[Paper Review] Accretion onto a Supermassive Black Hole Binary Before Merger

Mark Avara, Julian H. Krolik+4 more|arXiv (Cornell University)|May 29, 2023

Black Holes and Theoretical Physics8 citations

TL;DR

This paper presents the first relativistic 3D GRMHD simulation of equal-mass, non-spinning supermassive binary black holes inspiraling from 20M to ~9M, showing sustained accretion with a modest drop and rich electromagnetic signatures.

ABSTRACT

While supermassive binary black holes inspiral toward merger they may also experience significant accretion of matter from a surrounding disk. We study the dynamics of this system, simultaneously describing the evolving spacetime and magnetized plasma, and present the first relativistic calculation simulating two equal-mass, non-spinning black holes as they inspiral from a $20M$ ($G=c=1$) initial separation almost to merger, $\simeq 9M$ ($M$=binary mass). Our dynamical results imply important observational consequences: for instance, the accretion rate $\dot M$ onto the black holes first decreases and then reaches a plateau, dropping by only a factor of $\sim 3$ despite the rapid inspiral. An estimated bolometric light curve thus suggests some merging SMBBHs may be quite luminous past the predicted decoupling from the circumbinary disk. The minidisks through which the accretion reaches the black holes are very non-standard: Reynolds, not Maxwell, stresses dominate, and they oscillate between two states. In one part of the cycle, ``sloshing" streams transfer mass from one minidisk to the other through the L1 point at a rate $\sim 0.1 imes$ the accretion rate, carrying kinetic energy at a rate that can be as large as the peak minidisk bolometric luminosity. We also discover that episodic accretion drives minidisks with time-varying tilts. The unsigned poloidal magnetic flux on the black hole event horizon is roughly constant at a dimensionless level $ϕ\sim 2-3$, but doubles just before merger; if the black holes had significant spin, this flux indicates the potential for powerful jets with variability driven by binary dynamics, another prediction of potentially unique EM signatures. This simulation is the first to employ our multipatch infrastructure \pwmhd, decreasing computational expense to $\sim 3\%$ of conventional single-grid methods' cost.

Motivation & Objective

• Investigate gas dynamics and accretion onto a binary black hole system as it inspirals toward merger.
• Determine how the circumbinary disk feeds minidisks and the binary, including time-dependent accretion rates.
• Identify observable electromagnetic signatures associated with late-stage inspiral and approaching merger.
• Explore the role of magnetic fields, torques, and disk instabilities on accretion and emission.

Proposed method

• Solve general relativistic MHD equations in a time-dependent spacetime approximated by matching Schwarzschild patches to a 2.5PN/3.5PN spacetime for the binary.
• Use a new multipatch infrastructure (PatchworkMHD) with two patches (outer spherical CBD region and inner Cartesian minidisks region) to avoid coordinate singularities and reduce computational cost.
• Evolve magnetized gas with ideal MHD and a cooling prescription based on a target entropy to radiate dissipative heat.
• Represent the magnetic field evolution with constrained transport (FluxCT) and a divergence-cleaning routine at patch boundaries to minimize magnetic divergence errors.
• Initialize the circumbinary disk from a pre-equilibrated RunSE state, modify inner regions to fit the two-patch setup, and run for ~36 binary orbits (from 20M to ~9M).
• Provide multiple resolutions (PM.IN20s and PM.IN20sHR) to test convergence and compare with single-m mesh runs.

Experimental results

Research questions

• RQ1 How does gas flow from the circumbinary disk into the two minidisks as the binary inspirals from 20M to ~9M?
• RQ2 What are the time-dependent accretion rates onto each black hole and the total onto the binary, and how do they evolve with decreasing separation?
• RQ3 What electromagnetic signatures arise from minidisks, sloshing streams, and disk tilts during late-stage inspiral?
• RQ4 How does the magnetic flux on the black hole horizons evolve, and what are implications for jet production during merger?

Key findings

• The total accretion rate onto the binary decreases by about a factor of 3–4 as separation shrinks but remains roughly steady thereafter until the final stage.
• An estimated bolometric light curve follows the same pattern as the accretion rate, suggesting potential for significant EM emission near merger despite binary evolution.
• Minidisks are dominated by Reynolds stresses rather than Maxwell stresses and exhibit cyclic state changes with episodic ‘sloshing’ streams transferring mass across the gap at ~10% of the accretion rate, carrying substantial kinetic energy.
• Mass transfer via sloshing can deposit energy into shocks, producing dissipation at rates comparable to minidisk luminosities.
• Episodic accretion drives minidisks to acquire time-varying tilts relative to the orbital plane, contributing to cyclical light curve features.
• Unsigned poloidal magnetic flux on the horizon remains φ~2–3, doubling just before merger, indicating potential for powerful jets if spin is significant.