Radiative efficiency of collisionless accretion

The radiative efficiency, eta = L/(M) over dot c(2), of a slowly accreting black hole is estimated using a two-temperature model of accretion. The radiative efficiency depends on the magnetic field strength near the Schwarzschild radius. For weak magnetic fields, i.e., beta(-1) = B-2/8 pi p less than or similar to 10(-3), the low efficiency n similar to 10(-4) that is assumed in some theoretical models is achieved. For beta(-1) > 10(-3), a significant fraction of viscous heat is dissipated by electrons and radiated away resulting in eta > 10(-4). At equipartition magnetic fields, beta(-1) similar to 1, we estimate eta similar to 10(-1).