Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori

This paper presents an initial survey of the properties of accretion flows in the Kerr metric from three-dimensional, general relativistic magnetohydrodynamic simulations of accretion tori. We consider three fiducial models of tori around rotating, both prograde and retrograde, and nonrotating black holes; these three fiducial models are also contrasted with axisymmetric simulations and a pseudo-Newtonian simulation with equivalent initial conditions, to delineate the limitations of these approximations. There are both qualitative and quantitative differences in the fiducial models, with many of these effects attributable to the location of the marginally stable orbit, r_ms(a), both with respect to the initial torus and in absolute terms. In the retrograde model, the initial inner edge of the torus is close to r_ms, and little angular momentum need be lost to drive accretion, whereas in the prograde case the gas must slowly accrete over a significant distance and shed considerable angular momentum. Evolution is driven by the magnetorotational instability and the nonzero Maxwell stresses produced by the turbulence and results in a redistribution of the specific angular momentum to near-Keplerian values. The magnetic energy remains subthermal within the turbulent disk, but dominates in the final plunging flow into the hole. The Maxwell stress remains nonzero in this plunging flow inside of r_ms, and the fluid's specific angular momentum continues to drop. The accretion rate into the hole is highly time-variable and is determined by the rate at which gas from the turbulent disk is fed into the plunging flow past r_ms. The retrograde model, with the largest r_ms, shows the least variability in accretion rate. While accretion variability is a function of a, the turbulence itself is also intrinsically variable. A magnetized, backflowing corona and an evacuated, magnetized funnel are features of all models.