General Relativistic 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 that arranges itself into a circumbinary disk (CBD). Numerical simulations have shown that gas accretion onto the binary happens in streams flowing from the CBD into "minidisks" orbiting each of the two BHs, possibly generating jets. However, to date, simulations in full general relativity are too computationally expensive to be run long enough for the CBD to reach a steady accretion regime, while simulations adopting an approximate spacetime metric can be run for much longer, but break down when approaching merger.

In this talk, I will discuss a novel way to overcome the above limitations. First, the SphericalNR code is used to run a CBD simulation on a uniform grid with curvilinear coordinates until the accretion rate onto the binary becomes stable and turbulence settles in. Then, MHD fields are interpolated onto a Cartesian grid, and the resulting system is evolved down to merger using the IllinoisGRMHD code. Finally, shortly after merger, all the spacetime and MHD fields are interpolated onto a uniform grid with curvilinear coordinates, and the evolution proceeds once again with SphericalNR. A simple radiation cooling model is applied in all of the three stages of the simulation process.

I will present the first set of comprehensive inspiral-merger-postmerger simulations of equal-mass SMBBHs, focusing on the effects of black hole spin on the structure and dynamics of the minidisks and jets. Minidisks survive until close to merger, with the survival time depending on the size and direction of the spins. Two very energetic jets are formed, each one powered by a minidisk; this is especially evident if the spins are aligned with the orbital angular momentum. As the minidisks disappear, the jets are quenched and eventually switched off; a single jet is then generated shortly after merger. I will present both a qualitative and a quantitative analysis of the simulations, comparing the accretion rate, enclosed mass, and Poynting flux profiles at different locations and for different spin setups and discussing a few other relevant diagnostics. In summary, SMBBH systems are found to yield potentially observable EM signals.