ABSTRACT
Supermassive black hole (SMBH) binaries are expected to be a key focus of next-generation gravitational wave detectors. Observations confirm that most galaxies harbor an SMBH at their centers; therefore, SMBH binaries are a natural consequence of galaxy mergers. The merger-induced enhancement of the gaseous core typically suggests that the SMBH binary will be surrounded by a gaseous structure called a circumbinary disk. Using the GPU-enabled version of the Athena++ code, AthenaK, we recently conducted a series of 3D magnetohydrodynamics simulations. We propose the concept of "Magnetically Arrested (MAD) Circumbinary Accretion" when the separation of the SMBH binary is small. In this scenario, the tidally opened cavity is filled with vertically oriented strong magnetic fields, and the disk periodically ejects magnetic flux tubes, similar to the MAD state observed for a single black hole in general relativistic MHD simulations. To connect our findings with potential observations, we carried out simulations where the SMBH binary spirals inward and merges following the post-Newtonian approximation. We demonstrate that this MAD state is inevitable for all SMBH binaries before merger and that corresponding electromagnetic signals may be observable. Additionally, we present a separate suite of simulations that zoom in from galactic scales to sub-parsec scales, showing for the first time the formation of a multi-phase, multi-scale circumbinary accretion flow self-consistently formed from large-scale accretion.
