A slim accretion disk is an optically thick, moderately geometrically thick flow at a high accretion rate. Its inflow time can be shorter than its photon-diffusion time, so radial advection carries trapped radiation inward.
A slim accretion disk is optically thick and retains an important inward advective heat flux. It is distinct from the optically thin version of a radiatively inefficient accretion flow.
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