Magnetized accretion onto rapidly spinning binary black holes: mini-disk thermodynamics, magnetic transport, and dual jets

Abstract

Supermassive binary black holes embedded in gas-rich environments are promising multi-messenger sources for pulsar timing arrays and future space-borne gravitational-wave interferometers. Their electromagnetic emission is governed by nonlinear plasma dynamics around the binary and is expected to inherit variability associated with the orbital motion thereof. Because active galactic nuclei are intrinsically stochastic, identifying robust binary signatures requires predictive models that connect large-scale circumbinary flows to the black holes. Previous relativistic simulations of accreting binaries have mostly focused on smaller separations and lower spins. We perform three-dimensional general relativistic magnetohydrodynamic simulations of a relaxed, magnetized circumbinary disk accreting onto an equal-mass binary black hole with a separation of $30$ gravitational radii and dimensionless spin $χ=0.9$. At these separations, the mini-disks around each black hole are persistent mass reservoirs but still show pronounced amplitude modulations governed by the eccentric circumbinary disk and the sloshing gas between the mini-disks. We analyze how the magnetic flux is transported from the circumbinary disk to the horizons, launching powerful dual jets with energy extraction efficiencies reaching $\approx 40\%$. The horizon-threading flux and jet luminosity alternate between the two black holes, producing an on-off dual-jet state. The wide jet funnels interact above the binary and form a persistent current sheet favorable to reconnection. We explore the influence of mini-disk thermodynamics, comparing efficient and inefficient cooling inside the cavity. Hotter mini-disks are less massive, exhibit weaker coherent periodicity, and launch less luminous jets despite comparable horizon-threading magnetic flux.