arXiv subjects

Bence Kocsis

Publications and source records attributed to Bence Kocsis.

At least 19 recordsLinked to original sources

arXiv2026

Stellar discs and intermediate-mass black holes in galactic nuclei I. Fragmenting the disc in an isotropic stellar potential

The origin of the complex orbital structure of young massive stars at the Galactic centre remains an open question. If these stars formed in a single episode from a gaseous accretion disc, they may initially have constituted a single, coherently rotating stellar disc. We investigate whether perturbations from an unseen intermediate-mass black hole (IMBH) could fragment and/or disrupt such a disc into the multiple orbital components observed today. First, we derive a theoretical criterion for when and where the IMBH's torque overcomes the disc's self-torque and tears it apart. We then test this picture with direct $N$-body simulations of a stellar disc interacting with an inclined IMBH around a central supermassive black hole. We find that the outcome depends strongly on the IMBH's orbit and mass. A prograde IMBH rapidly aligns with the stellar disc, while a massive retrograde IMBH ($m_{\bullet} \simeq 0.67\,M_{\rm d}$) anti-aligns relative to the radially overlapping stars and efficiently fragments the original disc into three components in angular-momentum space: an inner disc, a misaligned overlapping region, and an unperturbed outer disc. The IMBH also excites eccentricities in the overlapping region, driving stars away from initially circular orbits. These features emerge for an IMBH mass of $2000\,{\rm M}_{\odot}$ and a disc mass of $3000\,{\rm M}_{\odot}$ within 10--20 Myr, a timescale comparable to the age of the young Galactic centre stellar population, and provide a plausible explanation for the observed multiple orbital planes, warped geometry, and broad eccentricity distribution.

astro-ph.GA↗

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/.

astro-ph.HE↗

arXiv2026

Properties of black hole mergers in disks of active galactic nuclei

Ground-based gravitational wave (GW) observatories have detected approximately 200 binary black hole (BH) mergers. The astrophysical origin of these events are debated, with evidence suggesting that at least a subset originated from dynamic environments characterized by frequent close encounters. Accretion disks in active galactic nuclei (AGNs) are of particular interest, as certain observed features could be more readily produced within such environments. In this paper, we investigate the expected properties of mergers in these environments, and their dependence on various parameters, using one-dimensional $N$-body simulations combined with a comprehensive semi-analytical model. In our fiducial model, the distributions of masses and mass ratios ($q$) are similar to those observed. However, they depend strongly on the lifetime and density of the AGN disk and on the number and accretion efficiency of BHs, with higher masses predicted as these quantities increase. The most massive mergers, such as GW231123, can be produced either by efficient gas accretion or by hierarchical mergers among $\geq 3$ generations of BHs. The observed negative correlation between $q$ and the average effective spin ($\chi_{\rm eff}$), along with the positive correlation between $\chi_{\rm eff}$ and the chirp mass ($M_{\rm chirp}$), can be explained by a combination of efficient gas accretion, which promotes spin alignment, and hierarchical mergers, which produce high-$|\chi_{\rm eff}|$ and low-$q$ binaries. Hierarchical mergers can also explain the negative correlation between $q$ and the dispersion of $\chi_{\rm eff}$, as well as the positive correlation between $|\chi_{\rm eff}|$ and $M_{\rm chirp}$. We present a comprehensive study on how the expected distribution of each of these quantities depends on model parameters and assumptions, which will aid the interpretation of observed GW population properties.

astro-ph.HE↗

arXiv2026

The Depletion of Collisionless Dark Matter Spikes

Dense concentrations of dark matter (DM) surrounding black holes provide a compelling opportunity to probe the nature of DM. In the classic Gondolo-Silk model, the adiabatic growth of a massive black hole (MBH) in a DM cusp produces a steep density spike ($\rho \propto r^{-7/3}$), potentially inducing measurable gravitational-wave dephasings in intermediate and extreme mass-ratio inspirals (IMRIs/EMRIs). We challenge this paradigm by considering a collisionless spike embedded in a realistic nuclear star cluster (NSC). Using 1D orbit-averaged Fokker-Planck (FP) simulations of isotropic NSCs, we show that mass segregation in a multi-mass stellar cusp accelerates relaxation, relative to single-mass models, thereby driving the DM to the lower density $r^{-3/2}$ Bahcall-Wolf profile within $\lesssim 1 \mathrm{Gyr}$. In the inner regions, where the FP description breaks down, we model strong triple interactions between DM particles and EMRIs using post-Newtonian 3-body simulations. We show that EMRIs eject DM particles via slingshots, depleting the inner spike over a few Gyrs. Because EMRI number densities are too low to drive two-body relaxation, and collisionless DM cannot efficiently repopulate the depleted phase space, this depletion is irreversible. While the extent of EMRI-induced depletion depends on the EMRI rate and mass, we find reductions in DM densities by several orders of magnitude. Hence, DM-induced dephasings for EMRIs may fall below the detectability threshold of LISA for MBHs at $z = 3$ (2.14 Gyr) with masses $\lesssim 10^{5}\,M_\odot$ (for an $\mathcal{O}(10) \, \mathrm{Gyr}^{-1}$ EMRI rate), extending to $\lesssim 10^6\,M_\odot$ for more optimistic rates of $\mathcal{O}(300-1000) \, \mathrm{Gyr}^{-1}$. Our findings substantially reduce the parameter space over which MBHs can host detectable collisionless DM spikes.

gr-qc↗

arXiv2025

Angular-momentum pairs in spherical systems: applications to the Galactic centre

Consider a system of point masses in a spherical potential. In such systems objects execute planar orbits covering two-dimensional rings or annuli, represented by the angular-momentum vectors, which slowly reorient due to the persistent weak gravitational interaction between different rings. This process, called vector resonant relaxation, is much faster than other processes which change the size/shape of the rings. The interaction is stron9gest between objects with closely aligned angular-momentum vectors. In this paper, we show that nearly parallel angular-momentum vectors may form stable bound pairs in angular-momentum space. We examine the stability of such pairs against an external massive perturber, and determine the critical separation analogous to the Hill radius or tidal radius in the three-body problem, where the angular-momentum pairs are marginally disrupted, as a function of the perturber's mass, the orbital inclination, and the radial distance. Angular-momentum pairs or multiples closer than the critical inclination will remain bound and evolve together in angular-momentum-direction space under any external influence, such as anisotropic density fluctuations, or massive perturbers. This study has applications in various astrophysical contexts, including galactic nuclei, in particular the Milky Way's Galactic centre, globular clusters, or planetary systems. In nuclear star clusters with a central super-massive black hole, we apply this criterion to the disc of young, massive stars, and show that clusters in angular-momentum space may be used to constrain the presence of intermediate-mass black holes or the mass of the nearby gaseous torus.

astro-ph.GA↗

arXiv2025

Black holes as telescopes: Discovering supermassive binaries through quasi-periodic lensed starlight

Supermassive black hole (SMBH) binary systems are unavoidable outcomes of galaxy mergers. Their dynamics encode information about their formation and growth, the composition of their host galactic nuclei, the evolution of galaxies, and the nature of gravity. Many SMBH binaries with separations pc-kpc have been found, but closer (sub-parsec) binaries remain to be confirmed. Identifying these systems may elucidate how binaries evolve past the ``final parsec'' until gravitational radiation drives them to coalescence. Here we show that SMBH binaries in non-active galactic nuclei can be identified and characterized by the gravitational lensing of individual bright stars, located behind them in the host galaxy. The rotation of `caustics' -- regions where sources are hugely magnified due to the SMBH binary's orbit and inspiral -- leads to Quasi-Periodic Lensing of Starlight (QPLS). The extreme lensing magnification of individual bright stars produces a significant variation in the host galaxies' luminosity; their lightcurve traces the orbit of the SMBH binary and its evolution. QPLS probes the population of sources observable by pulsar timing arrays and space detectors (LISA, TianQin), offering advance warning triggers for merging SMBHs for coincident or follow-up GW detections. SMBH population models predict $1-50\; [190-5,000] \left({n_\star}/{\rm pc}^{-3}\right)$ QPLS binaries with period less than $10\; [40]$ yr with comparable masses and $z<0.3$, where $n_\star$ is the stellar number density. Additionally, stellar and orbital motion will lead to frequent instances of single/double flares caused by SMBHBs with longer periods. This novel signature can be searched for in a wealth of existing and upcoming time-domain photometric data: identifying quasi-periodic variability in galactic lightcurves will reveal an ensemble of binary systems and illuminate outstanding questions around them.

astro-ph.GA↗

arXiv2025

Hydrodynamic simulations of black hole evolution in AGN discs I: orbital alignment of highly inclined satellites

The frequency of compact object interactions in AGN discs is naturally tied to the number of objects embedded within it. We investigate the evolution of black holes in the nuclear stellar cluster on inclined orbits to the AGN disc by performing adiabatic hydrodynamical simulations of isolated black hole disc crossings over a range of disc densities and inclinations $i\in[2^\circ,15^\circ]$. We find radiation dominates the pressure in the wake that forms around the BH across the full inclination and disc density range. We identify no well defined steady state wake morphology due to the thin geometry of the disc and the vertical exponential density drop off, where the wake morphology depends on the vertical depth of the transit within the disc. The inclination damping $\Delta i$ relative the pre-transit inclination behaves as a power law in $\sin(i)$ and the ambient Hill mass $m_\text{H,0}$ as $\Delta i/i \propto m_{\rm H,0}^{0.4} \sin(i)^{-2.7}$. The drag on the BH is dominated by the gravity of the wake for the majority of our inclination range until accretion effects become comparable at $\sin(i)\gtrsim30H_0/R_0$, where $H_0/R_0$ is the disc aspect ratio. At low inclinations ($\sin(i)\lesssim3H_0/R_0$) the wake morphology becomes more spherical, leading to a regime change in the inclination damping behaviour. Our results suggest that the inclination damping timescale is shorter than expected from only episodic Bondi-Hoyle-Lyttelton accretion events during each transit, implying inclined objects may captured by the AGN disc earlier in its lifetime than previously thought.

astro-ph.HE↗

arXiv2025

Hydrodynamic simulations of black hole evolution in AGN discs II: inclination damping for partially embedded satellites

We investigate the evolution of black holes on orbits with small inclinations ($i < 2^\circ$) to the gaseous discs of active galactic nuclei. We perform 3D adiabatic hydrodynamic simulations within a shearing frame, studying the damping of inclination by black hole-gas gravitation. We find that for objects with $i<3H_0R_0^{-1}$, where $H_0R_0^{-1}$ is the disc aspect ratio, the inclination lost per midplane crossing is proportional to the inclination preceding the crossing, resulting in a net exponential decay in inclination. For objects with $i>3H_0R_0^{-1}$, damping efficiency decreases for higher inclinations. We consider a variety of different AGN environments, finding that damping is stronger for systems with a higher ambient Hill mass: the initial gas mass within the BH sphere-of-influence. We provide a fitting formula for the inclination changes as a function of Hill mass. We find reasonable agreement between the damping driven by gas gravity in the simulations and the damping driven by accretion under a Hill-limited Bondi-Hoyle-Lyttleton prescription. We find that gas dynamical friction consistently overestimates the strength of damping, especially for lower inclination systems, by at least an order of magnitude. For regions in the AGN disc where coplanar binary black hole formation by gas dissipation is efficient, we find that the simulated damping timescales are especially short with $\tau_d < 10P_\mathrm{SMBH}$. We conclude that as the timescales for inclination damping are shorter than the expected interaction time between isolated black holes, the vast majority of binaries formed from gas capture should form from components with negligible inclination to the AGN disc.

astro-ph.HE↗

arXiv2025

3D Adiabatic Simulations of Binary Black Hole Formation in AGN

We investigate close encounters between initially unbound black holes (BHs) in the gaseous discs of active galactic nuclei (AGN), performing the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary BH formation. We discuss a suite of 135 simulations, considering 9 AGN disc environments and 15 BH impact parameters. We find that the gas distribution within the Hill sphere about an isolated embedded BH is akin to a spherically symmetric star with a low-mass convective envelope and a BH core, with large convective currents driving strong outflows away from the midplane. We find that Coriolis force acting on the outflow results in winds, analogous to cyclones, that counter-rotate with respect to the midplane flow within the Hill sphere. We confirm the existence of strong thermal blasts due to minidisc collisions during BH close encounters, as predicted in our previous 2D studies. We document binary formation across a wide range of environments, finding formation likelihood is increased when the gas mass in the Hill sphere is large, allowing for easier binary formation in the outer AGN disc. We provide a comprehensive overview of the SMBH's role in binary formation, investigating how binary formation in intermediate density environments is biased towards certain binary orientations. We offer two models for predicting dissipation by gas during close encounters, as a function of the ambient Hill mass alone, or with the periapsis depth. We use these models to motivate a prescription for binary formation likelihood that can be readily applied to Monte-Carlo simulations of AGN evolution.

astro-ph.HE↗

arXiv2025

Constraining Axion Dark Matter with Galactic-Centre Resonant Dynamics

We study the influence of fuzzy-dark-matter cores on the orbits of stars at the Galactic centre. This dark matter candidate condenses into dense, solitonic cores, and, if a super-massive black hole is present at the centre of such a core, its central part forms a `gravitational atom'. Here, we calculate the atom's contribution to the gravitational potential felt by a Galactic-centre star, for a general state of the atom. We study the angular-momentum dynamics this potential induces, and show that it is similar to vector resonant relaxation. Its influence is found to be potentially sufficiently strong that such a dynamical component should be accounted for in Galactic-centre modelling. For the Milky Way, the atom is expected to have some spherical asymmetry, and we use this to derive a stability condition for the disc of young, massive stars at the Galactic centre - if the atom's mass is too large, then the disc would be destroyed. Thus, the existence of this disc constrains the mass of the particles comprising the solitonic core. We study an example model of the core, where all of the rotation of the core's inner region is assumed to come from an $l=1$ state, and its amplitude is determined by the halo's spin parameter; such a core is found to be in tension with the stability of the clockwise stellar disc for $4.2\times 10^{-20}\,\textrm{eV} \leq m_a \leq 5.4\times 10^{-20}\,\textrm{eV}$ at $2\sigma$. Other core models would vary the constrained values of $m_a$ somewhat. These constraints will tighten significantly with future, improved data.

astro-ph.CO↗

arXiv2025

Resonant Locking Between Binary Systems Induced by Gravitational Waves

The interaction of gravitational waves (GWs) with matter is thought to be typically negligible in the Universe. We identify an exception in the case of resonant interactions, where GWs emitted by a background binary system, such as an inspiraling supermassive black hole (SMBH) binary, cause a resonant response in a stellar-mass foreground binary and the frequencies of the two systems become, and remain, synchronized. We point out that this previously unexplored dynamical phenomenon is not only possible, but can lead to $\mathcal{O}(30)$ binary systems becoming resonantly locked in the host galaxy of merging SMBHs of mass $10^{8.5-11}\Msun$, each of which has a significantly reduced merger time. We predict $\mathcal{O}(10^{10})$ binary systems have been locked in the Universe's history. Resonant locking could be detected through anomalous inspiral of binary systems.

gr-qc↗

arXiv2025

Prompt gravitational-wave mergers aided by gas in Active Galactic Nuclei: The hydrodynamics of binary-single black hole scatterings

Black hole binary systems embedded in AGN discs have been proposed as a source of the observed gravitational waves (GWs) from LIGO-Virgo-KAGRA. Studies have indicated binary-single encounters could be common place within this population, yet we lack a comprehensive understanding of how the ambient gas affects the dynamics of these three-body encounters. We present the first hydrodynamical simulations of black hole binary-single encounters in an AGN disc. We find gas is a non-negligible component of binary-single interactions, leading to unique dynamics, including the formation of quasi-stable hierarchical triples. The gas efficiently and reliably dissipates the energy of the three-body system, hardening the triple provided it remains bound after the initial encounter. The hardening timescale is shorter for higher ambient gas densities. Formed triple systems can be hardened reliably by $2-3$ orders of magnitude relative to the initial binary semi-major axis within less than a few AGN orbits, limited only by our resolution. We calculate that the gas hardening of the triple enhances the probability for a merger by a minimum factor of $3.5-8$ depending on our assumptions. In several cases, two of the black holes can execute periapses on the order of less than $10$ Schwarzschild radii, where the dynamics were fully resolved for previous close approaches. The likelihood of these prompt mergers increases when the gas density is larger. Our results suggest that current timescale estimates (without gas drag) for binary-single induced mergers are an upper bound. The shrinkage of the triple by gas has the prospect of increasing the chance for unique GW phenomena such as residual eccentricity, dephasing from a third object and double GW mergers.

astro-ph.GA↗

arXiv2024

Black Hole Merger Rates in AGN: contribution from gas-captured binaries

It has been suggested that merging black hole (BH) binaries in active galactic nucleus (AGN) discs formed through two-body scatterings via the gas-capture process may explain a significant fraction of BH mergers in AGN and a non-negligible contribution to the observed rate from LIGO-VIRGO-KAGRA. We perform Monte Carlo simulations of BH and binary BH formation, evolution and mergers across the observed AGN mass function using a novel physically motivated treatment for the gas-capture process derived from hydrodynamical simulations of BH-BH encounters in AGN and varying assumptions on the AGN disc physics. The results suggest that gas-captured binaries could result in merger rates of 0.73 - 7.1Gpc$^{-3}$yr$^{-1}$. Most mergers take place near the outer boundary of the accretion disk, but this may be subject to change when migration is considered. The BH merger rate in the AGN channel in the Universe is dominated by AGN with supermassive BH masses on the order of 10$^{7} M_\odot$ , with 90% of mergers occurring in the range 10$^{6} M_\odot$ - 10$^{8} M_\odot$ . The merging mass distribution is flatter than the initial BH mass power law by a factor $\Delta \xi$ = 1.1 to 1.2, as larger BHs can align with the disc and successfully form binaries more efficiently. Similarly, the merging mass ratio distribution is flatter, therefore the AGN channel could easily explain the high mass and unequal mass ratio detections such as GW190521 and GW190814. When modelling the BH binary formation process using a simpler dynamical friction treatment, we observe very similar results, where the primary bottleneck is the alignment time with the disk. We find the most influential parameters on the rates are the anticipated number of BHs and their mass function. We conclude that AGN remain an important channel for consideration, particularly for gravitational wave detections involving one or two high mass BHs.

astro-ph.HE↗

arXiv2024

Evolution of the disky second generation of stars in globular clusters on cosmological timescale

Context. Many Milky Way globular clusters (GCs) host multiple stellar populations, challenging the traditional view of GCs as single-population systems. It has been suggested that second-generation stars could form in a disk from gas lost by first-generation stars or from external accreted gas. Aims. We investigate how the introduction of a second stellar generation affects mass loss, internal mixing, and rotational properties of GCs in a time-varying Galactic tidal field and different orbital configurations. Methods. We conducted direct N-body simulations of GCs on three types of orbits derived from the observed Milky Way GCs. We evolved the clusters for 8 Gyr in the time-varying Galactic potential of the IllustrisTNG-100 cosmological simulation. After 2 Gyr, we introduced a second stellar generation, comprising 5% of the initial mass of the first generation, as a flattened disk of stars. For comparison, we ran control simulations using a static Galactic potential and isolated clusters. Results. We present the mass loss, structural evolution, and kinematic properties of GCs with two stellar generations, focusing on tidal mass, half-mass radii, velocity distributions, and angular momentum. Conclusions. Our results show that the mass loss of GCs depends primarily on their orbital parameters, with tighter orbits leading to higher mass loss. The Galaxy's growth resulted in tighter orbits, meaning GCs lost less mass than if its mass had always been constant. The initially flattened second-generation disk became nearly spherical within one relaxation time. However, whether its distinct rotational signature was retained depends on the orbit: for the long radial orbit, it vanished quickly; for the tube orbit, it lasted several Gyr; but for the circular orbit, rotation persisted until the present day

astro-ph.GA↗

arXiv2024

Extracting Astrophysical Information of Highly-Eccentric Binaries in the Millihertz Gravitational Wave Band

Wide, highly eccentric ($e>0.9$) compact binaries can naturally arise as progenitors of gravitational wave (GW) mergers. These systems are expected to have a significant population in the mHz band (e.g., $\sim 3-45$ detectable stellar-mass binary black holes with $e>0.9$ in the Milky Way), with their GW signals characterized by "repeated bursts" emitted upon each pericenter passage. In this study, we show that the detection of mHz GW signals from highly eccentric stellar mass binaries in the local universe can strongly constrain their orbital parameters. Specifically, it can achieve a relative measurement error of $\sim 10^{-6}$ for orbital frequency and $\sim 1\%$ for eccentricity (as $1-e$) in most of the detectable cases. On the other hand, the binary's mass ratio, distance, and intrinsic orbital orientation may be less precisely determined due to degeneracies in the GW waveform. We also perform mock LISA data analysis to evaluate the realistic detectability of highly eccentric compact binaries. Our results show that highly eccentric systems could be efficiently identified when multiple GW sources and stationary Gaussian instrumental noise are present in the detector output. This work highlights the potential of extracting the signal of "bursting'' LISA sources to provide valuable insights into their orbital evolution, surrounding environment, and formation channels.

astro-ph.HE↗

arXiv2024

Gas Assisted Binary Black Hole Formation in AGN Discs

We investigate close encounters by stellar mass black holes (BHs) in the gaseous discs of active galactic nuclei (AGN) as a potential formation channel of binary black holes (BBHs). We perform a series of 2D isothermal viscous hydrodynamical simulations within a shearing box prescription using the Eulerian grid code Athena++. We co-evolve the embedded BHs with the gas keeping track of the energetic dissipation and torquing of the BBH by gas gravitation and inertial forces. To probe the dependence of capture on the initial conditions, we discuss a suite of 345 simulations spanning local AGN disc density ($ρ_0$) and impact parameter ($b$) space. We identify a clear region in $b - ρ_0$ space where gas assisted BBH capture is efficient. We find that the presence of gas leads to strong energetic dissipation during close encounters between unbound BHs, forming stably bound eccentric BBHs. We find that the gas dissipation during close encounters increases for systems with increased disc density and deeper periapsis passages $r_p$, fitting a power law such that $ΔE \propto ρ_0^αr_p^β$ where $\{α,β\} = \{1.01\pm0.04,-0.43\pm0.03\}$. Alternatively, the gas dissipation is approximately $ΔE = 4.3 M_\text{d} v_\text{H} v_p$, where $M_\text{d} $ is the mass of a single BH minidisc just prior to the encounter when the binary separation is $2r_\text{H}$ (two binary Hill radii), $v_\text{H}$ and $v_p$ are the relative BH velocities at $2r_\text{H}$ and at the first closest approach, respectively. We derive a prescription for capture which can be used in semi-analytical models of AGN. We do not find the dissipative dynamics observed in these systems to be in agreement with the simple gas dynamical friction models often used in the literature.

astro-ph.GA↗

arXiv2024

Disc Novae: Thermodynamics of Gas Assisted Binary Black Hole Formation in AGN Discs

We investigate the thermodynamics of close encounters between stellar mass black holes (BHs) in the gaseous discs of active galactic nuclei (AGN), during which binary black holes (BBHs) may form. We consider a suite of 2D viscous hydrodynamical simulations within a shearing box prescription using the Eulerian grid code Athena++. We study formation scenarios where the fluid is either an isothermal gas or an adiabatic mixture of gas and radiation in local thermal equilibrium. We include the effects of viscous and shock heating, as well as optically thick cooling. We co-evolve the embedded BHs with the gas, keeping track of the energetic dissipation and torquing of the BBH by gas and inertial forces. We find that compared to the isothermal case, the minidiscs formed around each BH are significantly hotter and more diffuse, though BBH formation is still efficient. We observe massive blast waves arising from collisions between the radiative minidiscs during both the initial close encounter, and subsequent periapsis periods for successfully bound BBHs. These "disc novae" have a profound effect, depleting the BBH Hill sphere of gas and injecting energy into the surrounding medium. In analysing the thermal emission from these events, we observe periodic peaks in local luminosity associated with close encounters/periapses, with emission peaking in the optical/near-IR. In the AGN outskirts, these outbursts can reach 4% of the AGN luminosity in the IR band, with flares rising over 0.5-1year. Collisions in different disc regions, or when treated in 3D with magnetism, may produce more prominent flares.

astro-ph.HE↗

arXiv2024

Observability of dynamical tides in merging eccentric neutron star binaries

While dynamical tides only become relevant during the last couple of orbits for circular inspirals, orbital eccentricity can increase their impact during earlier phases of the inspiral by exciting tidal oscillations at each close encounter. We investigate the effect of dynamical tides on the orbital evolution of eccentric neutron star binaries using post-Newtonian numerical simulations and construct an analytic stochastic model that reproduces the numerical results. Our study reveals a strong dependence of dynamical tides on the pericenter distance, with the fractional energy transferred to dynamical tides over that dissipated in gravitational waves (GWs) exceeding $\sim1\%$ at separations $r_\mathrm{p}\lesssim50$ km for large eccentricities. We demonstrate that the effect of dynamical tides on orbital evolution can manifest as a phase shift in the GW signal. We show that the signal-to-noise ratio of the GW phase shift can reach the detectability threshold of $8$ with a single aLIGO detector at design densitivity for eccentric neutron star binaries at a distance of $40$ Mpc. This requires a pericenter distance of $r_\mathrm{p0}\lesssim68$ km ($r_\mathrm{p0}\lesssim76$ km) at binary formation with eccentricity close to $1$ for a reasonable tidal deformability and f-mode frequency of $500$ and $1.73$ kHz ($700$ and $1.61$ kHz), respectively. The observation of the phase shift will enable measuring the f-mode frequency of neutron stars independently from their tidal deformability, providing significant insights into neutron star seismology and the properties of the equation of state. We also explore the potential of distinguishing between equal-radius and twin-star binaries, which could provide an opportunity to reveal strong first-order phase transitions in the nuclear equation of state.

astro-ph.HE↗