Fully General Relativistic Magnetohydrodynamic Simulations of Accretion Flows onto Spinning Massive Black Hole Binary Mergers

Abstract

We perform the first suite of fully general relativistic magnetohydrodynamic simulations of spinning massive black hole binary mergers. We consider binary black holes with spins of different magnitudes aligned to the orbital angular momentum, which are immersed in a hot, magnetized gas cloud. We investigate the effect of the spin and degree of magnetization (defined through the fluid parameter $β^{-1}\equiv p_{\mathrm{mag}}/p_{\mathrm{fluid}}$) on the properties of the accretion flow. We find that magnetized accretion flows are characterized by more turbulent dynamics, as the magnetic field lines are twisted and compressed during the late inspiral. Post-merger, the polar regions around the spin axis of the remnant Kerr black hole are magnetically dominated, and the magnetic field strength is increased by a factor $\sim$10$^2$ (independently from the initial value of $β^{-1}$). The magnetized gas in the equatorial plane acquires higher angular momentum, and settles in a thin circular structure around the black hole. We find that mass accretion rates of magnetized configurations are generally smaller than in the unmagnetized cases by up to a factor $\sim$3. Black hole spins have also a suppressing effect on the accretion rate, as large as $\sim$48\%. As a potential driver for electromagnetic emission we follow the evolution of the Poynting luminosity, which increases after merger up to a factor $\sim2$ with increasing spin, regardless of the initial level of magnetization of the fluid. Our results stress the importance of taking into account both spins and magnetic fields when studying accretion processes onto merging massive black holes.