Year: 2003
Type: article
Abstract: Impressed by a widespread misunderstanding of the issue, we return to the old question concerning the location of the inner edge of the accretion disk around a black hole. We recall the fundamental results obtained in the 1970’s and 1980’s by Warsaw and Kyoto research groups that proved, in particular, that the inner edge does not coincide with the location of the innermost stable Keplerian circular orbit. We give some novel illustrations of this particular point and of some other fundamental results obtained before. To investigate the flow dynamics of the inner edge of an accretion disk, we carefully solved the structure of the transonic flow and plotted the effective potential profile based on the angular-momentum distribution calculated numerically. We showed that the flow does not have a potential minimum for accretion rates, $\dot{M} \gtrsim 10 L_\mathrm{E} / c^2$ (with $L_\mathrm{E}$ being the Eddington luminosity and $c$ being the speed of light). This property is realized even for relatively small viscosity parameters (i.e., $\alpha \sim 0.01$), because of the effect of the pressure gradient. In conclusion, an argument based on the last circular orbit of a test particle cannot give a correct inner boundary of the super-critical flow, and the inner edge should be determined in connection with radiation efficiency. The same argument can apply to optically thin ADAF. The interpretation of the observed QPO frequencies should be re-considered, since the assumption of a Keplerian rotation velocity can grossly over–or underestimate the disk rotation velocity, depending on the magnitude of the viscosity.
Authors: Ken-ya Watarai, Shin Mineshige
Institutions: Kyoto University, Yukawa Institute for Theoretical Physics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2042914279