  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>100R_g. In such flows, the radiative output comes mostly
from the convection part, and the radiative efficiency is independent of
accretion rate and equals ~0.001. 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 by factor
about 100 dimmer that neutron stars accreting with the same accretion rates.
