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Register a new userThe growth of stellar mass black hole binaries trapped in an accretion disk of active galactic nuclei
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Among the four black hole binary merger events detected by LIGO, six progenitor black holes have masses greater than 20,$M_odot$. The existence of such massive BHs calls for extreme metal-poor stars as the progenitors. An alternative possibility that a pair of stellar mass black holes each with mass $sim7,M_odot$ increases to $>20,M_odot$ via accretion from a disk surrounding a super massive black hole in an active galactic nucleus is considered. The growth of mass of the binary and the transfer of orbital angular momentum to the disk accelerates the merger. Based on the recent numerical work of Tang et al. (2017), it is found that, in the disk of a low mass AGN with mass $sim10^6,M_odot$ and Eddington ratio $>0.01$, the mass of an individual BH in the binary can grow to $>20,M_odot$ before coalescence provided that accretion takes place at a rate more than 10 times the Eddington value. The mechanism predicts a new class of gravitational wave sources involving the merger of two extreme Kerr black holes associated with active galactic nuclei and a possible electromagnetic wave counterpart.
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The recent advanced LIGO/Virgo detections of gravitational waves (GWs) from stellar binary black hole (BBH) mergers, in particular GW190521, which is potentially associated with a quasar, have stimulated renewed interest in active galactic nuclei (AGNs) as factories of merging BBHs. Compact objects evolving from massive stars are unavoidably enshrouded by a massive envelope to form accretion-modified stars (AMSs) in the dense gaseous environment of a supermassive black hole (SMBH) accretion disk. We show that most AMSs form binaries due to gravitational interaction with each other during radial migration in the SMBH disk, forming BBHs inside the AMS. When a BBH is born, its orbit is initially governed by the tidal torque of the SMBH. Bondi accretion onto BBH at a hyper-Eddington rate naturally develops and then controls the evolution of its orbits. We find that Bondi accretion leads to efficient removal of orbital angular momentum of the binary, whose final merger produces a GW burst. Meanwhile, the Blandford-Znajek mechanism pumps the spin energy of the merged BH to produce an electromagnetic counterpart (EMC). Moreover, hyper-Eddington accretion onto the BBH develops powerful outflows and triggers a Bondi explosion, which manifests itself as a EMC of the GW burst, depending on the viscosity of the accretion flow. Thermal emission from Bondi sphere appears as one of EMCs. BBHs radiate GWs with frequencies $sim 10^{2},$Hz, which are accessible to LIGO.
Accretion disks of active galactic nuclei (AGN) have been proposed as promising sites for producing both (stellar-mass) compact object mergers and extreme mass ratio inspirals. Along with the disk-assisted migration/evolution process, ambient gas materials inevitably accrete onto the compact objects. The description of this process is subject to significant theoretical uncertainties in previous studies. It was commonly assumed that either an Eddington accretion rate or a Bondi accretion rate (or any rate in between) takes place, although these two rates can differ from each other by several orders of magnitude. As a result, the mass and spin evolution of compact objects within AGN disks are essentially unknown. In this work, we construct a relativistic supercritical inflow-outflow model for black hole (BH) accretion. We show that the radiation efficiency of the supercritical accretion of a stellar-mass BH (sBH) is generally too low to explain the proposed electromagnetic counterpart of GW190521. Applying this model to sBHs embedded in AGN disks, we find that, although the gas inflow rates at Bondi radii of these sBHs are in general highly super-Eddington, a large fraction of inflowing gas eventually escapes as outflows so that only a small fraction accretes onto the sBH, resulting in mildly super-Eddington BH absorption in most cases. We also implement this inflow-outflow model to study the evolution of neutron stars (NS) and white dwarfs (WD) in AGN disks, taking into account corrections from star sizes and star magnetic fields. It turns out to be difficult for WDs to grow to the Chandrasekhar limit via accretion because WDs are spun up more efficiently to reach the shedding limit before the Chandrasekhar limit. For NSs the accretion-induced collapse is possible if NS magnetic fields are sufficiently strong, keeping the NS in a slow rotation state during accretion.
We consider a black hole (BH) density cusp in a nuclear star cluster (NSC) hosting a supermassive back hole (SMBH) at its center. Assuming the stars and BHs inside the SMBH sphere of influence are mass-segregated, we calculate the number of BHs that sink into this region under the influence of dynamical friction. We find that the total number of BHs increases significantly in this region due to this process for lower mass SMBHs by up to a factor of 5, but there is no increase in the vicinity of the highest mass SMBHs. Due to the high BH number density in the NSC, BH-BH binaries form during close approaches due to GW emission. We update the previous estimate of OLeary et al. for the rate of such GW capture events by estimating the $langle n^2rangle/langle nrangle^2$ parameter where $n$ is the number density. We find a BH merger rate for this channel to be in the range $sim0.01-0.1 , mathrm{Gpc^{-3}yr^{-1}}$. The total merger rate is dominated by the smallest galaxies hosting SMBHs, and the number of heaviest BHs in the NSC. It is also exponentially sensitive to the radial number density profile exponent, reaching $>100 , mathrm{Gpc^{-3}yr^{-1}}$ when the BH mass function is $m^{-2.3}$ or shallower and the heaviest BH radial number density is close to $r^{-3}$. Even if the rate is much lower than the range constrained by the current LIGO detections, the GW captures around SMBHs can be distinguished by their high eccentricity in the LIGO band.
Massive black hole binaries are naturally predicted in the context of the hierarchical model of structure formation. The binaries that manage to lose most of their angular momentum can coalesce to form a single remnant. In the last stages of this process, the holes undergo an extremely loud phase of gravitational wave emission, possibly detectable by current and future probes. The theoretical effort towards obtaining a coherent physical picture of the binary path down to coalescence is still underway. In this paper, for the first time, we take advantage of observational studies of active galactic nuclei evolution to constrain the efficiency of gas-driven binary decay. Under conservative assumptions we find that gas accretion toward the nuclear black holes can efficiently lead binaries of any mass forming at high redshift (> 2) to coalescence within the current time. The observed downsizing trend of the accreting black hole luminosity function further implies that the gas inflow is sufficient to drive light black holes down to coalescence, even if they bind in binaries at lower redshifts, down to z~0.5 for binaries of ~10 million solar masses, and z~0.2 for binaries of ~1 million solar masses. This has strong implications for the detection rates of coalescing black hole binaries of future space-based gravitational wave experiments.
Supermassive black hole binaries are likely to accrete interstellar gas through a circumbinary disk. Shortly before merger, the inner portions of this circumbinary disk are subject to general relativistic effects. To study this regime, we approximate the spacetime metric of close orbiting black holes by superimposing two boosted Kerr-Schild terms. After demonstrating the quality of this approximation, we carry out very long-term general relativistic magnetohydrodynamic simulations of the circumbinary disk. We consider black holes with spin dimensionless parameters of magnitude 0.9, in one simulation parallel to the orbital angular momentum of the binary, but in another anti-parallel. These are contrasted with spinless simulations. We find that, for a fixed surface mass density in the inner circumbinary disk, aligned spins of this magnitude approximately reduce the mass accretion rate by 14% and counter-aligned spins increase it by 45%, leaving many other disk properties unchanged.
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