Decoupling of a supermassive black hole binary from its magnetically arrested circumbinary accretion disk

Merging supermassive black hole (SMBH) binaries will likely be surrounded by a circumbinary accretion disk. Close to merger, a gravitational radiation-driven inspiral will happen on timescales faster than the effective viscous time at the disk cavity wall, leading to a decoupling of the inner binary dynamics from the surrounding gaseous environment. Here we perform the first magnetohydrodynamics simulation of this decoupling process from a magnetically arrested circumbinary accretion disk. In this regime, the central cavity is filled with a very strong vertical magnetic flux, regulating accretion onto the binary. Our simulations identify three main stages of this process: (i) large-scale magnetic flux loss prior to decoupling, (ii) Rayleigh-Taylor-driven accretion streams onto the binary during and after decoupling, which can power magnetic towerlike outflows, resembling dual jets, and (iii) postmerger, the cavity wall becomes unstable and the magnetic flux trapped inside the cavity will get ejected in large coherent outbreak episodes with implications for potential multimessenger transients to merging SMBH binaries.