The Role of the Outer Boundary Condition in Accretion Disk Models: Theory and Application

The influence of the outer boundary condition (OBC) on the dynamics andradiation of optically thin accretion flow is investigated. Bremsstrahlung andsynchrotron radiations amplified by Comptonization are taken into account andtwo-temperature plasma assumption is adopted. The three OBCs we adopted are thetemperatures of the electrons and ions and the specific angular momentum of theaccretion flow at a certain outer boundary. We find that when the generalparameters such as the mass accretion rate and the viscous parameter are fixed,the peak flux at various bands such as radio, IR and X-ray, can differ by aslarge as several orders of magnitude under different OBCs in our example. Ourresults indicate that OBC is both dynamically and radiatively importanttherefore should be regarded as a new ``parameter'' in accretion disk models.We apply the above results to the compact radio source Sgr A* and find that thediscrepancy between the mass accretion rate favored by ADAF models in theliterature and that favored by the three dimensional hydrodynamical simulationcan be naturally resolved by seriously considering the outer boundary conditionof the accretion flow.Comment: 23 pages, 9 figures,accepted by the Astrophysical Journa