The evolution of main sequence binaries resided in the galactic centre is influenced a lot by the central super massive black hole (SMBH). Due to this perturbation, the stars in a dense environment are likely to experience mergers or collisions through secular or non-secular interactions. In this work, we study the dynamics of the stellar binaries at galactic center, perturbed by another distant SMBH. Geometrically, such a four-body system is supposed to be decomposed into the inner triple (SMBH-star-star) and the outer triple (SMBH-stellar binary-SMBH). We survey the parameter space and determine the criteria analytically for the stellar mergers and the tidal disruption events (TDEs). For a relative distant and equal masses SMBH binary, the stars have more opportunities to merge as a result from the Lidov-Kozai(LK) oscillations in the inner triple. With a sample of tight stellar binaries, our numerical experiments reveal that a significant fraction of the binaries, ~70 per cent, experience merger eventually. Whereas the majority of the stellar TDEs are likely to occur at a close periapses to the SMBH, induced by the outer Kozai effect. The tidal disruptions are found numerically as many as ~10 per cent for a close SMBH binary that is enhanced significantly than the one without the external SMBH. These effects require the outer perturber to have an inclined orbit (>=40 degree) relatively to the inner orbital plane and may lead to a burst of the extremely astronomical events associated with the detection of the SMBH binary.
An enhanced rate of stellar tidal disruption events (TDEs) may be an important characteristic of supermassive black hole (SMBH) binaries at close separations. Here we study the evolution of the distribution of stars around a SMBH binary due to the eccentric Kozai-Lidov (EKL) mechanism, including octupole effects and apsidal precession caused by the stellar mass distribution and general relativity. We identify a region around one of the SMBHs in the binary where the EKL mechanism drives stars to high eccentricities, which ultimately causes the stars to either scatter off the second SMBH or get …
Supermassive black hole binaries (SMBHBs) in the 10 million to 10 billion $M_\odot$ range form in galaxy mergers, and live in galactic nuclei with large and poorly constrained concentrations of gas and stars. There are currently no observations of merging SMBHBs--- it is in fact possible that they stall at their final parsec of separation and never merge. While LIGO has detected high frequency GWs, SMBHBs emit GWs in the nanohertz to millihertz band. This is inaccessible to ground-based interferometers, but possible with Pulsar Timing Arrays (PTAs). Using data from local galaxies in the 2 Micr…
Supermassive black hole (BH) binaries would comprise the strongest sources of gravitational waves (GW) once they reach <<1 pc separations, for both pulsar timing arrays (PTAs) and space based (SB) detectors. While BH binaries coalescences constitute a natural outcome of the cosmological standard model and galaxy mergers, their dynamical evolution is still poorly understood and therefore their abundances at different stages. We use a dynamical model for the decay of BH binaries coupled with a cosmological simulation and semi-empirical approaches to the occupation of haloes by galaxies and BHs, …
Most large galaxies harbor supermassive black holes (SMBHs) in their centers, and galaxies merge. Consequently, binary SMBHs should be common in galactic nuclei. However, close binaries of SMBH (CB-SMBHs) with sub-parsec separation cannot be imaged directly using current facilities. Some indirect signatures, such as periodic signals in light curves and double peaks in emission-line profile, have been used to find CB-SMBH candidates, but ambiguities still exist and no definitive conclusions can be made. We have recently proposed a new method focusing on kinematic signatures that can be derived …
We analyze the outcome of the interaction between a stellar binary and a supermassive black hole binary (SMBHB) by performing a large number of gravitational scattering experiments. Most of the encounters result in either the ejection of an intact binary or the ejection of two individual stars following the tidal breakup of the binary. However, tidal disruption events (TDEs) and mergers constitute a few percent of the outcomes, and double, temporally distinct TDEs (i.e., separated by at least one orbit of the supermassive black hole binary) occur at the percent level. We also demonstrate that …
Changing-look active galactic nuclei (CL-AGNs) as a new subpopulation challenge some fundamental physics of AGNs because the timescales of the phenomenon can hardly be reconciled with accretion disk models. In this Letter{\textit{}}, we demonstrate the extreme case: close binaries of supermassive black holes (CB-SMBHs) with high eccentricities are able to trigger the CL transition through one orbit. In this scenario, binary black holes build up their own mini-disks by peeling gas off the inner edges of the circumbinary disk during the apastron phase, after which they tidally interact with the …
Gravitationally bound supermassive black hole binaries (SBHBs) are thought to be a natural product of galactic mergers and growth of the large scale structure in the universe. They however remain observationally elusive, thus raising a question about characteristic observational signatures associated with these systems. In this conference proceeding I discuss current theoretical understanding and latest advances and prospects in observational searches for SBHBs.
We show that a 10 year Gaia mission could astrometrically detect the orbital motion of ~1 sub-parsec separation supermassive black hole binary in the heart of nearby, bright active galactic nuclei (AGN). Candidate AGN lie out to a redshift of z=0.02 and in the V-band magnitude range $10 \lesssim m_V \lesssim 13$. The distribution of detectable binary masses peaks at a few times $10^7$ solar masses and is truncated above a few times $10^8$ solar masses.
Partially-depleted cores are practically ubiquitous in luminous early-type galaxies (M$_{B}\lesssim-$20.5 mag), and typically smaller than 1 kpc. In one popular scenario, supermassive black hole binaries --- established during dry (i.e. gas-poor) galaxy mergers --- kick out the stars from a galaxy's central region via three-body interactions. Here, this "binary black hole scouring scenario" is probed at its extremes by investigating the two galaxies reported to have the largest partially-depleted cores found to date: 2MASX~J09194427+5622012 and 2MASX~J17222717+3207571 (the brightest galaxy in …
We introduce a new model for the accretion and feedback of supermassive black hole (SMBH) binaries to the KETJU code, which enables us to resolve the evolution of SMBH binaries down to separations of tens of Schwarzschild radii in gas-rich galaxy mergers. Our subgrid binary accretion model extends the widely used Bondi--Hoyle--Lyttleton accretion into the binary phase and incorporates preferential mass accretion onto the secondary SMBH, which is motivated by results from small-scale hydrodynamical circumbinary disc simulations. We perform idealised gas-rich disc galaxy merger simulations using…
Close binaries of supermassive black holes (CB-SMBHs) with separations of $\lesssim 0.1$pc as the final stage of galaxy mergers are sources of low frequency gravitational waves (GW), however, they are still elusive observationally because they are not spatially resolved. Fortunately, reverberation as echoes of broad emission lines to ionizing continuum conveys invaluable information of the dynamics of broad-line regions (BLRs) governed by supermassive black holes in the central regions of active galactic nuclei (AGNs). In this paper, we demonstrate how to composite the hybrid 2-dimensional tra…
Pairs of supermassive black holes (SMBHs) at different stages are natural results of galaxy mergers in the hierarchical framework of galaxy formation and evolution. However, identifications of close binaries of SMBHs (CB-SMBHs) with sub-parsec separations in observations are still elusive. Recently, unprecedented spatial resolutions achieved by GRAVITY/GRAVITY+ onboard The Very Large Telescope Interferometer through spectroastrometry (SA) provide new opportunities to resolve CB-SMBHs. Differential phase curves of CB-SMBHs with two independent broad-line regions (BLRs) are found to have disting…
The coalescence of supermassive black holes (SMBHs) should generate the strongest sources of gravitational waves (GWs) in the Universe. However, the dynamics of their coalescence is the subject of much debate. In this study, we use a suite of $N$-body simulations to follow the merger of two nuclear star clusters (NSCs), each hosting a SMBH in their centre. We find that the presence of distinct star clusters around each SMBH has important consequences for the dynamical evolution of the SMBH binary: (i) The separation between the SMBHs decreases by a few orders of magnitude in the first few Myrs…
The flat stellar density cores of massive elliptical galaxies form rapidly due to sinking supermassive black holes (SMBH) in gas-poor galaxy mergers. After the SMBHs form a bound binary, gravitational slingshot interactions with nearby stars drive the core regions towards a tangentially biased stellar velocity distribution. We use collisionless galaxy merger simulations with accurate collisional orbit integration around the central SMBHs to demonstrate that the removal of stars from the centre by slingshot kicks accounts for the entire change in velocity anisotropy. The rate of strong (unbindi…
Supermassive black hole binaries (SMBHBs) are an inevitable consequence of galaxy mergers. At subparsec separations, they are practically impossible to resolve, and the most promising technique is to search for quasars with periodic variability. However, searches for quasar periodicity in time-domain data are challenging due to the stochastic variability of quasars. In this paper, we used Bayesian methods to disentangle periodic SMBHB signals from intrinsic damped random walk (DRW) variability in active galactic nuclei light curves. We simulated a wide variety of realistic DRW and DRW+sine lig…
Galaxy mergers produce supermassive black hole binaries, which emit gravitational waves prior to their coalescence. We perform three-dimensional hydrodynamic simulations to study the tidal disruption of stars by such a binary in the final centuries of its life. We find that the gas stream of the stellar debris moves chaotically in the binary potential and forms accretion disks around both black holes. The accretion light curve is modulated over the binary orbital period owing to relativistic beaming. This periodic signal allows to detect the decay of the binary orbit due to gravitational wave …
Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are among the densest systems in the Universe, harbouring millions of stars and compact objects (COs). Within a few parsecs from the SMBH, stars can form binaries. In this paper, we model the supernova (SN) process of massive binaries that are born in proximity of the SMBH and that produce CO binaries. These binaries can later merge via emission of gravitational waves as a consequence of the Lidov-Kozai mechanism. We study the dynamical evolution of these systems by means of high-precision $N$-body simulati…
The binding energy liberated by the coalescence of supermassive black hole (SMBH) binaries during galaxy mergers is thought to be responsible for the low density cores often found in bright elliptical galaxies. We use high-resolution $N$-body and Monte Carlo techniques to perform single and multi-stage galaxy merger simulations and systematically study the dependence of the central galaxy properties on the binary mass ratio, the slope of the initial density cusps, and the number of mergers experienced. We study both the amount of depleted stellar mass (or ``mass deficit'), $M_{\rm def}$, and t…
Disc-driven migration is a key evolutionary stage of supermassive black-hole binaries hosted in gas-rich galaxies. Besides promoting the inspiral, viscous interactions tend to align the spins of the black holes with the orbital angular momentum of the disc. We present a critical and systematic investigation of this problem, also known as the Bardeen-Petterson effect. We design a new iterative scheme to solve the non-linear dynamics of warped accretion discs under the influence of both relativistic frame dragging and binary companion. We characterize the impact of the disc "critical obliquity",…
Supermassive black hole binaries (SMBHBs) are a natural outcome of galaxy mergers and should form frequently in galactic nuclei. Sub-parsec binaries can be identified from their bright electromagnetic emission, e.g., Active Galactic Nuclei (AGN) with Doppler shifted broad emission lines or AGN with periodic variability, as well as from the emission of strong gravitational radiation. The most massive binaries (with total mass >10^8 M_sol) emit in the nanohertz band and are targeted by Pulsar Timing Arrays (PTAs). Here we examine the synergy between electromagnetic and gravitational wave signatu…
We introduce a technique for gravitational-wave analysis, where Gaussian process regression is used to emulate the strain spectrum of a stochastic background using population-synthesis simulations. This leads to direct Bayesian inference on astrophysical parameters. For PTAs specifically, we interpolate over the parameter space of supermassive black-hole binary environments, including 3-body stellar scattering, and evolving orbital eccentricity. We illustrate our approach on mock data, and assess the prospects for inference with data similar to the NANOGrav 9-yr data release.
It is generally believed that gaseous disks around supermassive black hole (SMBH) binaries in centers of galaxies can facilitate binary merger and give rise to observational signatures both in electromagnetic and gravitational wave domains. We explore general properties of circumbinary disks by reformulating standard equations for the viscous disk evolution in terms of the viscous angular momentum flux F_J. In steady state F_J is a linear function of the specific angular momentum, which is a generalization of (but is not equivalent to) the standard constant \dot M disk solution. If the torque …
Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predict…
Nuclear spiral arms are small-scale transient spiral structures found in the centers of galaxies. Similarly to their galactic-scale counterparts, nuclear spiral arms can perturb the orbits of stars. In the case of the Galactic Center (GC), these perturbations can affect the orbits of stars and binaries in a region extending to several hundred parsecs around the supermassive black hole (MBH), causing diffusion in orbital energy and angular momentum. This diffusion process can drive stars and binaries to close approaches with the MBH, disrupting single stars in tidal disruption events (TDEs), or…
We simulate an equal-mass merger of two Milky Way-size galaxy discs with moderate gas fractions at parsec-scale resolution including a new model for radiative cooling and heating in a multi-phase medium, as well as star formation and feedback from supernovae. The two discs initially have a $2.6\times10^6\mathrm{~M_{\odot}}$ supermassive black hole (SMBH) embedded in their centers. As the merger completes and the two galactic cores merge, the SMBHs form a a pair with a separation of a few hundred pc that gradually decays. Due to the stochastic nature of the system immediately following the merg…
Pulsar timing arrays (PTAs) are placing increasingly stringent constraints on the strain amplitude of continuous gravitational waves emitted by supermassive black hole binaries on subparsec scales. In this paper, we incorporate independent information about the dynamical masses $M_{bh}$ of supermassive black holes in specific galaxies at known distances and use this additional information to further constrain whether or not those galaxies could host a detectable supermassive black hole binary. We estimate the strain amplitudes from individual binaries as a function of binary mass ratio for two…
Gravitational waves (GWs) in the nano-hertz band are great tools for understanding the cosmological evolution of supermassive black holes (SMBHs) in galactic nuclei. We consider SMBH binaries in high-$z$ ultra-luminous infrared galaxies (ULIRGs) as sources of a stochastic GW background (GWB). ULIRGs are likely associated with gas-rich galaxy mergers containing SMBHs that possibly occur at most once in the life of galaxies, unlike multiple dry mergers at low redshift. Adopting a well-established sample of ULIRGs, we study the properties of the GWB due to coalescing binary SMBHs in these galaxie…
The Next-Generation Very Large Array (ngVLA) has the potential to be a workhorse for the discovery and study of paired supermassive black holes either at large separations (dual) or in tightly bound systems (binary). In this chapter, we outline the science case for the study of these supermassive pairs, and summarize discovery methods that can be used at radio wavelengths to discover them: including morphological, spectral, and time-domain identifications. One critical aspect of this work is that multi-messenger binary black hole studies may be possible with the ngVLA when combined with gravit…
It has been well-established that particular centrophilic orbital families in non-spherical galaxies can, in principle, drive a black hole binary to shrink its orbit through three-body scattering until the black holes are close enough to strongly emit gravitational waves. Most of these studies rely on orbital analysis of a static SMBH-embedded galaxy potential to support this view; it is not clear, however, how these orbits transform as the second SMBH enters the center, so our understanding of which orbits actually interact with a SMBH binary is not ironclad. Here, we analyze two flattened ga…
Distinguishing the photon output of an accreting supermassive black hole binary system from that of a single supermassive black hole accreting at the same rate is intrinsically difficult because the majority of the light emerges from near the ISCOs of the black holes. However, there are two possible signals that can distinctively mark binaries, both arising from the gap formed in circumbinary accretion flows inside approximately twice the binary separation. One of these is a "notch" cut into the thermal spectra of these systems in the IR/optical/UV, the other a periodically-varying excess hard…
Hypervelocity stars (HVSs) that are observed in the Galactic halo, are believed to be accelerated to large velocities by a process of tidal disruption of binary stars passing close to a supermassive black hole (SMBH) which resides in the center of the Galaxy. It is, however, still unclear, where these relatively young stars were born and which dynamical process pushed them to nearly radial orbits around the SMBH. In this paper we investigate the possibility that the young binaries originated from a thin eccentric disc, similar to the one observed in the Galactic center nowadays. By means of di…
The evolution of main sequence binaries that reside in the Galactic Centre can be heavily influenced by the central supermassive black hole (SMBH). Due to these perturbative effects, the stellar binaries in dense environments are likely to experience mergers, collisions or ejections through secular and/or non-secular interactions. More direct interactions with the central SMBH are thought to produce hypervelocity stars (HVSs) and tidal disruption events (TDEs). In this paper, we use N-body simulations to study the dynamics of stellar binaries orbiting a central SMBH primary with an outer SMBH …
We illustrate a novel signature of black hole binaries from their effect on the kinematics of water maser emission in their environments. With the help of simulations, we establish the condition for clumps to mase based on their coherence lengths calibrated to those of the known maser galaxy NGC 4258. This is then used to identify masing clumps around a binary black hole system, and quantify the kinematic and spectral differences relative to the single black hole case. For some generic circumstances, blue-shifted masers around a binary black hole are found to preferentially follow the Kepleria…
A significant number of stellar-mass black-hole (BH) binaries may merge in galactic nuclei or in the surrounding gas disks. With purposed space-borne gravitational-wave observatories, we may use such a binary as a signal carrier to probe modulations induced by a central supermassive BH (SMBH), which further allows us to place constraints on the SMBH's properties. We show in particular the de Sitter precession of the inner stellar-mass binary's orbital angular momentum (AM) around the AM of the outer orbit will be detectable if the precession period is comparable to the duration of observation,…
We investigate the pre-disruption gravitational dynamics and post-disruption hydrodynamics of the tidal disruption of stars by supermassive black hole (SMBH) binaries. We focus on binaries with relatively low mass primaries ($10^6M_{\odot}$), moderate mass ratios, and separations with reasonably long gravitational wave inspiral times (tens of Myr). First, we generate a large ensemble (between 1 and 10 million) of restricted three-body integrations to quantify the statistical properties of tidal disruptions by circular SMBH binaries of initially-unbound stars. Compared to the reference case of …
Astrophysical Population Coordinates for Supermassive Black Hole Binaries in Pulsar Timing Array Inference
State Key Laboratory of Radio Astronomy and Technology, Xinjiang Astronomical Observatory, CAS（中国科学院新疆天文台射电天文与技术重点实验室）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）；Xinjiang Key Laboratory of Radio Astrophysics（新疆射电天体物理学重点实验室）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）
The supermassive black holes originally in the nuclei of two merging galaxies will form a binary in the remnant core. The early evolution of the massive binary is driven by dynamical friction before the binary becomes "hard" and eventually reaches coalescence through gravitational wave emission. { We consider the dynamical friction evolution of massive binaries consisting of a secondary hole orbiting inside a stellar cusp dominated by a more massive central black hole.} In our treatment we include the frictional force from stars moving faster than the inspiralling object which is neglected in …
There are increasing interests in binary supermassive black holes (SMBHs), but merging binaries with separations smaller than ~1 light days (~10^2 gravitational radii for 10^8 Msun), which are rapidly evolving under control of gravitational waves, are elusive in observations. In this paper, we discuss fates of mini-disks around component SMBHs for three regimes: 1) low rates (advection-dominated accretion flows: ADAFs); 2) intermediate rates; 3) super-Eddington accretion rates. Mini-disks with intermediate rates are undergoing evaporation through thermal conduction of hot corona forming a hybr…
Motivated by observational searches for sub-parsec supermassive black hole binaries (SBHBs) we develop a modular analytic model to determine the likelihood for detection of SBHBs by ongoing spectroscopic surveys. The model combines the parametrized rate of orbital evolution of SBHBs in circumbinary disks with the selection effects of spectroscopic surveys and returns a multivariate likelihood for SBHB detection. Based on this model we find that in order to evolve into the detection window of the spectroscopic searches from larger separations in less than a Hubble time, $10^8M_\odot$ SBHBs must…
Pulsar timing arrays (PTAs) are presently the only means to search for the gravitational wave stochastic background from supermassive black hole binary populations, considered to be within the grasp of current or near future observations. However, the stringent upperlimit set by the Parkes PTA (Shannon et al. 2013, 2015) has been interpreted as excluding at $> 90\%$ confidence the current paradigm of binary assembly through galaxy mergers and hardening via stellar interactions, suggesting evolution is accelerated (by stars and/or gas) or stalled. Using Bayesian hierarchical modelling, we consi…
Supermassive black hole binaries (SMBHBs) should form frequently in galactic nuclei as a result of galaxy mergers. At sub-parsec separations, binaries become strong sources of low-frequency gravitational waves (GWs), targeted by Pulsar Timing Arrays (PTAs). We used recent upper limits on continuous GWs from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11yr dataset to place constraints on putative SMBHBs in nearby massive galaxies. We compiled a comprehensive catalog of ~44,000 galaxies in the local universe (up to redshift ~0.05) and populated them with hypotheti…
One of the proposed explanations for the broad, double-peaked Balmer emission lines observed in the spectra of some active galactic nuclei (AGNs) is that they are associated with sub-parsec supermassive black hole (SMBH) binaries. Here, we test the binary broad-line region hypothesis through several decades of monitoring of the velocity structure of double-peaked H-alpha emission lines in 13 low-redshift, mostly radio-loud AGNs. This is a much larger set of objects compared to an earlier test by Eracleous et al. (1997) and we use much longer time series for the three objects studied in that pa…
In the present paper, we explore opportunities of applying the GRAVITY at the Very Large Telescope Interferometry (VLTI) with unprecedented spatial resolution to identify close binaries of supermassive black holes (CB-SMBHs) in active galactic nuclei (AGNs). Each SMBH is assumed to be separately surrounded by their own broad-line regions (BLRs) composed of clouds with virialized motion. Composition of the binary orbital motion and the virial motion of clouds in each BLR determines the projected velocity fields and hence differential phase curves, which are obviously different from that of a si…
Supermassive black hole binaries (SMBHs) are expected to result from galaxy mergers, and thus are natural byproducts (and probes) of hierarchical structure formation in the Universe. They are also the primary expected source of low-frequency gravitational wave emission. We search for binary BHs using time-variable velocity shifts in broad Mg II emission lines of quasars with multi-epoch observations. First, we inspect velocity shifts of the binary SMBH candidates identified in Ju et al. (2013), using SDSS spectra with an additional epoch of data that lengthens the typical baseline to ~10 yr. W…
The formation of massive black hole binaries (MBHBs) is an unavoidable outcome of galaxy evolution via successive mergers. However, the mechanism that drives their orbital evolution from parsec separations down to the gravitational wave (GW) dominated regime is poorly understood, and their final fate is still unclear. If such binaries are embedded in gas-rich and turbulent environments, as observed in remnants of galaxy mergers, the interaction with gas clumps (such as molecular clouds) may efficiently drive their orbital evolution. Using numerical simulations, we test this hypothesis by study…
