Accretion Onto Supermassive Black Hole Binaries As They Inspiral: Illumination via 3D-GRMHD Simulations

While several promising candidate supermassive binary black holes (BBH) have been identified, detailed physical modeling of these complex systems will be necessary to distinguish them from the single black hole AGN population. Although the large range of temporal and physical scales and wide parameter space have historically made their modeling prohibitive when performed via full 3D general relativistic magnetohydrodynamic simulations (3D-GRMHD), I will demonstrate the necessity of this type of modeling, and briefly describe the novel numerical tools we employ to lessen this burden. I will present our recent 3D-GRMHD simulations which push the boundaries of temporal duration, high numerical resolution, global realism including self-consistent initial conditions, include the effects of black hole spin and jet launching, and which are made possible using our multi-domain/multi-physics code, PatchworkMHD. I will describe the discovery of unexpected 3D hydrodynamical and magnetic behavior that is guiding us towards unique binary observational signatures, not expected to be present in ‘normal’ AGN spectra and light curves. An essential part of the modeling of these systems is accounting correctly for decoupling of the BBH from the surrounding accretion disk at late stages of the inspiral. I will present new insights into this process gleaned from 3D-GRMHD simulations, the longest such simulations to date. To prepare for future time-domain observational surveys like Vera Rubin Observatory’s LSST, and targeted searches of individual systems hopefully detected soon using pulsar timing arrays (PTAs), finding these unique observational signatures will be key and help us to confirm the identity of BBH systems and secure them as a corner-stone of the multi-messenger era.