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 AGN disks are $\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$ for mass functions (MF) $M^{-1(-2)}$. If a fraction $f_{AGN}$ of the observed rate of BH-BH mergers (${\cal}{R}_{BH-BH}$) come from AGN, we expect a rate of NS-BH (NS-NS) mergers from the AGN channel of ${\cal}{R}_{BH-NS} \sim f_{AGN}[10,300]\rm{Gpc}^{-3} \rm{yr}^{-1}$,(${\cal}{R}_{NS-NS} \leq f_{AGN}400\rm{Gpc}^{-3} \rm{yr}^{-1}$). Allowing for the ratio of NS-NS/BH-BH LIGO search volumes, from preliminary O3 results we can rule out the AGN channel as a dominant contribution to observed NS-NS mergers. The number of lower mass gap events in this channel is a strong function of the nuclear MF and mass segregation efficiency. Compact object merger ratios derived from LIGO can restrict models of MF, mass segregation and populations embedded in AGN disks. The expected number of EM counterparts to NS-BH mergers in AGN disks at $z<1$ is $\sim [30,900]{\rm{yr}}^{-1}(f_{AGN}/0.1)$. EM searches for flaring due to embedded events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them.
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…
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…
Double white dwarf (double-WD) binaries may merge within a Hubble time and produce high-mass WDs. Compared to other high-mass WDs, the double-WD merger products have higher velocity dispersion because they are older. With the power of Gaia data, we show strong evidence for double-WD merger products among high-mass WDs by analyzing the transverse-velocity distribution of more than a thousand high-mass WDs (0.8--1.3 $M_\odot$). We estimate that the fraction of double-WD merger products in our sample is about 20 %. We also obtain a precise double-WD merger rate and its mass dependence. Our merger…
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…
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…
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 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…
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…
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 …
Mergers of two carbon-oxygen white dwarfs have long been suspected to be progenitors of Type Ia Supernovae. Here we present our modifications to the cosmological smoothed particle hydrodynamics code Gadget to apply it to stellar physics including but not limited to mergers of white dwarfs. We demonstrate a new method to map a one-dimensional profile of an object in hydrostatic equilibrium to a stable particle distribution. We use the code to study the effect of initial conditions and resolution on the properties of the merger of two white dwarfs. We compare mergers with approximate and exact b…
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…
Mergers of white dwarfs (WDs) may lead to a variety of transient astrophysical events, SNIa being one possible outcome. Lyutikov & Toonen (2017, 2019) argued that mergers of WDs result, under various parameter regimes, in unusual central engine-powered supernova and a type of short Gamma Ray Bursts that show extended emission tails. Observations by Gvaramadze et al. (2019) of the central star and the nebula J005311 match to the details the model of Lyutikov & Toonen (2017, 2019) for the immediate product of a merger of a heavy ONeMg WD with CO WD (age, luminosity, stellar size, hydrogen defici…
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…
Following the GRB 170817A prompt emission lasting a fraction of a second, $10^8$ s of data in the X-rays, optical, and radio wavelengths have been acquired. We here present a model that fits the spectra, flux, and time variability of all these emissions, based on the thermal and synchrotron cooling of the expanding matter ejected in a binary white dwarf merger. The $10^{-3} M_\odot$ of ejecta, expanding at velocities of $10^9$ cm s$^{-1}$, are powered by the newborn massive, fast rotating, magnetized white dwarf with a mass of $1.3 M_\odot$, a rotation period of $\gtrsim 12$ s, and a dipole ma…
The merger of two carbon-oxygen white dwarfs can lead either to a spectacular transient, stable nuclear burning or a massive, rapidly rotating white dwarf. Simulations of mergers have shown that the outcome strongly depends on whether the white dwarfs are similar or dissimilar in mass. In the similar-mass case, both white dwarfs merge fully and the remnant is hot throughout, while in the dissimilar case, the more massive, denser white dwarf remains cold and essentially intact, with the disrupted lower mass one wrapped around it in a hot envelope and disk. In order to determine what constitutes…
It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star--white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by \cite{mar19}. By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic fiel…
Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly-rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf bin…
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…
We present a nucleosynthesis study of the merger of a 0.4 solar masses helium white dwarf with a 0.8 solar masses carbon-oxygen white dwarf, coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.
We search for merger products among the 25 most massive white dwarfs in the Montreal White Dwarf Database 100 pc sample through follow-up spectroscopy and high-cadence photometry. We find an unusually high fraction, 40%, of magnetic white dwarfs among this population. In addition, we identify four outliers in transverse velocity and detect rapid rotation in five objects. Our results show that $56^{+9}_{-10}$\% of the $M\approx1.3~M_{\odot}$ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculatio…
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 …
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…
One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of $0.9 \mathrm{M_\odot}$ and $1.1 \mathrm{M_\odot}$ combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion un…
Type Ia supernovae (SNe Ia) are thought to result from thermonuclear explosions of carbon-oxygen white dwarf stars. Existing models generally explain the observed properties, with the exception of the sub-luminous 1991-bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but hitherto simulations have failed to produce an explosion. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to an underluminous explosion, though at the expense of requiring a single common-envelope phase, and component mass…
We systematically investigate the mergers of neutron star-white dwarf binaries from beginning to end, with focus on the properties of the inflows and outflows in accretion disks and their electromagnetic emissions. Using population synthesis models, we determine a subset of these binaries in which the white dwarf companion undergoes unstable mass transfer and complete tidal disruption, forming a large accretion disk around the neutron star. The material evolves according to an advection-dominated accretion disk model with nuclear burning, neutrino-emissions, and disk-surface wind ejection. The…
Stellar mergers and chemical element mixing: implications for the metamorphic stellar evolution in AGN disks
Canadian Institute for Theoretical Astrophysics, University of Toronto（多伦多大学理论天体物理研究所）；TAPIR, MC 350-17, California Institute of Technology（加州理工学院）；Department of Astronomy and Astrophysics, University of California, Santa Cruz（加州大学圣克鲁兹分校天文与天体物理系）；Institute for Advanced Study, Tsinghua University（清华大学高等研究院）；Department of Astronomy, Westlake University（西湖大学天文系）
We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some…
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 evolution of the remnant of the merger of two white dwarfs is still an open problem. Furthermore, few studies have addressed the case in which the remnant is a magnetic white dwarf with a mass larger than the Chandrasekhar limiting mass. Angular momentum losses might bring the remnant of the merger to the physical conditions suitable for developing a thermonuclear explosion. Alternatively, the remnant may be prone to gravitational or rotational instabilities, depending on the initial conditions reached after the coalescence. Dipole magnetic braking is one of the mechanisms that can drive s…
Supernovae interacting with Si and S-rich circumstellar matter from double white dwarf mergers
National Astronomical Observatory of Japan, National Institutes of Natural Sciences（日本国立天文台，日本国立科学研究所）；Graduate Institute for Advanced Studies, SOKENDAI（综合研究大学院大学）；School of Physics and Astronomy, Monash University（蒙纳士大学物理与天文学学院）；International Center of Supernovae (ICESUN), Yunnan Key Laboratory of Supernova Research, Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台超新星国际研究中心（ICESUN），云南省超新星研究重点实验室）
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. …
Merger of a white dwarf binary creates a differentially rotating object which is expected to generate strong magnetic fields. Kinetic energy stored in differential rotation is partially dissipated in the magnetically dominated corona, which forms a hot variable outflow with ejection velocity comparable to $10^9$ cm s$^{-1}$. The outflow should carry significant mass and energy for hours to days, creating an expanding fireball with the following features. (i) The fireball is initially opaque and its internal energy is dominated by the trapped thermal radiation. The stored heat is partially conv…
The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $α$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towar…
The diverse outcomes of binary white dwarf mergers and connections to Galactic LISA sources
University of California, San Diego（加州大学圣地亚哥分校）；Carnegie Mellon University（卡内基梅隆大学）；Canadian Institute for Theoretical Astrophysics, University of Toronto（多伦多大学加拿大理论天体物理研究所）
Violent mergers revisited: The origin of the fastest stars in the Galaxy
Max-Planck-Institut für Astrophysik（马克斯·普朗克天体物理学研究所）；University of California, Berkeley（加州大学伯克利分校）；Universität Potsdam（波茨坦大学）；Trinity College Dublin（都柏林圣三一学院）；Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室）；Center for Astrophysics | Harvard & Smithsonian（哈佛与史密森尼天体物理中心）；Queen’s University Belfast（贝尔法斯特女王大学）；Zentrum für Astronomie der Universität Heidelberg（海德堡大学天文中心）；Heidelberger Institut für Theoretische Studien（海德堡理论研究所）
The merger of two white dwarfs is expected to result in a central fast rotating core surrounded by a debris disk, in which magnetorotational instabilities give rise to a hot magnetized corona and a magnetized outflow. The dissipation of magnetic energy via reconnection could lead to the acceleration of cosmic-rays in the expanding material, which would result in high energy neutrinos. We discuss the possibility of using these neutrino signals as probes of the outflow dynamics, magnetic energy dissipation rate and cosmic-ray acceleration efficiency. Importantly, the accompanying high-energy gam…
We perform stellar evolution calculations of the remnant of the merger of two He white dwarfs (WDs). Our initial conditions are taken from hydrodynamic simulations of double WD mergers and the viscous disc phase that follows. We evolve these objects from shortly after the merger into their core He-burning phase, when they appear as hot subdwarf stars. We use our models to quantify the amount of H that survives the merger, finding that it is difficult for $\gtrsim 10^{-4}\;{\rm M}_\odot$ of H to survive, with even less being concentrated in the surface layers of the object. We also study the ro…
It has been recently proposed that the ejected matter from white dwarf (WD) binary mergers can produce transient, optical and infrared emission similar to the "kilonovae" of neutron star (NS) binary mergers. To confirm this we calculate the electromagnetic emission from WD-WD mergers and compare with kilonova observations. We simulate WD-WD mergers leading to a massive, fast rotating, highly magnetized WD with an adapted version of the smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial conditions for the ejecta such as escape velocity, mass and initial position and dist…
We use a novel method to predict the contribution of normal star-forming galaxies, merger-induced bursts, and obscured AGN, to IR luminosity functions (LFs) and global SFR densities. We use empirical halo occupation constraints to populate halos with galaxies and determine the distribution of normal and merging galaxies. Each system can then be associated with high-resolution hydrodynamic simulations. We predict the distribution of observed luminosities and SFRs, from different galaxy classes, as a function of redshift from z=0-6. We provide fitting functions for the predicted LFs, quantify th…
We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium…
We present the results of a set of high-resolution simulations of the merging process of two white dwarfs. In order to do so, we use an up-to-date Smoothed Particle Hydrodynamics code which incorporates very detailed input physics and an improved treatment of the artificial viscosity. Our simulations have been done using a large number of particles (4x10^5) and cover the full range of masses and chemical compositions of the coalescing white dwarfs. We also compare the time evolution of the system during the first phases of the coalescence with that obtained using a simplified treatment of mass…
Despite their unique astrophysical relevance, the outcome of white dwarf binary mergers has so far only been studied for a very restricted number of systems. Here we present the results of a survey with more than two hundred simulations systematically scanning the white dwarf binary parameter space. We consider white dwarf masses ranging from 0.2 to 1.2 $M_\odot$ and account for their different chemical compositions. We find excellent agreement with the orbital evolution predicted by mass transfer stability analysis. Much of our effort in this paper is dedicated to determining which binary sys…
Close binaries of double white dwarfs (DWDs) inspiral and merge by emitting gravitational wave (GW). Orbital motion of some of these binaries are expected to be observed at low frequency band by space-borne laser interferometric detectors of GW. The merger remnant may suffer thermonuclear runaway and explode as type Ia supernova if they are massive enough. As GW sources the remnants have so far been scarcely studied. Here we propose a new mechanism of GW emission from DWD merger remnants which may be observed by planned GW detectors in decihertz frequency band. A remnant is temporarily expecte…
