Hydrodynamical Simulations of Black Hole Binary Formation in AGN Disks

Li, Jiaru; Dempsey, Adam M.; Li, Hui; Lai, Dong; Li, Shengtai

doi:10.3847/2041-8213/acb934

Cited by 27 publications

(9 citation statements)

References 66 publications

(148 reference statements)

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“…Simulations have shown that during their encounters, disk-embedded binaries can become bound after their separation reach of order J. Li et al 2022aJ. Li et al , 2023, which means their capture impact parameter is effectively ∼R H .…”

Section: Binary Capture Timescalementioning

confidence: 99%

The Population of Massive Stars in Active Galactic Nuclei Disks

Chen,

Lin

2024

ApJ

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Gravitational instability in the outskirts of active galactic nuclei (AGN) disks leads to disk fragmentation and formation of ∼300 M ⊙ supermassive stars with potentially long lifetimes. Alternatively, stars can be captured ex situ and grow from gas accretion in the AGN disk. However, the number density distribution throughout the disk is limited by thermal feedback as their luminosities provide the dominant heating source. We derive equilibrium stellar surface density profiles under two limiting contexts: in the case where the stellar lifetimes are prolonged, due to the recycling of hydrogen-rich disk gas, only the fraction of gas converted into heat is removed from the disk accretion flow. Alternatively, if stellar composition recycling is inefficient and stars can evolve off the main sequence, the disk accretion rate is quenched toward smaller radii resembling a classical starburst disk, albeit the effective removal rate depends not only on the stellar lifetime, but also the mass of stellar remnants. For AGNs with central supermassive black hole masses of ∼106–108 M ⊙ accreting at ∼0.1 Eddington efficiency, we estimate a total number of 103–105 massive stars and the rate of stellar mergers to be 10−3 to 1 yr−1. We initiate the detailed study of the interaction between a swarm of massive stars through hydro and N-body simulations to provide better prescriptions of dynamical processes in AGN disks, and to constrain more accurate estimates of the stellar population.

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Section: Binary Capture Timescalementioning

confidence: 99%

The Population of Massive Stars in Active Galactic Nuclei Disks

Chen,

Lin

2024

ApJ

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“…The unusual disk environment causes stars to accrete and grow in mass much more rapidly than elsewhere in the interstellar medium, changing the stars' structure, allowing them to reach much higher masses, and making BH remnants a more common outcome upon their death (Cantiello et al 2021;Dittmann et al 2021;Jermyn et al 2021). Once trapped in the AGN disk, BHs can go through radial migration and undergo close encounters with stars or compact objects, which has been shown in both analytical studies (e.g., Tagawa et al 2020;DeLaurentiis et al 2023), and hydrodynamics simulations (Li et al 2023;Rowan et al 2023aRowan et al , 2023bWhitehead et al 2023) to result in the formation of bound binaries. These binaries can be both retrograde and prograde and also both circular and eccentric, depending on the impact parameter of the encounter.…”

Section: Introductionmentioning

confidence: 97%

“Tidal Peeling Events”: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole

Xin,

Haiman,

Perna

et al. 2024

ApJ

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Close encounters between stellar-mass black holes (BHs) and stars occur frequently in dense star clusters and in the disks of active galactic nuclei. Recent studies have shown that in highly eccentric close encounters, the star can be tidally disrupted by the BH in a microtidal disruption event (microTDE), resulting in rapid mass accretion and possibly bright electromagnetic signatures. Here we consider a scenario in which the star might approach the stellar-mass BH in a gradual, nearly circular inspiral, under the influence of dynamical friction in a circum-binary gas disk or three-body interactions in a star cluster. We perform hydrodynamics simulations of this scenario using the smoothed particle hydrodynamics code PHANTOM. We find that under certain circumstances (for initial eccentricity e 0 ≳ 0.4 and penetration factor β = 1, or e 0 < 0.4 and β ≲ 0.67), the mass of the star is slowly stripped away by the BH. We call this gradual tidal disruption a “tidal-peeling event.” Additionally, we discover that some low-eccentricity microTDEs (e 0 < 0.4 and β = 1) are a new form of fast luminous transients similar to parabolic microTDEs. Depending on the initial distance and eccentricity of the encounter, these low-eccentricity microTDEs might exhibit significant accretion rates and orbital evolution distinct from those of a typical (eccentric) microTDE.

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“…In an AGN disk, BHs are embedded due to capture via dynamical interactions between the nuclear star cluster and the AGN disk (Ostriker 1983;Miralda-Escudé & Kollmeier 2005;Wang et al 2024) and by in situ star formation (Levin & Beloborodov 2003;Goodman & Tan 2004;Milosavljević & Loeb 2004;Chen et al 2023b;Derdzinski & Mayer 2023). The AGN disk environment helps to bring the BHs closer together (Bellovary et al 2016;Derdzinski et al 2019Derdzinski et al , 2021Tagawa et al 2020b;Li et al 2021;Grishin et al 2023) and hence form binaries (Li et al 2022(Li et al , 2023Boekholt et al 2023;DeLaurentiis et al 2023;Qian et al 2024;Rowan et al 2023Rowan et al , 2024Rozner et al 2023;Whitehead et al 2023a;Whitehead et al 2023b), which then merge over relatively short timescales. Comparisons to the observed BH masses, spins, and merger rate indicate that a sizable fraction of the observed mergers may indeed originate in AGN disks (Tagawa et al 2021a;Gayathri et al 2021;Ford & McKernan 2022).…”

Section: Introductionmentioning

confidence: 99%

Shock Cooling and Breakout Emission for Optical Flares Associated with Gravitational-wave Events

Tagawa,

Kimura,

Haiman

et al. 2024

ApJ

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The astrophysical origin of stellar-mass black hole (BH) mergers discovered through gravitational waves (GWs) is widely debated. Mergers in the disks of active galactic nuclei (AGNs) represent promising environments for at least a fraction of these events, with possible observational clues in the GW data. An additional clue to unveil AGN merger environments is provided by possible electromagnetic emission from postmerger accreting BHs. Associated with BH mergers in AGN disks, emission from shocks emerging around jets launched by accreting merger remnants is expected. Here we compute the properties of the emission produced during breakout and the subsequent adiabatic expansion phase of the shocks, and we then apply this model to optical flares suggested to be possibly associated with GW events. We find that the majority of the reported flares can be explained by breakout and shock cooling emission. If the optical flares are produced by shock cooling emission, they would display moderate color evolution, possibly color variations among different events, and a positive correlation between delay time and flare duration and would be preceded by breakout emission in X-rays. If the breakout emission dominates the observed lightcurve, we predict the color to be distributed in a narrow range in the optical band and the delay time from GW to electromagnetic emission to be longer than ∼2 days. Hence, further explorations of delay time distributions, flare color evolution, and associated X-ray emission will be useful to test the proposed emission model for the observed flares.

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Hydrodynamical Simulations of Black Hole Binary Formation in AGN Disks

Cited by 27 publications

References 66 publications

The Population of Massive Stars in Active Galactic Nuclei Disks

The Population of Massive Stars in Active Galactic Nuclei Disks

“Tidal Peeling Events”: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole

Shock Cooling and Breakout Emission for Optical Flares Associated with Gravitational-wave Events

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