GRMHD simulations of accretion flows onto unequal-mass, precessing massive binary black hole mergers

TL;DR

This study uses general relativistic magnetohydrodynamics simulations to investigate how spin orientation affects gas dynamics around merging unequal-mass black hole binaries, revealing spin-dependent accretion modulation and potential observational signatures.

Contribution

It provides the first detailed analysis of accretion flow geometry and variability in unequal-mass, precessing black hole mergers using GRMHD simulations.

Findings

Spin orientation significantly alters accretion flow geometry.

Quasiperiodic accretion rate modulations depend on spin alignment.

Stronger accretion variability correlates with higher spin precession parameter.

Abstract

In this work, we use general relativistic magnetohydrodynamics simulations to explore the effect of spin orientation on the dynamics of gas in the vicinity of merging black holes. We present a suite of eight simulations of unequal-mass, spinning black hole binaries embedded in magnetized clouds of matter. Each binary evolution covers approximately 15 orbits before the coalescence. The geometry of the accretion flows in the vicinity of the black holes is significantly altered by the orientation of the individual spins with respect to the orbital angular momentum, with the primary black hole dominating the mass accretion rate $\dot{M}$. We observe quasiperiodic modulations of $\dot{M}$ in most of the configurations, whose amplitude is dependent on the orientation of the black hole spins. We find the presence of a relation between the average amplitude of $\dot{M}$ and the spin precession parameter $\chi_{\mathrm{p}}$ showing that spin misalignment systematically leads to stronger modulation, whereas configurations with spins aligned to the orbital angular momentum damp out the quasiperiodicity. This finding suggests a possible signature imprinted in the accretion luminosity of precessing binaries approaching merger and has possible consequences on future multimessenger observations of massive binary black hole systems.