Accretion and Orbital Inspiral in Gas-Assisted Supermassive Black Hole Binary Mergers

Abstract

Many galaxies are expected to harbor binary supermassive black holes (SMBHs) in their centers. Their interaction with the surrounding gas results in the accretion and exchange of angular momentum via tidal torques, facilitating binary inspiral. Here, we explore the non-trivial coupling between these two processes and analyze how the global properties of externally supplied circumbinary disks depend on the binary accretion rate. By formulating our results in terms of the angular momentum flux driven by internal stresses, we come up with a very simple classification of the possible global disk structures, which differ from the standard constantṀ accretion disk solution. The suppression of accretion by the binary tides, leading to a significant mass accumulation in the inner disk, accelerates binary inspiral. We show that once the disk region strongly perturbed by the viscously transmitted tidal torque exceeds the binary semimajor axis, the binary can merge in less than its mass-doubling time due to accretion. Thus, unlike the inspirals driven by stellar scattering, the gas-assisted merger can occur even if the binary is embedded in a relatively low-mass disk (lower than its own mass). This is important for resolving the "last parsec" problem for SMBH binaries and understanding powerful gravitational wave sources in the universe. We argue that the enhancement of accretion by the binary found in some recent simulations cannot persist for a long time and should not affect the long-term orbital inspiral. We also review existing simulations of SMBH binary-disk coupling and propose a numerical setup which is particularly well suited to verifying our theoretical predictions.

Proper motion in lensed radio jets at redshift 3: A possible dual super-massive black hole system in the early Universe

“…To some extent, this is expected given the increased merger rate (expected at high redshift), but to catch one in the act of forming would also mean that the in-spiral time needs to be slower than what is currently predicted (e.g. Rafikov 2016). As this conclusion is currently based on the statistics of this single possible detection, further studies of other lensed radio sources at high angular resolution with VLBI are needed to determine if there are other cases with proper motion in the radio jets, and whether such motion is also consistent with a DAGN system.…”

Evolution of supermassive black hole binaries and tidal disruption rates in non-spherical galactic nuclei

“…Three-body interactions between the pair of SMBHs and stars of the galactic nucleus klezhnin@princeton.edu, eugvas@lpi.ru typically result in an ejection of the lighter of the three bodies (the star), extracting the energy from the binary orbital motion (Quinlan 1996). Torques from the circumbinary gaseous discs also lead to the shrinking of the binary (e.g., Mayer 2013;Rafikov 2016;Tang et al 2017), although this process appears to be effective only when the separation becomes small enough (Lodato et al 2009). Finally, when the separation between the two SMBHs reaches 10 −3 pc, the emission of gravitational waves (GW) becomes efficient enough and leads to the ultimate coalescence of the SMBHs.…”

Non-steady-state long-term evolution of supermassive black hole binaries surrounded by accretion discs

“…Finally, we compare our results with the work of Rafikov (2016), which presented analytical estimates of the circumbinary disc structure, assuming steady-state evolution. Note that Rafikov (2016) did not include the binary torque directly in the evolutionary equations.…”

“…Since this process is (at least) a 2D effect, we cannot directly compute it, and we need to use a recipe to include it in our code. Some possibilities could be using a constant accretion (Kocsis et al 2012a,b), a fraction of the external accretion (Rafikov 2016), or a fraction of the steady-state accretion from a standard single BH Shakura-Sunyaev disc. Using any of these alternative implies the inflow is tied directly to the viscous evolution of the circumbinary disc.…”

“…A and B in each snapshot, at different binary separations, for our simulations with γ = 10 −4 . The bottom panel shows our numerical r (R S ) The radial dependence of the accretion rate in our simulations, compared to the semi-analytic results ofRafikov (2016) which are expected to be valid far away from the binary. The solid curves show the simulation results for three different binary separations, as labeled, while the dashed curves show analytic fits to the results ofRafikov (2016) from Eq.…”