mass through TDEs when compared to higher mass black holes. This growth of lower mass black holes through TDEs can enable us to place interesting constraints on their spins; we find maximum spins of $a_\bullet \approx 0.9$ for black holes with masses below $\sim10^{5.5}$ M$_\odot$.

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arXiv2024

Growth of Massive Black-Holes in FFB Galaxies at Cosmic Dawn

The scenario of feedback-free starbursts (FFB), which predicts excessively bright galaxies at cosmic dawn as observed using JWST, may provide a natural setting for black hole (BH) growth. This involves the formation of intermediate-mass seed BHs and their runaway mergers into super-massive BHs with high BH-to-stellar mass ratios and low AGN luminosities. We present a scenario of merger-driven BH growth in FFB galaxies and study its feasibility. BH seeds form within the building blocks of the FFB galaxies, namely, thousands of compact star clusters, each starbursting in a free-fall time of a few Myr before the onset of stellar and supernova feedback. The BH seeds form by rapid core collapse in the FFB clusters, in a few free-fall times, sped up by the migration of massive stars due to the young, broad stellar mass function and stimulated by a `gravo-gyro' instability due to internal cluster rotation and flattening. BHs of $10^4 M_\odot$ are expected in $10^6 M_\odot$ FFB clusters within sub-kpc galactic disks at $z \sim 10$. The BHs then migrate to the galaxy center by dynamical friction, hastened by the compact FFB stellar galactic disk configuration. Efficient mergers of the BH seeds will produce $10^{6-8} M_\odot$ BHs with a BH-to-stellar mass ratio $\sim 0.01$ by $z \sim 4-7$, as observed. The growth of the central BH by mergers can overcome the bottleneck introduced by gravitational wave recoils if the BHs inspiral within a relatively cold disk or if the escape velocity from the galaxy is boosted by a wet compaction event. Such events, common in massive galaxies at high redshifts, can also help by speeding up the inward BH migration and by providing central gas to assist with the final parsec problem. The cold disk version of the FFB scenario provides a feasible route for the formation of supermassive BHs.

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arXiv2024

How to Escape from a Trap: Outcomes of Repeated Black Hole Mergers in AGN

Stellar-mass black holes (BHs) embedded in active galactic nuclei (AGN) may be major sources of astrophysical gravitational waves (GWs), contributing both to the observed LIGO-Virgo-KAGRA population of binary BH mergers and to future populations of LISA-band extreme mass ratio inspirals (EMRIs). The ability of these BHs to pair up into binaries, inspiral, and produce GWs will be shaped by the existence of migration traps, regions in the AGN where hydrodynamic torques vanish. Previous works have studied the existence and location of migration traps in AGN disks. Here, we investigate how individual BHs may escape such traps as an outcome of mergers, potentially suppressing hierarchical growth. We find that while GW recoil kicks are strong enough to kick merged BHs onto inclined orbits, gas drag quickly realigns them into the AGN disk. A more robust escape mechanism is gap opening: once a BH grows above a critical mass, its gravity disturbs the AGN gas sufficiently to eliminate the trap. In low-mass AGN relevant for LISA, gaps open easily and the resulting ``wet EMRI'' masses are unlikely to reflect protracted hierarchical mergers. In combination with our previous work, we find that migration traps only exist in a relatively narrow range of AGN luminosities between [10^{43.5},10^{45.5}] erg/s. We identify an even narrower AGN luminosity range for which stellar mass BHs can grow into the pair instability mass gap and beyond. This characteristic luminosity scale may assist in indirect tests of the ``AGN channel'' for binary BH mergers.

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arXiv2024

Resonant Capture of Stars by Black Hole Binaries: Extreme Eccentricity Excitation

Massive black hole (MBH) binaries in galactic nuclei are one of the leading sources of $\sim$ mHz gravitational waves (GWs) for future missions such as $\rm{\textit{LISA}}$. However, the poor sky localization of GW interferometers will make it challenging to identify the host galaxy of MBH mergers absent an electromagnetic counterpart. One such counterpart is the tidal disruption of a star that has been captured into mean motion resonance with the inspiraling binary. Here we investigate the production of tidal disruption events (TDEs) through capture into, and subsequent evolution in, orbital resonance. We examine the full nonlinear evolution of planar autoresonance for stars that lock in to autoresonance with a shrinking MBH binary. Capture into the 2:1 resonance is guaranteed for any realistic astrophysical parameters (given a relatively small MBH binary mass ratio), and the captured star eventually attains an eccentricity $e\approx 1$, leading to a TDE. Stellar disks can be produced around MBHs following an active galactic nucleus episode, and we estimate the TDE rates from resonant capture produced when a secondary MBH begins inspiralling through such a disk. In some cases, the last resonant TDE can occur within a decade of the eventual $\rm{\textit{LISA}}$ signal, helping to localize the GW event.

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arXiv2024

Counting the Unseen I: Nuclear Density Scaling Relations for Nucleated Galaxies

The volumetric rate of tidal disruption events (TDEs) encodes information on the still-unknown demographics of central massive black holes (MBHs) in low-mass galaxies ($\lesssim 10^9$~M$_\odot$). Theoretical TDE rates from model galaxy samples can extract this information, but this requires accurately defining the nuclear stellar density structures. This region is typically dominated by nuclear star clusters (NSCs), which have been shown to increase TDE rates by orders of magnitude. Thus, we assemble the largest available sample of pc-scale 3-D density profiles that include NSC components. We deproject the PSF-deconvolved surface brightness profiles of 91 nearby galaxies of varying morphology and combine these with nuclear mass-to-light ratios estimated from measured colors or spectral synthesis to create 3-D mass density profiles. We fit the inner 3-D density profile to find the best-fit power-law density profile in each galaxy. We compile this information as a function of galaxy stellar mass to fit new empirical density scaling relations. These fits reveal positive correlations between galaxy stellar mass and central stellar density in both early- and late-type galaxies. We find that early-type galaxies have somewhat higher densities and shallower profiles relative to late-type galaxies at the same mass. We also use the density profiles to estimate the influence radius of each galaxy's MBH and find that the sphere of influence was likely resolved in most cases. These new relations will be used in future works to build mock galaxy samples for dynamical TDE rate calculations, with the aim of constraining MBH demographics in low-mass galaxies.

astro-ph.GA↗

arXiv2024

Repeating partial disruptions and two-body relaxation

Two-body relaxation may drive stars onto near-radial orbits around a massive black hole, resulting in a tidal disruption event (TDE). In some circumstances, stars are unlikely to undergo a single terminal disruption, but rather to have a sequence of many grazing encounters with the black hole. It has long been unclear what is the physical outcome of this sequence: each of these encounters can only liberate a small amount of stellar mass, but may significantly alter the orbit of the star. We study the phenomenon of repeating partial tidal disruptions (pTDEs) by building a semi-analytical model that accounts for mass loss and tidal excitation. In the empty loss cone regime, where two-body relaxation is weak, we estimate the number of consecutive partial disruptions that a star can undergo, on average, before being significantly affected by two-body encounters. We find that in this empty loss cone regime, a star will be destroyed in a sequence of weak pTDEs, possibly explaining the tension between the low observed TDE rate and its higher theoretical estimates.

astro-ph.HE↗

arXiv2024

Elevated Rates of Tidal Disruption Events in Active Galactic Nuclei

Advances in time domain astronomy have produced a growing population of flares from galactic nuclei, including both tidal disruption events (TDEs) and flares in active galactic nuclei (AGN). Because TDEs are uncommon and AGN variability is abundant, large-amplitude AGN flares are usually not categorized as TDEs. While TDEs are normally channelled by the collisional process of two-body scatterings over relaxation timescale, the quadrupole moment of a gas disk alters the stellar orbits, allowing them to collisionlessly approach the central massive black hole (MBH). This leads to an effectively enlarged loss cone, the \emph{loss wedge}. Earlier studies found a moderate enhancement, up to a factor $\sim 2-3$, of TDE rates $\dot{N}_{\rm 2b} $ for a static axisymmetric perturbation. Here we study the loss wedge problem for an evolving AGN disk, which can capture large number of stars into the growing loss wedge over much shorter times. The rates $\dot{N}_{\rm cl}$ of collisionless TDEs produced by these time-evolving disks are much higher than the collisional rates $\dot{N}_{\rm 2b}$ in a static loss wedge. We calculate the response of a stellar population to the axisymmetric potential of an adiabatically growing AGN disk and find that the highest rates of collisionless TDEs are achieved for the largest (i) MBH masses $M_{\bullet}$ and (ii) disk masses $M_{\rm d}$. For $M_{\bullet}\sim 10^7 M_\odot$ and $M_{\rm d} \sim 0.1 M_{\bullet}$, the rate enhancement can be up to a factor $\dot{N}_{\rm cl}/\dot{N}_{\rm 2b} \sim 10$. The orbits of collisionless TDEs sometimes have a preferred orientation in apses, carrying implications for observational signatures of resulting flares.

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