Three-dimensional magnetohydrodynamic simulations of spherical accretion

We present three-dimensional numerical magnetohydrodynamic simulations of radiatively inefficient spherical accretion onto a black hole. The simulations are initialized with a Bondi flow and with a weak, dynamically unimportant, large-scale magnetic field. As the gas flows in, the magnetic field is amplified. When the magnetic pressure approaches equipartition with the gas pressure, the field begins to reconnect and the gas is heated up. The heated gas is buoyant and moves outward, causing line stretching of the frozen-in magnetic field. This leads to further reconnection and more heating and buoyancy-induced motions, so that the flow makes a transition to a state of self-sustained convection. The radial structure of the flow changes dramatically from its initial Bondi profile, and the mass accretion rate onto the black hole decreases significantly. Motivated by the numerical results, we develop a simplified analytical model of a radiatively inefficient spherical flow in which convective transport of energy to large radii plays an important role. In this convection-dominated Bondi flow, the accretion velocity is highly subsonic, and the density varies with radius as rho proportional to R-1/2 rather than the standard Bondi scaling rho proportional to R-3/2. We estimate that the mass accretion rate onto the black hole correspondingly scales as (M) over dot similar to (R-in/R-a)(M) over dot(Bondi), where R-in is a small multiple of the Schwarzschild radius of the black hole and R-a is the accretion radius at which the ambient gas in the surrounding medium is gravitationally captured by the black hole. Since the factor R-in/R-a is typically very small, (M) over dot is significantly less than the Bondi accretion rate. Convection-dominated Bondi flows may be relevant for understanding many astrophysical phenomena, e.g., post-supernova fallback and radiatively inefficient accretion onto supermassive black holes, stellar-mass black holes, and neutron stars.