GRMHD simulations of gas accretion onto merging supermassive black hole binaries*

The merger of two galaxies is expected to result in a supermassive binary black hole ('SMBBH') system surrounded by gas. SMBBH emit gravitational radiation, which could be detected by next-generation gravitational wave ('GW') detectors (LISA, PTAs). However, SMBBH could also emit electromagnetic ('EM') radiation through accretion of gas onto the binary; detecting such radiation and predicting its signatures by means of numerical simulations is key to telling SMBBH and regular AGNs apart and to direct GW detectors to the correct location in the sky.

Due to its large angular momentum, the gas surrounding the binary arranges itself into a circumbinary disk ('CBD'). Numerical simulations have shown that accretion happens in streams of material flowing from the lump into "mini-disks" orbiting each of the two black holes; however, to date, simulations in full general relativity are too computationally expensive to be run long enough for the system to reach a steady accretion regime and are thus unrealistic. On the other hand, simulations adopting an approximate spacetime metric can be run for much longer, but they become less and less accurate as the binary shrinks until they break down close to merger.

In this talk, I will present a novel way to overcome the above limitations. First, a long-term, full-3D, magnetized CBD simulation is run with the SphericalNR code adopting curvilinear coordinates and a post-Newtonian metric, excising the region containing the binary; this simulation is run long enough for the accretion rate to stabilize and for turbulence to develop, thus providing an astrophysically realistic scenario. Then, MHD quantities are interpolated onto a Cartesian grid and the excised region is filled; finally, the system is evolved using the IllinoisGRMHD code. EM radiative cooling is taken into account by estimating the cooling timescale in different ways in the CBD, in the mini-disks and in the cavity between the CBD and the mini-disks. I will highlight the effect of both cooling and black hole spin on the features and dynamics of the mini-disks by showing the results of the first set of SMBBH merger simulations with realistic initial conditions; all simulations are currently running on the Frontera supercomputer.