Spectral models of convection-dominated accretion flows

For small values of the dimensionless viscosity parameter, namely alpha less than or similar to0.1, the dynamics of nonradiating accretion hows is dominated by convection; convection strongly suppresses the accretion of matter onto the central object and transports a luminosity similar to (10(-3)-10(-2))Mc(2) from Small to large radii in the flow. A fraction of this convective luminosity is likely to be radiated at large radii via thermal bremsstrahlung emission. We show that this leads to a correlation between the frequency of maximal bremsstrahlung emission and the luminosity of the source, v(peak) proportional to L-2/3. Accreting black holes with X-ray luminosities 10(-4)L(Edd) greater than or similar toL(x) (0.5-10 keV) greater than or similar to 10(-7)L(Edd) are expected to have hard X-ray spectra, with photon indices Gamma similar to 2, and sources with L-x less than or similar to 10(-9)L(Edd) are expected to have soft spectra, with Gamma similar to 3.5. This is testable with Chandra and XMM.