Spectral Models of Convection-Dominated Accretion Flows

For small values of the dimensionless viscosity parameter, namely α ∼< 0.1, the dynamics of non-radiating accretion flows is dominated by convection; convection strongly suppresses the accretion of matter onto the central object and transports a luminosity ∼ 10 − 10Ṁc 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, νpeak ∝ L . Accreting black holes with X-ray luminosities 10LEdd ∼> LX(0.5 − 10keV) ∼> 10 LEdd are expected to have hard X-ray spectra, with photon indices Γ ∼ 2, and sources with LX ∼< 10 LEdd are expected to have soft spectra, with Γ ∼ 3.5. This is testable with Chandra and XMM.