How dim could accreting black holes be?

Recent hydrodynamical simulations of radiatively inefficient black hole accretion flows with low viscosity have demonstrated that these flows differ significantly from those described by an advection-dominated model. The black hole flows are advection-dominated only in their inner parts but convectively dominated at radii R greater than or similar to 10(2)R(G). In such flows, the radiative output comes mostly from the convection part, and the radiative efficiency is independent of accretion rate and equals epsilon (BH) = 10(-3). This value gives a limit for how dim an accreting black hole could be. It agrees with recent Chandra observations, which indicate that accreting black holes in low-mass X-ray binaries are dimmer by a factor of about 100 than neutron stars accreting with the same accretion rates.