Radiative Efficiency of Collisionless Accretion

Radiative efficiency, η ≡ L/Ṁc, 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, β ≡ B/8πp ∼< 10 , the low efficiency η ∼ 10 assumed in some theoretical models is achieved. For β > 10, a significant fraction of viscous heat is dissipated by electrons and radiated away resulting in η > 10. At equipartition magnetic fields, β ∼ 1, we estimate η ∼ 10. Subject headings: accretion − hydromagnetics − turbulence