How Dim Could Accreting Black Holes Be?

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Abstract
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 102RG. In such flows, the radiative output comes mostly from the convection part, and the radiative efficiency is independent of accretion rate and equals 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.

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