Spectral Models of Convection‐dominated Accretion Flows

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Abstract
For small values of the dimensionless viscosity parameter, namely α 0.1, the dynamics of nonradiating accretion flows is dominated by convection; convection strongly suppresses the accretion of matter onto the central object and transports a luminosity ~ c2 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 ∝ L2/3. Accreting black holes with X-ray luminosities 10-4LEdd LX(0.5-10 keV) 10-7LEdd are expected to have hard X-ray spectra, with photon indices Γ ~ 2, and sources with LX 10-9LEdd are expected to have soft spectra, with Γ ~ 3.5. This is testable with Chandra and XMM.

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