The heaviest neutron stars and lightest black holes expected to be produced by stellar evolution leave the mass-range $2.2$ M$_{\odot}\lesssim m \lesssim 5$ M$_\odot$ largely unpopulated. Objects found in this so-called lower mass gap likely originate from a distinct astrophysical process. Such an object, with mass $2.6$ M$_\odot$ was recently detected in the binary merger GW190814 through gravitational waves by LIGO/Virgo. Here we show that black holes in the mass gap are naturally assembled through mergers and accretion in AGN disks, and can subsequently participate in additional mergers. We compute the properties of AGN-assisted mergers involving neutron stars and black holes, accounting for accretion. We find that mergers in which one of the objects is in the lower mass gap represent up to $4$% of AGN-assisted mergers detectable by LIGO/Virgo. The lighter object of GW190814, with mass $2.6$ M$_\odot$, could have grown in an AGN disk through accretion. We find that the unexpectedly high total mass of 3.4 M$_\odot$ observed in the neutron star merger GW190425 may also be due to accretion in an AGN disk.
We review recent progress in studying accretion flows on to supermassive black holes (SMBH). Much of this removes earlier assumptions of symmetry and regularity, such as aligned and prograde disc rotation. This allows a much richer variety of effects, often because cancellation of angular momentum allows rapid infall. Potential applications include lower SMBH spins allowing faster mass growth and suppressing gravitational-wave reaction recoil in mergers, gas-assisted SMBH mergers, and near-dynamical accretion in galaxy centres.
We present the results of 2D, moving mesh, viscous hydrodynamical simulations of accretion onto merging supermassive black hole (SMBH) binaries. We include viscous heating, shock heating, and radiative cooling, and simulate the transition from the "pre-decoupling" epoch, where the inspiral timescale is longer than the viscous timescale, to the "post-decoupling" epoch, where the inspiral timescale is shorter than the viscous timescale. We find that there is no abrupt halt to the accretion at decoupling, but rather the accretion shows a slow decay, with significant accretion well after the expec…
Stellar-mass binary black hole mergers are poised to represent the majority of gravitational-wave (GW) observations by Advanced LIGO and Virgo. Probing their origin will be difficult due to the expected lack of electromagnetic emission and limited localization accuracy. Associations with rare host galaxy types -- such as active galactic nuclei (AGN) -- can nevertheless be identified statistically through spatial correlation. We show that (i) fractional contributions $f_{\rm agn}=50-100\%$ from AGN hosts to the total BBH merger rate can be statistically established with 70-300 detected events (…
The growing number of stellar-mass binary black hole mergers discovered by Advanced LIGO and Advanced Virgo are starting to constrain the binaries' origin and environment. However, we still lack sufficiently accurate modeling of binary formation channels to obtain strong constraints, or to identify sub-populations. One promising formation mechanism that could result in different black hole properties is binaries merging within the accretion disks of Active Galactic Nuclei (AGN). Here we show that the black holes' orbital alignment with the AGN disks preferentially selects heavier black holes. …
Recently, Graham et al. (2020) identified ZTF19abanrhr as a candidate electromagnetic counterpart to the binary black hole merger GW190521. The authors argue that the observations are consistent with a kicked binary black hole interacting with the accretion disk of the activate galactic nucleus AGN J124942.3+344929. If a real association (rather than happenstance), this has implications for the sources of LIGO/Virgo binary mergers, future prospects for electromagnetic counterparts, and measurements of the expansion rate of the Universe. In this Letter, we provide a re-analysis of the multi-mes…
Active Galactic Nuclei (AGNs) hosting disk water megamasers are well known to be obscured by large amounts of gas, likely due to the presence along the line of sight of an almost edge-on disky structure orbiting the supermassive black hole. Correcting for the high obscuration is crucial to infer parameters intrinsic to the source, like its luminosity. We present a broadband X-ray spectral analysis of a water megamaser AGN in an early merger (NGC 5765B), combining Chandra and NuSTAR data. NGC 5765B is highly Compton-thick and reflection-dominated, following the general trend among disk megamase…
Advanced LIGO \& 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) disks of gas around super-masssive black holes. Here we investigate the relative number of compact object mergers in AGN disk models, including BH, neutron stars (NS) and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disk grows $\propto t^{1/3}$ which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS-BH mergers in…
We present results from a multi-wavelength analysis searching for multiple AGN systems in nearby (z<0.077) triple galaxy mergers. Combining archival Chandra, SDSS, WISE, and VLA observations, we quantify the rate of nearby triple AGN, as well as investigate possible connections between SMBH accretion and merger environments. Analyzing the multi-wavelength observations of 7 triple galaxy mergers, we find that 1 triple merger has a single AGN (NGC 3341); we discover, for the first time, 4 likely dual AGN (SDSS J1027+1749, SDSS J1631+2352, SDSS J1708+2153, and SDSS J2356-1016); we confirm one tri…
GRB 170817A, detected by Fermi-GBM 1.7\,s after the merger of a neutron star (NS) binary, provides the first direct evidence for a link between such a merger and a short-duration gamma-ray burst. The X-ray observations after GRB 170817A indicate a possible X-ray flare with a peak luminosity $L_{\rm peak} \sim 2\times 10^{39}\,{\rm erg\,s}^{-1}$ near day 156. Here we show that this X-ray flare may be understood based on a slim disc around a compact object. On the one hand, there exists the maximal accretion rate $\dot M_{\rm max}$ for the slim disc, above which an optically thick outflow is sig…
Accretion disks around supermassive black holes are promising sites for stellar mass black hole mergers detectable with LIGO. Here we present the results of Monte-Carlo simulations of black hole mergers within 1-d AGN disk models. For the spin distribution in the disk bulk, key findings are: (1) The distribution of $χ_{\rm eff}$ is naturally centered around $\tildeχ_{\rm eff} \approx 0.0$, (2) the width of the $χ_{\rm eff}$ distribution is narrow for low natal spins. For the mass distribution in the disk bulk, key findings are: (3) mass ratios $\tilde{q} \sim 0.5-0.7$, (4) the maximum merger m…
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 (AGN) should always exceed merger rates in quiescent galactic nuclei (nuclear star clusters, NSCs) around supermassive BHs (SMBHs) without accretion disks. This is primarily due to average binary lifetimes in AGN that are significantly shorter than in NSCs. The lifetime difference comes from rapid hardening of BBHs in AGN, such that their semi-major axes are smaller than the hard-soft b…
The majority of gravitational wave (GW) events detected so far by LIGO/Virgo originate from binary black hole (BBH) mergers. Among the different binary evolution paths, the merger of BBHs in accretion discs of active galactic nuclei (AGNs) is a possible source of GW detections. We consider an idealised analytical model of the orbital evolution of BBHs embedded in an AGN accretion disc. In this framework, the disc-binary interaction increases the orbital eccentricity and decreases the orbital separation, driving the BBH into a regime where GW emission eventually leads to coalescence. We compute…
Dual AGN are important for understanding galaxy-merger-triggered fueling of black holes and hierarchical growth of structures. The least explored type of dual AGN are those associated with mergers of two dwarf galaxies. According to observations and cosmological simulations, dwarf galaxies are the most abundant type of galaxies in the early Universe and the galaxy merger rate is dominated by dwarfs. However, these mergers are generally too distant to be directly observed, and low-redshift dwarf-dwarf merger-related dual AGN are notoriously hard to find. In this paper, we present the first resu…
There is some weak evidence that the black hole merger named GW190521 had a non-zero eccentricity. In addition, the component black holes' masses exceeded the limit predicted by stellar evolution. The large masses can be explained by successive mergers, which may be efficient in gas disks surrounding active galactic nuclei (AGN), but it is difficult to maintain an eccentric orbit all the way to the merger, as basic physics would argue for circularization. Here we show that AGN-disk environments can lead to an excess of eccentric mergers, if the interactions between single and binary black hole…
In this proceeding we explore a pathway to radio-loudness under the hypothesis that retrograde accretion onto giant spinning black holes leads to the launch of powerful jets, as seen in radio loud QSOs and recently in LAT/Fermi and BAT/Swift Blazars. Counter-rotation of the accretion disc relative to the BH spin is here associated to gas-poor galaxy mergers progenitors of giant (missing-light) ellipticals. The occurrence of retrograde accretion enters as unifying element that may account for the radio-loudness/galaxy morphology dichotomy observed in AGN.
Two Mpc-size contact discontinuities have recently been identified in the XMM-Newton and Suzaku X-ray observations of the outskirts of the Perseus cluster (Walker et al. 2020). These structures have been tentatively interpreted as "sloshing cold fronts", which are customarily associated with differential motions of the cluster gas, perturbed by a merger. In this study we consider an alternative scenario, namely, that the most prominent discontinuity near the cluster virial radius is the result of the collision between the accretion shock and the "runaway" merger shock. We also discuss the poss…
The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of Active Galactic Nuclei (AGN), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 Luminous and Ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGN that are Com…
The accretion disks of active galactic nuclei (AGN) 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 AGN are the only GW channel where an electromagnetic (EM) counterpart must occur (whether detectable or not). Considering AGN 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 fi…
We present post-Newtonian $N$-body simulations on mergers of accreting stellar-mass black holes (BHs), where such general relativistic effects as the pericenter shift and gravitational wave (GW) emission are taken into consideration. The attention is concentrated on the effects of the dynamical friction and the Hoyle-Lyttleton mass accretion by ambient gas. We consider a system composed of ten BHs with initial mass of $30~M_\odot$. As a result, we show that mergers of accreting stellar-mass BHs are classified into four types: a gas drag-driven, an interplay-driven, a three body-driven, or an a…
The recently discovered gravitational wave sources GW190521 and GW190814 have shown evidence of BH mergers with masses and spins that could be outside of the range expected from isolated stellar evolution. These merging objects could have undergone previous mergers. Such hierarchical mergers are predicted to be frequent in active galactic nuclei (AGN) disks, where binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Here we compare the properties of these observed events to the theoretical models of mergers in AGN disks, which are obtained by performing one-d…
We present results from our X-ray analysis of the first systematic search for triple AGN in nearby (z<0.077) triple galaxy mergers. We analyze archival Chandra observations of 7 triple galaxy mergers with BAYMAX (Bayesian Analysis of Multiple AGN in X-rays), fitting each observation with single, dual, and triple X-ray point source models. In doing so, we conclude that 1 triple merger has one X-ray point source (SDSS J0858+1822, although it's unlikely to be an AGN); 5 triple mergers are likely composed of two X-ray point sources (NGC 3341, SDSS J1027+1749, SDSS J1631+2352, SDSS J1708+2153, and …
From Liu and Han (2014), the accretion-dominated jet power has a linear proportionality with the accretion rate, whereas the power law index is <=0.5 at lower accretion rate. Attributing the jet power in low accretion rate AGN to the black hole spin, it implies that the jet power has a flatter spectrum than the accretion-dominated jet versus the accretion rate. The black hole must be spinning rapidly for producing such jet power efficiently, and this may allow us to find high-spin black holes in the radio-loud low-luminosity AGN.
We present new numerical relativity results of neutron star mergers with chirp mass $1.188M_\odot$ and mass ratios $q=1.67$ and $q=1.8$ using finite-temperature equations of state (EOS), approximate neutrino transport and a subgrid model for magnetohydrodynamics-induced turbulent viscosity. The EOS are compatible with nuclear and astrophysical constraints and include a new microphysical model derived from ab-initio calculations based on the Brueckner-Hartree-Fock approach. We report for the first time evidence for accretion-induced prompt collapse in high-mass-ratio mergers, in which the tidal…
The astrophysical origin of gravitational wave (GW) transients is a timely open question in the wake of discoveries by LIGO/Virgo. In active galactic nuclei (AGNs), binaries form and evolve efficiently by interaction with a dense population of stars and the gaseous AGN disk. Previous studies have shown that stellar-mass black hole (BH) mergers in such environments can explain the merger rate and the number of suspected hierarchical mergers observed by LIGO/Virgo. The binary eccentricity distribution can provide further information to distinguish between astrophysical models. Here we derive the…
Remnant accretion disks formed in compact object mergers are an important ingredient in the understanding of electromagnetic afterglows of multi-messenger gravitational-wave events. Due to magnetically and neutrino driven winds, a significant fraction of the disk mass will eventually become unbound and undergo r-process nucleosynthesis. While this process has been studied in some detail, previous studies have typically used approximate initial conditions for the accretion disks, or started from purely hydrodynamical simulations. In this work, we analyse the properties of accretion disks formed…
The likely outcome of a compact object merger event is a central black hole surrounded by a rapidly accreting torus of debris. This disk of debris is a rich source of element synthesis, the outcome of which is needed to predict electromagnetic counterparts of individual events and to understand the contribution of mergers to galactic chemical evolution. Here we study disk outflow nucleosynthesis in the context of a two-dimensional, time-dependent black hole-neutron star merger accretion disk model. We use two time snapshots from this model to examine the impact of the evolution of the neutrino…
A compact accretion disk may be formed in the merger of two neutron stars or of a neutron star and a stellar-mass black hole. Outflows from such accretion disks have been identified as a major site of rapid neutron-capture (r-process) nucleosynthesis and as the source of 'red' kilonova emission following the first observed neutron-star merger GW170817. We present long-term general-relativistic radiation magnetohydrodynamic simulations of a typical post-merger accretion disk at initial accretion rates of $\dot{M}\sim 1\,M_\odot\,\text{s}^{-1}$ over 400ms post-merger. We include neutrino radiati…
Neutron star mergers are believed to occur in accretion disks around supermassive black holes. Here we show that a putative jet launched from the merger of a binary neutron star (BNS) or a neutron star--black hole (NSBH) merger occurring at the migration trap in an active galactic nucleus (AGN) disk would be choked. The jet energy is deposited within the disk materials to power a hot cocoon. The cocoon is energetic enough to break out from the AGN disk and produce a bright X-ray shock breakout transient peaking at $\sim0.15\,{\rm d}$ after the merger. The peak luminosity is estimated as $\sim …
By a method of population synthesis we construct a model of dark matter (DM) accretion onto binary black holes (BHs), and investigate the merger rate of the binary BHs. We find that the merger rate can weakly increase (less than 10\%). However, the DM accretion can efficiently enhance the masses of binary BHs. In our model, the result for $Z = 0.01$ without the DM accretion can not explain the GW170104, GW170814, and GW150914, while with the DM accretion, it can cover all observations well. For the higher metallicity ($Z = 0.02$), our model can not explain the mergers of high-mass binary BHs l…
Binary active galactic nuclei (AGNs) provide clues to how gas-rich mergers trigger and fuel AGNs and how supermassive black hole (SMBH) pairs evolve in a gas-rich environment. While significant effort has been invested in their identification, the detailed properties of binary AGNs and their host galaxies are still poorly constrained. In a companion paper, we examined the nature of ionizing sources in the double nuclei of four kpc-scale binary AGNs with redshifts between 0.1~0.2. Here, we present their host galaxy morphology based on F336W (U-band) and F105W (Y-band) images taken by the Wide F…
Electromagnetic Counterparts of Stellar-mass Binary Black Hole Mergers in AGN Accretion Disks: Theoretical Models and Observational Status
University of Science and Technology of China（中国科学技术大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；Guizhou University（贵州大学）
Accretion disc theory predicts that an AGN disc becomes self-gravitating and breaks up into stars at an outer radius $R_{\rm sg}$ ~ 12 light-days, with effectively no free parameter. We present evidence that the longer observed AGN light echoes are all close to 12d in the AGN rest frames. These observations give a stringent test of AGN disc theory. Further monitoring should offer insight into the formation angular momentum of the gas forming the disc. For distant AGN, observed lags significantly longer than 12d give lower limits on their redshifts.
This paper estimates the specific accretion-rate distribution of AGN using a sample of 4821 X-ray sources from both deep and shallow surveys. The specific accretion-rate distribution is defined as the probability of a galaxy with a given stellar mass and redshift hosting an active nucleus with a certain specific accretion rate. We find that the probability of a galaxy hosting an AGN increases with decreasing specific accretion rate. There is evidence that this trend reverses at low specific accretion rates, $λ<10^{-4}-10^{-3}$ (in Eddington units). There is also a break close to the Eddington …
We perform the first suite of fully general relativistic magnetohydrodynamic simulations of spinning massive black hole binary mergers. We consider binary black holes with spins of different magnitudes aligned to the orbital angular momentum, which are immersed in a hot, magnetized gas cloud. We investigate the effect of the spin and degree of magnetization (defined through the fluid parameter $β^{-1}\equiv p_{\mathrm{mag}}/p_{\mathrm{fluid}}$) on the properties of the accretion flow. We find that magnetized accretion flows are characterized by more turbulent dynamics, as the magnetic field li…
Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic timescale $\sim 10^5$ yr. We show that this is the timescale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion disc feeding event.
We investigate optical, infrared, and radio active galactic nucleus (AGN) signs in the merger remnant Arp 187, which hosts luminous jets launched in the order of $10^5$ yr ago but whose present-day AGN activity is still unknown. We find AGN signs from the optical BPT diagram and infrared [OIV]25.89 $μ$m line, originating from the narrow line regions of AGN. On the other hand, Spitzer/IRS show the host galaxy dominated spectra, suggesting that the thermal emission from the AGN torus is considerably small or already diminished. Combining the black hole mass, the upper limit of radio luminosity o…
Black hole mergers detectable with LIGO can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum and the spin spectrum of black holes (BH) in AGN disks. The predicted merger rate spans $\sim 10^{-4}-10^{4} \rm{Gpc}^{-1} \rm{yr}^{-1}$, so upper limits from LIGO ($<212\rm{Gpc}^{-1}\rm{yr}^{-1}$) already constrain it. The predicted mass spectrum has the form of a broken power-law consisting of a pre-existing BH powerlaw mass spectrum and a harder powerlaw mass spectrum resulting from mergers. The predicted spin spectrum is multi-peaked with the ev…
We explore the effects of r-process nucleosynthesis on fall-back accretion in neutron star(NS)-NS and black hole-NS mergers, and the resulting implications for short-duration gamma-ray bursts (GRBs). Though dynamically important, the energy released during the r-process is not yet taken into account in merger simulations. We use a nuclear reaction network to calculate the heating (due to beta-decays and nuclear fission) experienced by material on the marginally-bound orbits nominally responsible for late-time fall-back. Since matter with longer orbital periods t_orb experiences lower densities…
Massive black hole (MBH) coalescences are powerful sources of low-frequency gravitational waves. To study these events in the cosmological context we need to trace the large-scale structure and cosmic evolution of a statistical population of galaxies, from dim dwarfs to bright galaxies. To cover such a large range of galaxy masses, we analyse two complementary simulations: Horizon-AGN with a large volume and low resolution which tracks the high-mass (> 1e7 Msun) MBH population, and NewHorizon with a smaller volume but higher resolution that traces the low-mass (< 1e7 Msun) MBH population. Whil…
