arXiv subjects

Barak Rom

Publications and source records attributed to Barak Rom.

9 recordsLinked to original sources

arXiv2026

Dynamics in Nuclear Stellar Clusters: The Impact of Collisions and Disrupted Binaries

The nuclear stellar clusters surrounding supermassive black holes (SMBHs) host millions of stars and stellar remnants. We study how stellar collisions and binary disruptions, alongside two-body scattering and gravitational-wave (GW) emission, shape the stellar distribution and regulate the abundance of stars on tightly bound orbits. We show the following. (a) Stars in the inner region of the cluster follow a steady-state density profile scaling as $n(r)\propto r^{-5/4}$, set by the balance between collisional depletion and binary replenishment. This profile is largely independent of whether two-body scattering or GW emission drives the orbital evolution prior to the collisions. (b) For SMBHs with $M\lesssim2 \times 10^7 M_\odot$, roughly half of the stars injected by the Hills mechanism eventually collide. The rest are tidally disrupted while on orbits with periods of order months to years. (c) For more massive SMBHs, these short-orbital-period tidal disruption events are suppressed, and most injected stars are ultimately destroyed by collisions. (d) Stellar extreme-mass-ratio inspirals (sEMRIs) can form around SMBHs with $M\gtrsim2\times10^6 M_\odot$, but are typically terminated by collisions before circularizing. Our model highlights the dynamical challenge stellar collisions pose for the formation of sEMRIs and, consequently, for stellar models of quasiperiodic eruptions. Applied to the Galactic Center, the collision-regulated density profile is consistent with the observed stellar distribution slope. Based on this profile, we estimate the stellar mass within the orbit of S2, finding it consistent with the observational upper limit, account for the recently discovered star S301, predict that most stars near Sgr$~{\rm A}^*$ follow eccentric orbits, and determine their typical eccentricities.

astro-ph.GA↗

arXiv2026

Red Giant Destruction by Stellar and Black Hole Collisions in Galactic Nuclei

We study the impact of collisions involving red giants (RGs) in the dense stellar environments of galactic nuclei. We analytically estimate when collisions with main-sequence stars or stellar-mass black holes can strip a RG's envelope via ram pressure or accretion-driven shocks, or eject its helium core through gravitational recoil. At high velocities, $v\gtrsim10^3~{\rm km/s}$, collisions with main-sequence stars efficiently deplete the RG population. At lower velocities, collisions with stellar-mass BHs typically dominate over stellar encounters, but the overall RG destruction rate is low and does not significantly affect the RG population. Nonetheless, these collisions produce low-mass helium white dwarfs, which are the stripped cores of the disrupted RGs, at a rate of $\sim 500~~{\rm Gyr}^{-1}$. Helium white dwarfs can produce an interesting class of white dwarf tidal disruption events around $\sim 10^{5-6} M_\odot$ massive black holes where Carbon-Oxygen white dwarfs cannot be tidally disrupted outside the horizon. Applied to our own Galactic Center, we quantify the impact of collisions on the observed population of RGs, as well as the effects of their intrinsic scarcity due to short RG lifetimes. We find that the RG projected density flattens within $\sim1$'', primarily due to collisions for fainter RGs and their short lifetimes for more luminous RGs.

astro-ph.HE↗

arXiv2025

Universal Waveforms for Extreme Mass-Ratio Inspiral

We engage with the challenge of calculating the waveforms of gravitational waves emitted by spinless binary black hole merger in extreme mass-ratio limit. We model the stellar-mass black hole as a test-particle, initially on a circular orbit, that undergoes adiabatic inspiral until it reaches the innermost stable circular orbit (ISCO), after which it follows a geodesic trajectory. We compute the gravitational waveforms emitted during both phases -- before and after the ISCO crossing -- and demonstrate how to accurately connect them. While the waveforms are calculated adiabatically up to the ISCO, the associated phase error near the ISCO scales as $\nu^{1/5}$ and remains below one radian for sufficiently small mass-ratios $\nu$. Our complete waveform is universal in the sense that all computationally expensive calculations are performed once, and its application to any binary merger can be obtained by appropriately re-scaling time, phase, and amplitude. We compare our results with existing models in the literature and show that our complete waveforms are accurate enough all the way from separations that are an order of one gravitational radii outside the ISCO, to the merger.

gr-qc↗

arXiv2025

Mass Segregation and Transient Formation in Nuclear Stellar Clusters

Supermassive black holes at the centers of galaxies occasionally disrupt stars or consume stellar-mass black holes (BHs) that wander too close, producing observable electromagnetic or gravitational wave signals. We examine how mass segregation impacts the rates and distributions of such events. Assuming a relaxed stellar cluster, composed of stars and stellar-mass BHs, we show that the tidal disruption rate of massive stars ($m\gtrsim M_\odot$) is enhanced relative to their abundance in the stellar population. For stars up to $m\approx3M_\odot$, this enhancement is roughly $m/M_\odot$ and it is driven by segregation within the sphere of influence. Stars with masses $m\gtrsim3M_\odot$, if relaxed, are predominantly scattered by more massive stellar-mass BHs, leading to a constant enhancement factor of $\approx 9$, independent of mass. This aligns with observational evidence suggesting an over-representation of massive stars in tidal disruption events. For stellar-mass BHs, we predict an enhancement factor scaling as $m_\bullet^{1/2}$ for plunges and $m_\bullet^{3/2}$ for extreme-mass-ratio inspirals (EMRIs). The power of one-half in both cases reflects the shorter relaxation times of heavier BHs, allowing them to segregate into the sphere of influence from greater distances, thereby increasing their abundance. The additional power in the EMRIs' rate arises from the tendency of heavier BHs to circularize and sink inward more efficiently. Finally, we estimate the rate of main-sequence star inspirals and find that it favors low-mass stars ($m\lesssim M_\odot$). This seems compatible with the observationally estimated rate of quasiperiodic eruptions.

astro-ph.HE↗

arXiv2024

Dynamics around supermassive black holes: Extreme mass-ratio inspirals as gravitational-wave sources

Supermassive black holes and their surrounding dense stellar environments nourish a variety of astrophysical phenomena. We focus on the distribution of stellar-mass black holes around the supermassive black hole and the consequent formation of extreme-mass-ratio inspirals (EMRIs). We derive a steady-state distribution, considering the effects of two-body scattering and gravitational-wave emission, and calculate the EMRI formation rate, eccentricity distribution, and EMRI-to-plunge ratio. Our model predicts: (a) a stronger segregation than previously estimated at the outskirts of the sphere of influence (at $\sim0.01-2\rm pc$ for a Milky Way-like galaxy); (b) an increased EMRI-to-plunge ratio, favoring EMRIs at galaxies where stellar-mass black holes are scarce; (c) a detection of about $2\times10^3$ resolvable EMRIs, with a signal-to-noise ratio above $20$, along a $4\ \rm yr$ LISA mission time; and (d) a confusion noise, induced by a cosmological population of unresolved EMRIs, reducing the LISA sensitivity in the $1-5\ \rm mHz$ frequency range by up to a factor of $\approx2$, relative to the instrumental noise.

astro-ph.GA↗

arXiv2024

Semi-Analytical Fokker Planck Models for Nuclear Star Clusters

We study the dynamics of nuclear star clusters, the dense stellar environments surrounding massive black holes in the centers of galaxies. We consider angular momentum diffusion due to two-body scatterings among stellar objects and energy advection due to gravitational wave emission upon interaction with the central massive black hole. Such dynamics is described by a two-dimensional Fokker-Planck equation in energy-angular momentum space. Focusing on the transition between the diffusion-dominated region and the advection-dominated one, we utilize self-similarity to obtain a full solution for the Fokker-Planck equation. This solution provides the density and flux of the stellar objects in nuclear star clusters. This improves the rate estimates for extreme mass-ratio inspirals, and has interesting implications for a new class of galactic center transients called quasi-periodic eruptions.

astro-ph.HE↗

arXiv2023

Formation of Merging Stellar-Mass Black Hole Binaries by Gravitational Wave Emission in Active Galactic Nucleus Disks

Many stellar-mass Black Holes (sBHs) are expected to orbit supermassive black holes at galactic centers. For galaxies with Active Galactic Nuclei (AGN), it is likely that the sBHs reside in a disk. We study the formation of sBH binaries via gravitational wave emission in such disks. We examine analytically the dynamics of two sBHs orbiting a supermassive black hole, estimate the capture cross section, and derive the eccentricity distribution of bound binaries at different frequency bands. We find that the majority of the merging sBH binaries, assembled in this manner, can be measured as highly eccentric, detectable in the LIGO-Virgo-KAGRA (LVK) band from their formation, with $(1-e)\ll1$, through their circularization and up to their merger; the remaining binaries circularize to small eccentricities ($e\lesssim0.3$) before entering the LVK band. More eccentric mergers would be observed for sBHs with higher random velocities, closer to the supermassive black hole, or at lower observing frequency bands, as planned in future gravitational wave detectors such as the Einstein Telescope and LISA.

astro-ph.HE↗

arXiv2023

Energy Flux and Particle Flux in Steady-State Solutions of Nuclear Star Clusters

We examine the effects of two-body interactions in a nuclear star cluster surrounding a supermassive black hole. We evaluate the energy flux, analogously to the particle flux calculation of Bahcall and Wolf (1976). We show that there are two types of power-law steady-state solutions: one with zero energy flux and constant particle flux and the other with constant energy flux and zero particle flux. We therefore prove that a zero particle flux solution, which corresponds to the case of an accreting supermassive black hole, can be obtained by requiring a constant energy flux. Consequently, this solution can be derived by simple dimensional analysis, bypassing the need for detailed calculation. Finally, we show that this characteristic, of zero particle flux for constant energy flux and vice versa, is not unique to the Keplerian potential of a supermassive black hole but holds for any central potential of the form $\phi\propto r^{-\beta}$.

astro-ph.GA↗

arXiv2022

Extreme Mass-Ratio Binary Black Hole Merger: Characteristics of the Test-Particle Limit

We study binary black hole mergers in the extreme mass-ratio limit. We determine the energy, angular momentum, and linear momentum of the post-merger, remnant black hole. Unlike previous works, we perform our analysis directly in the test-particle limit by solving the Regge-Wheeler-Zerilli wave equation with a source that moves along a geodesic. We rely on the fact that toward the merger, small mass-ratio binary systems follow a quasiuniversal geodesic trajectory. This formalism captures the final premerger stages of small mass-ratio binaries and thus provides a straightforward universal description in a region inaccessible to numerical relativity simulations. We present a general waveform template that may be used in the search for gravitational wave bursts from small and intermediate mass-ratio binary systems. Finally, this formalism gives a formal proof that the recoil velocity is quadratic in the symmetric mass ratio $\nu$. Specifically, the velocity is given by $V/c\approx 0.0467 \nu^2$. This result is about $4\%$ larger than previously estimated. Most of this difference stems from the inclusion of higher multipoles in our calculation.

gr-qc↗