How Dim Accreting Black Holes Could 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 advectiondominated only in their inner parts, but convectively dominated at radii R ∼ > 10RG. In such flows, the radiative output comes mostly from the convection part, and the radiative efficiency is independent of accretion rate and equals ǫBH ≃ 10 . 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 than neutron stars accreting with the same accretion rates. Subject headings: accretion, accretion disks — black hole physics — convection Department of Astronomy & Astrophysics, Göteborg University & Chalmers University of Technology, 412-96 Göteborg, Sweden Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester NY 14623-1299. Institute of Astronomy, 48 Pyatnitskaya Ulitsa, 109017 Moscow, Russia.