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Abstract Tidal disruption events (TDEs) offer a unique probe of supermassive black hole (SMBH) demographics, but their observed rates remain difficult to reconcile with standard single-SMBH models. In this work, we use simulations of SMBH binaries, including the combined effects of eccentric Kozai–Lidov oscillations and two-body relaxation, to explore how TDE rates scale with SMBH mass and redshift. We find that binary systems exhibit increasing TDE rates with mass, in contrast to the declining trend expected for single SMBHs. These binary-driven rates match those observed in post-starburst galaxies, suggesting that a subset of TDE hosts may contain SMBH binaries. TDE light curves in some massive galaxies exhibit unexpectedly short durations, suggesting that the disrupting SMBH may be less massive than implied by host galaxy scaling relations, consistent with disruptions by the less massive black hole in a binary. By convolving our mass-dependent rates with the SMBH mass function, we predict redshift-dependent TDE rates, which we show can be used to constrain the SMBH binary fraction. Our results provide a testable framework for interpreting TDE demographics in upcoming wide-field surveys such as Legacy Survey of Space and Time and Roman. 

Abstract We propose a formation pathway linking black holes (BHs) observed in gravitational-wave (GW) mergers, wide BH–stellar systems uncovered by Gaia, and accreting low-mass X-ray binaries (LMXBs). In this scenario, a stellar-mass BH binary undergoes isolated binary evolution and merges while hosting a distant, dynamically unimportant tertiary stellar companion. The tertiary becomes relevant only after the merger, when the remnant BH receives a GW recoil kick. Depending on the kick velocity and system configuration, the outcome can be: (1) a bright electromagnetic (EM) counterpart to the GW merger; (2) an LMXB; (3) a wide BH–stellar companion system resembling the Gaia BH population; or (4) an unbound isolated BH. Modeling the three-body dynamics, we find that ∼0.02% of LIGO–Virgo–KAGRA (LVK) mergers may be followed by an EM counterpart within ∼10 days, produced by tidal disruption of the star by the BH. The flare is likely brightest in the optical–UV and lasts for days to weeks; in some cases, partial disruption causes recurring flares with a period of ∼2 months. We further estimate that this channel can produce ∼1%–10% of Gaia BH systems in the Milky Way. This scenario provides the first physically motivated link between GW sources, Gaia BHs, and some X-ray binaries, and predicts a rare but robust pathway for EM counterparts to binary BH mergers, potentially detectable in LVK’s O5 run. 

Abstract We present a catalog of ∼10,000 resolved triple star systems within 500 pc of the Sun, constructed using Gaia data. The triples include main-sequence, red giant, and white dwarf components spanning separations of 10–50,000 au. A well-characterized selection function allows us to constrain intrinsic demographics of the triple star population. We find that (a) all systems are compatible with being hierarchical and dynamically stable; (b) mutual orbital inclinations are isotropic for wide triples but show modest alignment as the systems become more compact; (c) primary masses follow a Kroupa initial mass function weighted by the triple fraction; (d) inner binary orbital periods, eccentricities, and mass ratios mirror those of isolated binaries, including a pronounced twin excess (mass ratios greater than 0.95) out to separations of 1000+ au, suggesting a common formation pathway; (e) tertiary mass ratios follow a power-law distribution with slope −1.4; (f) tertiary orbits are consistent with a log-normal period distribution and thermal eccentricities, subject to dynamical stability. Informed by these observations, we develop a publicly available prescription for generating mock triple star populations. Finally, we estimate the catalog’s completeness and infer the intrinsic triple fraction, which rises steadily with primary mass: from 5% at ≲0.5M_⊙to 35% at 2M_⊙. The public catalog provides a robust testbed for models of triple star formation and evolution. 

Abstract The Galactic center hosts a rotating disk of young stars between 0.05 and 0.5 pc of Sgr A*. The “S stars” at a distance <0.04 pc, however, are on eccentric orbits with nearly isotropically distributed inclinations. The dynamical origin of the S-star cluster has remained a theoretical challenge. Using a series ofN-body simulations, we show that a recent massive black hole merger with Sgr A* can self-consistently produce many of the orbital properties of the Galactic nuclear star cluster within 0.5 pc. A black hole merger results in a gravitational-wave recoil kick, which causes the surrounding cluster to form an apse-aligned eccentric disk. We show that stars near the inner edge of an eccentric disk migrate inward and are driven to high eccentricities and inclinations due to secular torques similar to the eccentric Kozai–Lidov mechanism. In our fiducial model, starting with a thin eccentric disk withe= 0.3, the initially unoccupied region within 0.04 pc is populated with high-eccentricity, high-inclination S stars within a few Myr. This formation channel requires a black hole of mass $2_{-1.2}^{+3}\times 10^5{M}_{\odot }$to have merged with Sgr A* within the last 10 Myr. 

Abstract The dynamical formation of binary black holes (BBHs) in globular clusters (GCs) may contribute significantly to the observed gravitational-wave (GW) merger rate. Furthermore, the Laser Interferometer Space Antenna (LISA) may detect many BBH sources from GCs at mHz frequencies, enabling the characterization of such systems within the Milky Way and nearby Universe. In this work, we use Monte CarloN-body simulations to construct a realistic sample of Galactic clusters, thus estimating the population, detectability, and parameter measurement accuracy of BBHs hosted within them. In particular, we show that the GW signal from 0.7 ± 0.7, 2.0 ± 1.7, 3.6 ± 2.3, and 13.4 ± 4.7 BBHs in Milky Way GCs can exceed the signal-to-noise ratio (SNR) threshold of SNR = 30, 5, 3, and 1 for a 10 yr LISA observation, with ∼50% of detectable sources exhibiting high eccentricities (e ≳ 0.9). Moreover, the Fisher matrix and Bayesian analyses of the GW signals indicate that these systems typically feature highly resolved orbital frequencies (δf_orb/f_orb ∼ 10⁻⁷to 10⁻⁵) and eccentricities (δe/e ∼ 10⁻³to 0.1), as well as a measurable total mass when SNR exceeds ∼20. Notably, we show that high-SNR BBHs can be confidently localized to specific Milky Way GCs with a sky localization accuracy ofδΩ ∼ 1 deg², and we address the large uncertainties in their distance measurement (δR ∼ 0.3–20 kpc). The detection and localization of even a single BBH in a Galactic GC would allow accurate tracking of its long-term orbital evolution, enable a direct test of the role of GCs in BBH formation, and provide a unique probe into the evolutionary history of Galactic clusters. 

Abstract The formation of cataclysmic variables (CVs) has long been modeled as a product of common envelope evolution (CEE) in isolated binaries. However, a significant fraction of intermediate-mass stars—the progenitors of the white dwarfs (WDs) in CVs—are in triples. We therefore investigate the importance of triple star dynamics in CV formation. Using Gaia astrometry and existing CV catalogs, we construct a sample of ∼50 CVs in hierarchical triples within 1 kpc of the Sun, containing main-sequence and WD tertiaries at separations of 100–30,000 au. We infer that at least 10% of CVs host wide tertiaries. To interpret this discovery, we evolve a population of 2000 triples using detailed three-body simulations, 47 of which become CVs. We predict that 20% of CVs in triples form without ever experiencing CEE, where the WD and donor are brought together by the eccentric Kozai-Lidov mechanism after the formation of the WD. These systems favor larger donor stars and longer birth orbital periods (8–20 hr) than typical CVs. Among systems that do undergo CEE, about half would not have interacted without the presence of the tertiary. Triple formation channels both with and without CEE require initially wide inner orbits (≳1 au), which in turn require larger tertiary separations to be stable. Consistent with this prediction, we find that the observed Gaia CV triples have wider separations on average than normal wide binaries selected in the same way. Our work underscores the importance of triples in shaping interacting binary populations including CVs, ultracompact binaries, and low-mass X-ray binaries. 

Many gravitational wave (GW) sources are expected to have non-negligible eccentricity in the millihertz band. These highly eccentric compact object binaries may commonly serve as a progenitor stage of GW mergers, particularly in dynamical channels where environmental perturbations bring a binary with large initial orbital separation into a close pericenter passage, leading to efficient GW emission and a final merger. This work examines the stochastic GW background from highly eccentric (e ≳0.9 ), stellar-mass sources in the mHz band. Our findings suggest that these binaries can contribute a substantial GW power spectrum, potentially exceeding the LISA instrumental noise at ∼3 - 7 mHz . This stochastic background is likely to be dominated by eccentric sources within the Milky Way, thus introducing anisotropy and time dependence in LISA's detection. However, given efficient search strategies to identify GW transients from highly eccentric binaries, the unresolvable noise level can be substantially lower, approaching ∼2 orders of magnitude below the LISA noise curve. Therefore, we highlight the importance of characterizing stellar-mass GW sources with extreme eccentricity, especially their transient GW signals in the millihertz band. 

Abstract Most galaxies, including the Milky Way, harbor a central supermassive black hole (SMBH) weighing millions to billions of solar masses. Surrounding these SMBHs are dense regions of stars and stellar remnants, such as neutron stars (NSs) and black holes (BHs). NSs and possibly BHs receive large natal kicks at birth on the order of hundreds of kilometers per second. The natal kicks that occur in the vicinity of an SMBH may redistribute the orbital configuration of the compact objects and alter their underlying density distribution. We model the effects of natal kicks on a Galactic center (GC) population of massive stars and stellar binaries with different initial density distributions. Using observational constraints from stellar orbits near the GC, we place an upper limit on the steepness of the initial stellar profile and find it to be core-like. In addition, we predict that 30%–70% of compact objects become unbound from the SMBH due to their kicks and will migrate throughout the Galaxy. Different BH kick prescriptions lead to distinct spatial and kinematic distributions. We suggest that the Nancy Grace Roman Space Telescope may be able to distinguish between these distributions and thus be able to differentiate between natal kick mechanisms. 

Abstract The dynamical formation channels of gravitational wave (GW) sources typically involve a stage when the compact object binary source interacts with the environment, which may excite its eccentricity, yielding efficient GW emission. For the wide eccentric compact object binaries, the GW emission happens mostly near the pericenter passage, creating a unique, burst-like signature in the waveform. This work examines the possibility of stellar-mass bursting sources in the mHz band for future LISA detections. Because of their long lifetime (∼10⁷yr) and promising detectability, the number of mHz bursting sources can be large in the local Universe. For example, based on our estimates, there will be ∼3–45 bursting binary black holes in the Milky Way, with ∼10²–10⁴bursts detected during the LISA mission. Moreover, we find that the number of bursting sources strongly depends on their formation history. If certain regions undergo active formation of compact object binaries in the recent few million years, there will be a significantly higher bursting source fraction. Thus, the detection of mHz GW bursts not only serves as a clue for distinguishing different formation channels, but also helps us understand the star formation history in different regions of the Milky Way.