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1. Dynamical Friction Models for Black Hole Binary Formation in Active Galactic Nucleus Disks

   https://doi.org/10.3847/1538-4357/ad1b53  

   Qian, Kecheng ; Li, Jiaru ; Lai, Dong ( February 2024 , The Astrophysical Journal)

   Stellar-mass black holes (sBHs) embedded in gaseous disks of active galactic nuclei (AGN) can be important sources of detectable gravitational radiation for LIGO/Virgo when they form binaries and coalesce due to orbital decay. In this paper, we study the effect of dynamical friction (DF) on the formation of BH binaries in AGN disks usingN-body simulations. We employ two simplified models of DF, with the force on the BH depending on Δv, the velocity of the sBH relative to the background Keplerian gas. We integrate the motion of two sBHs initially on circular orbits around the central supermassive black hole (SMBH) and evaluate the probability of binary formation under various conditions. We find that both models of DF (with different dependence of the frictional coefficient on ∣Δv∣) can foster the formation of binaries when the effective friction timescaleτsatisfies Ω_Kτ≲ 20–30 (where Ω_Kis the Keplerian frequency around the SMBH): prograde binaries are formed when the DF is stronger (smallerτ), while retrograde binaries dominate when the DF is weaker (largerτ). We determine the distribution of both prograde and retrograde binaries as a function of initial orbital separation and the DF strength. Using our models of DF, we show that for a given sBH number density in the AGN disk, the formation rate of sBH binaries increases with decreasingτand can reach a moderate value with a sufficiently strong DF.

    

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2. Hydrodynamical Simulations of Black Hole Binary Formation in AGN Disks

   https://doi.org/10.3847/2041-8213/acb934  

   Li, Jiaru ; Dempsey, Adam M. ; Li, Hui ; Lai, Dong ; Li, Shengtai ( February 2023 , The Astrophysical Journal Letters)

   We study close encounters between two single black holes (BHs) embedded in an AGN disk using a series of global 2D hydrodynamics simulations. We find that when the disk density is sufficiently high, bound BH binaries can be formed by the collision of their circum-single disks. Our analysis demonstrates that, after a BH pair passes the pericenter of their relative trajectory, post-collision gas drag may slow down the BHs, possibly forcing the two BHs to stay tightly bound. A binary formed by a close encounter can have a compact semimajor axis, large eccentricity, and retrograde orbital angular momentum. We provide a fitting formula that can accurately predict whether a close encounter can form a binary based on the gas mass and the incoming energy of the encounter. This fitting formula can be easily implemented in other long-term simulations that study the dynamical evolution of BHs in active galactic nucleus disks.

    

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

   https://doi.org/10.3847/1538-4357/ad11d3  

   Xin, Chengcheng ; Haiman, Zoltán ; Perna, Rosalba ; Wang, Yihan ; Ryu, Taeho ( January 2024 , The Astrophysical Journal)

   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 codePHANTOM. We find that under certain circumstances (for initial eccentricitye₀≳ 0.4 and penetration factorβ= 1, ore₀< 0.4 andβ≲ 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 (e₀< 0.4 andβ= 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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4. Merging black holes in dwarf galaxies: calculating binary black hole coalescence time-scales from simulations for LISA detection

   https://doi.org/10.1093/mnras/stad311  

   De Cun, Victoria I. ; Bellovary, Jillian M. ; Katz, Michael L. ( January 2023 , Monthly Notices of the Royal Astronomical Society)

   Supermassive black holes (SMBHs) merging in dwarf galaxies will be detectable by the Laser Interferometer Space Antenna (LISA) in the mid-2030s. Previous cosmological hydrodynamic simulations have shown the prediction of massive BHs merging in dwarf galaxies, but these simulations are limited by their resolution and cannot follow BH pairs all the way to coalescence. We calculate the delay time between BH pairing and merger based on the properties of the BHs and their host galaxies, and use these properties to calculate gravitational wave strains for eleven different binary BHs that merge inside dwarf galaxies from eight cosmological simulations. This delay time calculation accounts for dynamical friction due to gas and stars, loss-cone scattering, and hardening of the binary due to gravitational radiation. Out of the eleven BH mergers in the simulations, five BH pairs will merge within 0.8–8 Gyr of forming a close pair and could be observed by LISA, and the remaining six are unresolved due to resolution limitations of the simulation. As all five of the resolved close pairs merge within a Hubble time, we make the broad estimate that close SMBH pairs in dwarf galaxies will merge and be detectable by LISA, but this estimate depends on either the presence of gas during orbital decay or a solution to the dynamical buoyancy problem in cored potentials.

    

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5. Jets with a Twist: The Emergence of FR0 Jets in a 3D GRMHD Simulation of Zero-angular-momentum Black Hole Accretion

   https://doi.org/10.3847/1538-4357/ad0974  

   Lalakos, Aretaios ; Tchekhovskoy, Alexander ; Bromberg, Omer ; Gottlieb, Ore ; Jacquemin-Ide, Jonatan ; Liska, Matthew ; Zhang, Haocheng ( March 2024 , The Astrophysical Journal)

   Spinning supermassive black holes (BHs) in active galactic nuclei magnetically launch relativistic collimated outflows, or jets. Without angular momentum supply, such jets are thought to perish within 3 orders of magnitude in distance from the BH, well before reaching kiloparsec scales. We study the survival of such jets at the largest scale separation to date, via 3D general relativistic magnetohydrodynamic simulations of rapidly spinning BHs immersed into uniform zero-angular-momentum gas threaded by a weak vertical magnetic field. We place the gas outside the BH sphere of influence, or the Bondi radius, chosen to be much larger than the BH gravitational radius,R_B= 10³R_g. The BH develops dynamically important large-scale magnetic fields, forms a magnetically arrested disk (MAD), and launches relativistic jets that propagate well outsideR_Band suppress BH accretion to 1.5% of the Bondi rate,${\dot{M}}_{\mathrm{B}}$. Thus, low-angular-momentum accretion in the MAD state can form large-scale jets in Fanaroff–Riley (FR) type I and II galaxies. Subsequently, the disk shrinks and exits the MAD state: barely a disk (BAD), it rapidly precesses, whips the jets around, globally destroys them, and lets 5%–10% of${\dot{M}}_{\mathrm{B}}$reach the BH. Thereafter, the disk starts rocking back and forth by angles 90°–180°: the rocking accretion disk (RAD) launches weak intermittent jets that spread their energy over a large area and suppress BH accretion to ≲2%${\dot{M}}_{\mathrm{B}}$. Because the BAD and RAD states tangle up the jets and destroy them well insideR_B, they are promising candidates for the more abundant, but less luminous, class of FR0 galaxies.

    

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