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  1. ABSTRACT

    Stars embedded in active galactic nucleus (AGN) discs or captured by them may scatter onto the supermassive black hole (SMBH), leading to a tidal disruption event (TDE). Using the moving-mesh hydrodynamics simulations with arepo, we investigate the dependence of debris properties in in-plane TDEs in AGN discs on the disc density and the orientation of stellar orbits relative to the disc gas (pro- and retro-grade). Key findings are: (1) Debris experiences continuous perturbations from the disc gas, which can result in significant and continuous changes in debris energy and angular momentum compared to ‘naked’ TDEs. (2) Above a critical density of a disc around an SMBH with mass M• [ρcrit ∼ 10−8 g cm−3 (M•/106 M⊙)−2.5] for retrograde stars, both bound and unbound debris is fully mixed into the disc. The density threshold for no bound debris return, inhibiting the accretion component of TDEs, is $\rho _{\rm crit,bound} \sim 10^{-9}{\rm g~cm^{-3}}(M_{\bullet }/10^{6}\, {\rm M}_{\odot })^{-2.5}$. (3) Observationally, AGN-TDEs transition from resembling naked TDEs in the limit of ρdisc ≲ 10−2ρcrit,bound to fully muffled TDEs with associated inner disc state changes at ρdisc ≳ ρcrit,bound, with a superposition of AGN + TDE in between. Stellar or remnant passages themselves can significantly perturb the inner disc. This can lead to an immediate X-ray signature and optically detectable inner disc state changes, potentially contributing to the changing-look AGN phenomenon. (4) Debris mixing can enrich the average disc metallicity over time if the star’s metallicity exceeds that of the disc gas. We point out that signatures of AGN-TDEs may be found in large AGN surveys.

     
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  2. ABSTRACT

    Stars and stellar remnants orbiting a supermassive black hole (SMBH) can interact with an active galactic nucleus (AGN) disc. Over time, prograde orbiters (inclination i < 90°) decrease inclination, as well as semimajor axis (a) and eccentricity (e) until orbital alignment with the gas disc (‘disc capture’). Captured stellar-origin black holes (sBH) add to the embedded AGN population that drives sBH–sBH mergers detectable in gravitational waves using LIGO–Virgo–KAGRA or sBH–SMBH mergers detectable with Laser Interferometer Space Antenna. Captured stars can be tidally disrupted by sBH or the SMBH or rapidly grow into massive ‘immortal’ stars. Here, we investigate the behaviour of polar and retrograde orbiters (i ≥ 90°) interacting with the disc. We show that retrograde stars are captured faster than prograde stars, flip to prograde orientation (i < 90°) during capture, and decrease a dramatically towards the SMBH. For sBH, we find a critical angle iret ∼ 113°, below which retrograde sBH decay towards embedded prograde orbits (i → 0°), while for io > iret sBH decay towards embedded retrograde orbits (i → 180°). sBH near polar orbits (i ∼ 90°) and stars on nearly embedded retrograde orbits (i ∼ 180°) show the greatest decreases in a. Whether a star is captured by the disc within an AGN lifetime depends primarily on disc density, and secondarily on stellar type and initial a. For sBH, disc capture time is longest for polar orbits, low-mass sBH, and lower density discs. Larger mass sBH should typically spend more time in AGN discs, with implications for the spin distribution of embedded sBH.

     
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  3. ABSTRACT

    Galactic nuclei are promising sites for stellar origin black hole (BH) mergers, as part of merger hierarchies in deep potential wells. We show that binary black hole (BBH) merger rates in active galactic nuclei (AGNs) should always exceed merger rates in quiescent galactic nuclei (nuclear star clusters, NSCs) around supermassive black holes (SMBHs) without accretion discs. This is primarily due to average binary lifetimes in AGNs that are significantly shorter than those in NSCs. The lifetime difference comes from rapid hardening of BBHs in AGNs, such that their semimajor axes are smaller than the hard–soft boundary of their parent NSC; this contrasts with the large average lifetime to merger for BBHs in NSCs around SMBHs, due to binary ionization mechanisms. Secondarily, merger rates in AGNs are enhanced by gas-driven binary formation mechanisms. Formation of new BHs in AGN discs is a minor contributor to the rate differences. With the gravitational wave detection of several BBHs with at least one progenitor in the upper mass gap, and signatures of dynamical formation channels in the χeff distribution, we argue that AGNs could contribute $\sim 25{\!-\!}80{{\ \rm per\ cent}}$ of the LIGO–Virgo measured rate of $\sim 24\, \rm {Gpc}^{-3} \rm {yr}^{-1}$.

     
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  4. Abstract Active galactic nuclei (AGNs) are promising environments for the assembly of merging binary black hole (BBH) systems. Interest in AGNs as nurseries for merging BBHs is rising, following the detection of gravitational waves from a BBH system from the purported pair-instability mass gap, most notably GW190521. AGNs have also been invoked to explain the formation of the high-mass-ratio system GW190814. We draw on simulations of BBH systems in AGNs to propose a phenomenological model for the distribution of black hole spins of merging binaries in AGN disks. The model incorporates distinct features that make the AGN channel potentially distinguishable from other channels, such as assembly in the field and in globular clusters. The model parameters can be mapped heuristically to the age and density of the AGN disks. We estimate the extent to which different populations of mergers in AGNs can be distinguished. If the majority of merging black holes are assembled in AGNs, future gravitational-wave observations may provide insights into the dynamics of AGN disks. 
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  5. Abstract

    Stars are likely embedded in the gas disks of active galactic nuclei (AGN). Theoretical models predict that in the inner regions of the disk, these stars accrete rapidly, with fresh gas replenishing hydrogen in their cores faster than it is burned into helium, effectively stalling their evolution at hydrogen burning. We produce order-of-magnitude estimates of the number of such stars in a fiducial AGN disk. We find numbers of order 102–4, confined to the innerrcap∼ 3000rs∼ 0.03 pc. These stars can profoundly alter the chemistry of AGN disks, enriching them in helium and depleting them in hydrogen, both by order-unity amounts. We further consider mergers between these stars and other disk objects, suggesting that star–star mergers result in rapid mass loss from the remnant to restore an equilibrium mass, while star–compact object mergers may result in exotic outcomes and even host binary black hole mergers within themselves. Finally, we examine how these stars react as the disk dissipates toward the end of its life, and find that they may return mass to the disk fast enough to extend its lifetime by a factor of several and/or may drive powerful outflows from the disk. Post-AGN, these stars rapidly lose mass and form a population of stellar mass black holes around 10M. Due to the complex and uncertain interactions between embedded stars and the disk, their plausible ubiquity, and their order-unity impact on disk structure and evolution, they must be included in realistic disk models.

     
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  6. ABSTRACT

    Observations by LIGO–Virgo of binary black hole mergers suggest a possible anticorrelation between black hole mass ratio (q = m2/m1) and the effective inspiral spin parameter χeff, the mass-weighted spin projection on to the binary orbital angular momentum. We show that such an anticorrelation can arise for binary black holes assembled in active galactic nuclei (AGNs) due to spherical and planar symmetry-breaking effects. We describe a phenomenological model in which (1) heavier black holes live in the AGN disc and tend to spin-up into alignment with the disc; (2) lighter black holes with random spin orientations live in the nuclear spheroid; (3) the AGN disc is dense enough to rapidly capture a fraction of the spheroid component, but small in radial extent to limit the number of bulk disc mergers; (4) migration within the disc is non-uniform, likely disrupted by feedback from migrators or disc turbulence; (5) dynamical encounters in the disc are common and preferentially disrupt binaries that are retrograde around their centre of mass, particularly at stalling orbits, or traps. Comparisons of predictions in (q, χeff) parameter space for the different channels may allow us to distinguish their fractional contributions to the observed merger rates.

     
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  7. ABSTRACT

    As active galactic nuclei (AGN) ‘turn on’, some stars end up embedded in accretion discs around supermassive black holes (SMBHs) on retrograde orbits. Such stars experience strong headwinds, aerodynamic drag, ablation, and orbital evolution on short time-scales. The loss of orbital angular momentum in the first ∼0.1 Myr of an AGN leads to a heavy rain of stars (‘starfall’) into the inner disc and on to the SMBH. A large AGN loss cone (θAGN, lc) can result from binary scatterings in the inner disc and yield tidal disruption events (TDEs). Signatures of starfall include optical/UV flares that rise in luminosity over time, particularly in the inner disc. If the SMBH mass is $M_{\rm SMBH} \gtrsim 10^{8}\, \mathrm{M}_{\odot }$, flares truncate abruptly and the star is swallowed. If $M_{\rm SMBH}\lt 10^{8}\, \mathrm{M}_{\odot }$, and if the infalling orbit lies within θAGN, lc, the flare is followed by a TDE that can be prograde or retrograde relative to the AGN inner disc. Retrograde AGN TDEs are overluminous and short-lived as in-plane ejecta collide with the inner disc and a lower AGN state follows. Prograde AGN TDEs add angular momentum to inner disc gas and so start off looking like regular TDEs but are followed by an AGN high state. Searches for such flare signatures test models of AGN ‘turn on’, SMBH mass, as well as disc properties and the embedded population.

     
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  8. Abstract

    The accretion disks of active galactic nuclei (AGNs) are promising locations for the merger of compact objects detected by gravitational wave (GW) observatories. Embedded within a baryon-rich, high-density environment, mergers within AGNs are the only GW channel where an electromagnetic (EM) counterpart must occur (whether detectable or not). Considering AGNs with unusual flaring activity observed by the Zwicky Transient Facility (ZTF), we describe a search for candidate EM counterparts to binary black hole (BBH) mergers detected by LIGO/Virgo in O3. After removing probable false positives, we find nine candidate counterparts to BBH mergers during O3 (seven in O3a, two in O3b) with ap-value of 0.0019. Based on ZTF sky coverage, AGN geometry, and merger geometry, we expect ≈3(NBBH/83)(fAGN/0.5) potentially detectable EM counterparts from O3, whereNBBHis the total number of observed BBH mergers andfAGNis the fraction originating in AGNs. Further modeling of breakout and flaring phenomena in AGN disks is required to reduce our false-positive rate. Two of the events are also associated with mergers with total masses >100M, which is the expected rate for O3 if hierarchical (large-mass) mergers occur in the AGN channel. Candidate EM counterparts in future GW observing runs can be better constrained by coverage of the Southern sky as well as spectral monitoring of unusual AGN flaring events in LIGO/Virgo alert volumes. A future set of reliable AGN EM counterparts to BBH mergers will yield an independent means of measuring cosmic expansion (H0) as a function of redshift.

     
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  9. null (Ed.)
    ABSTRACT Advanced LIGO and Advanced Virgo are detecting a large number of binary stellar origin black hole (BH) mergers. A promising channel for accelerated BH merger lies in active galactic nucleus (AGN) discs of gas around supermasssive BHs. Here, we investigate the relative number of compact object (CO) mergers in AGN disc models, including BH, neutron stars (NS), and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disc grows ∝ t1/3 which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS–BH mergers in AGN discs are $\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$ for mass functions (MF) M−1(− 2). If a fraction fAGN of the observed rate of BH–BH mergers (RBH–BH) come from AGN, the rate of NS–BH (NS–NS) mergers in the AGN channel is ${R}_{\mathrm{ BH}\!-\!\mathrm{ NS}} \sim f_{\mathrm{ AGN}}[10,300]\, \rm {Gpc}^{-3}\, \rm {yr}^{-1},({\mathit{ R}}_{NS\!-\!NS} \le \mathit{ f}_{AGN}400\, \rm {Gpc}^{-3}\, \rm {yr}^{-1}$). Given the ratio of NS–NS/BH–BH LIGO search volumes, from preliminary O3 results the AGN channel is not the dominant contribution to observed NS–NS mergers. The number of lower mass gap events expected is a strong function of the nuclear MF and mass segregation efficiency. CO merger ratios derived from LIGO can restrict models of MF, mass segregation, and populations embedded in AGN discs. The expected number of electromagnetic (EM) counterparts to NS–BH mergers in AGN discs at z < 1 is $\sim [30,900]\, {\rm {yr}}^{-1}(f_{\mathrm{ AGN}}/0.1)$. EM searches for flaring events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them. 
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