General Relativistic Magnetohydrodynamic Simulations of Accretion Disks Around Tilted Binary Black Holes of Unequal Mass

Abstract

We perform general relativistic simulations of magnetized, accreting disks onto spinning black hole binaries with mass ratio q ≡ M1,irr/M2,irr = 1, 2 and 4. The magnitude of the individual dimensionless black hole spins are all χ = 0.26 and lie either along the initial orbital plane or 45 • above it. We evolve these systems throughout the late inspiral, merger and postmerger phases to identify the impact of the black hole spins and the binary mass ratio on any jet and their electromagnetic (Poynting) signatures. We find that incipient jets are launched from both black holes regardless of the binary mass ratio and along the spin directions as long as the force-free parameter B 2 /(8 πρ0) in the funnel and above their poles is larger than one. At large distances the two jets merge into a single one. This effect likely prevents the electromagnetic detection of individual jets. As the accretion rate reaches a quasistationary state during the late predecoupling phase, we observe a sudden amplification of the outgoing Poynting luminosity that depends on the binary mass ratio. Following merger, the sudden change in the direction of the spin of the black hole remnant with respect to the spins of its progenitors causes a reorientation of the jet. The remnant jet drives a single, high-velocity, outward narrow beam collimated by a tightly wound, helical magnetic field which, in turn, boosts the Poynting luminosity. This effect is nearly mass-ratio independent. During this process, a kink is produced in the magnetic field lines, confining the jet. The kink propagates along the jet but rapidly decays, leaving no memory of the spin-shift. Our results suggest that the merger of misaligned, low-spinning, black hole binary mergers in low-mass disks may not provide a viable scenario to explain X-shaped radio galaxies if other features are not taken into account. However, the sudden changes in the outgoing luminosity at merger may help to identify mergers of black holes in active galactic nuclei, shedding light on black hole growth mechanisms and the observed co-evolution of their host galaxies.

The Decoupling of Binaries from Their Circumbinary Disks

“…4 See also recent work along the same vein, which also included the aforementioned postmerger effects (Krauth et al 2023). Additionally, virtually all magnetohydrodynamic simulations of binary black holes, using an approximate but prescribed binary black hole metric or evolving the spacetime dynamically, have chosen initial conditions based on when the binary and disk are expected to decouple (e.g., Noble et al 2012;Farris et al 2012;Gold et al 2014;Khan et al 2018;Gutiérrez et al 2022;Combi et al 2022;Ruiz et al 2023). Overestimating the binary separation at "decoupling" by a factor of ∼2-3 may cause a mismatch between simulations and the physical systems that they intend to model.…”

The Decoupling of Binaries from Their Circumbinary Disks

“…nary black hole metric or evolving the spacetime dynamically, have chosen initial conditions based on when the binary and disk are expected to decouple (e.g., Noble et al 2012;Farris et al 2012;Gold et al 2014;Khan et al 2018;Combi et al 2022;Gutiérrez et al 2022;Ruiz et al 2023). Overestimating the binary separation at "decoupling" by a factor of ∼ 2 − 3 may cause a mismatch between simulations and the physical systems that they intend to model.…”