5. Concluding remarks
Our understanding of the accretion processes on compact objects is still far from complete. In the present paper, we demonstrated that the outcome of solutions of spherical hydrodynamic accretion processes in terms of spectral properties can differ very much depending upon the behavior of matter at the inner boundary. We have discussed the possibility that the weak hard tail in the soft states of a black hole is a result of the quasi-spherical flows and the hard tail should be absent in neutron stars and naked singularities. We also showed (Fig. 1b) that on account of their subsonic inner boundary condition, neutron stars produce much hotter radiation that the black holes or naked singularities. It is interesting to note that although the spherical flows are radiatively inefficient, they definitely have to dissipate their energy at the boundary layer, namely, in the post-shock region if the central object is a neutron star. The quasi-periodic oscillation from neutron stars could very well be due to the oscillation of this post-shock region (Molteni et al 1996). This is true even when some magnetic field is present on the neutron star surface as the radial flows may penetrate fields easier. For a black hole accretion such restriction on efficiency is not present as the flow can carry along most of its energy through the horizon.
Although our study is strictly applicable only for spherical flows, should some angular momentum be present, the solution would look similar to what is shown in Fig. 4a, which is a combination of Keplerian and sub-Keplerian flows (with or without shocks) as discussed in Chakrabarti & Titarchuk (1995) and Chakrabarti (1996b). Close to the inner boundary, the circular orbits are unstable anyway, and thus the flow would become quasi-spherical even in presence of rotation.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998