The role of the outer boundary condition in accretion disk models: theory and application

Abstract

The influence of the outer boundary condition (OBC) on the dynamics and radiation of optically thin accretion flow is investigated. Bremsstrahlung and synchrotron radiations amplified by Comptonization are taken into account and two-temperature plasma assumption is adopted. The three OBCs we adopted are the temperatures of the electrons and ions and the specific angular momentum of the accretion flow at a certain outer boundary. We find that when the general parameters 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 as large as several orders of magnitude under different OBCs in our example. Our results indicate that OBC is both dynamically and radiatively important therefore 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 the discrepancy between the mass accretion rate favored by ADAF models in the literature and that favored by the three dimensional hydrodynamical simulation can be naturally resolved by seriously considering the outer boundary condition of the accretion flow.