Spectral Models of Convection‐dominated Accretion Flows

Abstract

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.