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1. Black hole, neutron star, and white dwarf merger rates in AGN discs

   https://doi.org/10.1093/mnras/staa2681  

   McKernan, B; Ford, K E; O’Shaughnessy, R (September 2020, Monthly Notices of the Royal Astronomical Society)

   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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2. Growing black holes through successive mergers in galactic nuclei – I. Methods and first results

   https://doi.org/10.1093/mnras/stad1634  

   Atallah, Dany; Trani, Alessandro A; Kremer, Kyle; Weatherford, Newlin C; Fragione, Giacomo; Spera, Mario; Rasio, Frederic A (June 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present a novel, few-body computational framework designed to shed light on the likelihood of forming intermediate-mass (IM) and supermassive (SM) black holes (BHs) in nuclear star clusters (NSCs) through successive BH mergers, initiated with a single BH seed. Using observationally motivated NSC profiles, we find that the probability of an $${\sim }100\hbox{-}\mathrm{M}_\odot$$ BH to grow beyond $${\sim }1000 \, \mathrm{M}_\odot$$ through successive mergers ranges from $${\sim }0.1~{{\ \rm per\ cent}}$$ in low-density, low-mass clusters to nearly 90  per cent in high-mass, high-density clusters. However, in the most massive NSCs, the growth time-scale can be very long ($$\gtrsim 1\,$$ Gyr); vice versa, while growth is least likely in less massive NSCs, it is faster there, requiring as little as $${\sim }0.1\,$$Gyr. The increased gravitational focusing in systems with lower velocity dispersions is the primary contributor to this behaviour. We find that there is a simple ‘7-strikes-and-you’re-in’ rule governing the growth of BHs: Our results suggest that if the seed survives 7–10 successive mergers without being ejected (primarily through gravitational wave recoil kicks), the growing BH will most likely remain in the cluster and will then undergo runaway, continuous growth all the way to the formation of an SMBH (under the simplifying assumption adopted here of a fixed background NSC). Furthermore, we find that rapid mergers enforce a dynamically mediated ‘mass gap’ between about $${50\!-\!300 \, \mathrm{M}_\odot }$$ in an NSC. 

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3. Binary black hole merger rates in AGN discs versus nuclear star clusters: loud beats quiet

   https://doi.org/10.1093/mnras/stac2861  

   Ford, K E; McKernan, Barry (November 2022, Monthly Notices of the Royal Astronomical Society)

   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. AT2024lhc and AT2024kmq in the landscape of featureless tidal disruption events

   https://doi.org/10.1093/mnras/stag920  

   Yao, Yuhan; Chornock, Ryan; Mummery, Andrew; Margutti, Raffaella; Gilfanov, Marat; Guolo, Muryel; Coughlin, Eric R; Lu, Wenbin; Chakraborty, Joheen; Pasham, Dheeraj R; et al (May 2026, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We study AT2024kmq and AT2024lhc, two tidal disruption events (TDEs) with blue featureless spectra associated with high-mass black holes ($$M_{\rm BH}\sim 10^8\, \mathrm{ M}_\odot$$). Both events show optical precursors consistent with shock dissipation from stream self-intersection. Their X-ray emission is luminous ($$L_{\rm X}\sim 10^{44}\, {\rm erg\, s^{-1}}$$), highly variable (with minimum observed variability time-scales of 1.3 and 4.8 h for factor of ∼3 flux changes), long-lasting (>1 yr), emerging no later than the optical peak, and well characterized by power laws with $$1.7\lt \Gamma \lt 3$$ (where $$f_\nu \propto \nu ^{1-\Gamma }$$). The X-ray properties and radio non-detections support a compact corona ($$\lesssim 10 r_{\rm g}$$) producing Comptonized X-ray emission. Using all published featureless TDEs, we find statistically significant bimodality in the distribution of their peak ultraviolet/optical blackbody luminosities and radii. We assemble a comparison TDE sample with early-time X-ray observations with eROSITA, in which we find different $$M_{\rm BH}$$ distributions in TDEs with different X-ray spectral evolution properties: low-mass black holes ($$M_{\rm BH} \sim 10^6 \mathrm{ M}_\odot$$) remain soft ($$\Gamma \gt 4$$) within $$t\lesssim 2$$ yr, intermediate masses ($$\sim 10^7 \mathrm{ M}_\odot$$) transition from soft to hard at $$\sim$$1 yr, while high masses ($$\sim 10^8 \mathrm{ M}_\odot$$) are hard ($$1.5\lt \Gamma \lesssim 3$$) from the outset. We interpret this result as evidence that the soft-to-hard state transition in TDEs occurs at the critical threshold of $$\dot{M}_{\rm acc} \sim 0.03 \dot{M}_{\rm Edd}$$ (similar to X-ray binaries), using the fact that the transition time-scale predicted by simple disc theory scales with black hole mass as $$t_{\rm tr}\propto M_{\rm BH}^{-3/4}$$. 

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5. Massive black hole mergers with orbital information: predictions from the ASTRID simulation

   https://doi.org/10.1093/mnras/stac1432  

   Chen, Nianyi; Ni, Yueying; Holgado, A Miguel; Matteo, Tiziana Di; Tremmel, Michael; DeGraf, Colin; Bird, Simeon; Croft, Rupert; Feng, Yu (June 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We examine massive black hole (MBH) mergers and their associated gravitational wave signals from the large-volume cosmological simulation Astrid . Astrid includes galaxy formation and black hole models recently updated with an MBH seed population between 3 × 104h−1M⊙ and 3 × 105h−1M⊙ and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to 1.5 ckpc h−1. We calculate the initial eccentricities of MBH orbits directly from the simulation at kpc-scales, and find orbital eccentricities above 0.7 for most MBH pairs before the numerical merger. After approximating unresolved evolution on scales below $${\sim 200\, \text{pc}}$$, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time ($$\sim 500\, \text{Myr}$$) due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries (MBH < 2Mseed). As a result, only $$\lesssim 20{{\rm per \,cent}}$$ of seed MBH pairs merge at z > 3 after considering both unresolved DF evolution and binary hardening. These z > 3 seed-mass mergers are hosted in a biased population of galaxies with the highest stellar masses of $$\gt 10^9\, {\rm M}_\odot$$. With the higher initial eccentricity prediction from Astrid , we estimate an expected merger rate of 0.3−0.7 per year from the z > 3 MBH population. This is a factor of ∼7 higher than the prediction using the circular orbit assumption. The Laser Interferometer Space Antenna events are expected at a similar rate, and comprise $$\gtrsim 60\,{\rm{per\,cent}}$$ seed-seed mergers, $$\sim 30\,{\rm{per\,cent}}$$ involving only one seed-mass MBH, and $$\sim 10\,{\rm{per\,cent}}$$ mergers of non-seed MBHs. 

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