Circumbinary Disk Accretion into Spinning Black Hole Binaries

Abstract

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.

Mini-disk accretion onto spinning black hole binaries: quasi-periodicities and outflows

“…For relativistic binaries, the tidal truncation radius is approximately at r t ∼ 0.4r 12 (t) (Bowen et al 2017), similar to the Newtonian value, estimated to be ∼ 0.3 r 12 (Paczynski 1977;Papaloizou & Pringle 1977;Artymowicz & Lubow 1994). Since spin is a second order effect in the effective potential (Lopez Armengol et al 2021), mild values of spin do not change the truncation radius significantly for binary separations greater than 20M . The most relevant difference between our two simulations S06 and S0 is the location of the ISCO: r ISCO (χ = 0.6) = 2.82 M BHi for S06, and r ISCO (χ = 0.0) = 5.0 M BHi for S0 (both in given here in harmonic coordinates).…”

“…3.1. Overview of the system and previous studies In the steady-state of an equal-mass SMBBH, a lump orbits the edge of the circumbinary disk at an average frequency Ω lump = 0.28Ω B (Noble et al 2012;D'Orazio et al 2016;Farris et al 2015b,a;Lopez Armengol et al 2021;Noble et al 2021) modulating the accretion into the cavity. When one of the BHs passes near the lump, it peels off part of lump's inner edge, forming a stream that feeds the black hole with a beat frequency of Ω beat := Ω B − Ω lump ∼ 0.72 Ω B .…”

“…This feature is usually referred as the lump Noble et al 2012Noble et al , 2021. When the lump is formed, it behaves as a coherent m = 1 density mode orbiting the system at a frequency ∼ 0.25 Ω bin , where Ω bin is the binary frequency (Noble et al 2021;Lopez Armengol et al 2021). The time-dependence of the gas available for the accretion onto the black holes is then dominated by the lump.…”

“…More realistic simulations using MHD have been performed in Newtonian gravity Shi & Krolik 2015), in approximated GR (Noble et al 2012;Zilhão et al 2015;Lopez Armengol et al 2021;Noble et al 2021;Bowen et al 2018;Bowen et al 2019), and in full numerical relativity Farris et al 2011;Paschalidis et al 2021;Cattorini et al 2021;. MHD simulations are more computationally expensive than twodimensional α−viscous simulations because they demand three-dimensional domains and fine resolutions.…”

“…Spins also have a key role in the accretion of matter into BHs (Krolik et al 2005) and BBHs. For instance, accretion rate per unit mass near the circumbinary disk's inner edge depends on the spin, altering the mass profile in the inner part of the disk (Lopez Armengol et al 2021). Moreover, the character of the flow within the mini-disks depends on the ratio between the radius at which they are fed and the radius of the innermost stable circular orbit (ISCO) (Bowen et al 2018;Paschalidis et al 2021), which is strongly dependent on the spin.…”