On the structure of advective accretion disks at high luminosity

Global solutions of optically thick advective accretion disks around black holes are constructed. The solutions are obtained by solving numerically a set of ordinary differential equations corresponding to a steady, axisymmet ric, geometrically thin disk. We pay special attention to consistently sati sfying the regularity conditions at singular points of the equations. For t his reason, we analytically expand the solution at the singular point and u se coefficients of the expansion in our iterative numerical procedure. We o btain consistent transonic solutions for a wide range of values of the visc osity parameter alpha and mass accretion rate. We compare results for two d ifferent prescriptions for the viscosity: the first is to assume that the s hear stress is proportional to the pressure, and the other is to assume tha t it is proportional to the gradient of the angular velocity. We find that there are two singular points in the solutions corresponding to a shear str ess proportional to the pressure. The inner singular point is located close to the last stable orbit around the black hole. This point changes its typ e from a saddle to node depending on the value of a and the accretion rate. The outer singular point is located at a larger radius and is always of th e saddle type. We argue that, contrary to the previous investigations, a no dal-type inner singular point does not introduce multiple solutions. Only o ne integral curve, which corresponds to the unique global solution, simulta neously passes the inner and outer singular points independently of the typ e of inner singular point. Solutions for the case when shear stress is prop ortional to the angular velocity gradient have one singular point which is always of the saddle type and corresponds to the unique global solution. Th e structure of accretion disks corresponding to the two prescriptions for t he viscous stress are similar.