Spectral Models of Convection‐dominated Accretion Flows

For small values of the dimensionless viscosity parameter, namely$\alpha\lesssim 0.1$, the dynamics of non-radiating accretion flows isdominated by convection; convection strongly suppresses the accretion of matteronto the central object and transports a luminosity $\sim 10^{-3}-10^{-2} \dotM c^2$ from small to large radii in the flow. A fraction of this convectiveluminosity is likely to be radiated at large radii via thermal bremsstrahlungemission. We show that this leads to a correlation between the frequency ofmaximal bremsstrahlung emission and the luminosity of the source, $\nu_{\rmpeak} \propto L^{2/3}$. Accreting black holes with X-ray luminosities $10^{-4}L_{Edd}\gtrsim L_X(0.5-10{\rm keV}) \gtrsim 10^{-7}L_{Edd}$ are expected tohave hard X-ray spectra, with photon indices $\Gamma\sim2$, and sources with$L_X\lesssim 10^{-9}L_{Edd}$ are expected to have soft spectra, with$\Gamma\sim3.5$. This is testable with {\it Chandra} and {\it XMM}.Comment: final version accepted by ApJ; significant modifications from previous versio