The stress at the ISCO of black-hole accretion discs is not a free parameter

Understand

Radiation from the "plunging region" (the space between the innermost stable circular orbit (ISCO) and the black-hole (BH) surface) of an accretion flow onto a BH is supposed to add a small but significant contribution to the X-ray spectra of X-ray binary systems.

• The plunging region and its electromagnetic emission has been recently described by numerical and analytic calculations which lead to the conclusion that in the plunging region radiation is generated by the energy release through the action of a "leftover" stress in the vicinity of the ISCO and that the amount of this leftover can be chosen as a free parameter of the accretion-flow description.
• The present article aims to demonstrate that this is not true because the stress in the whole accretion flow onto a black hole is determined by its global transonic character enforced by the space-time structure of the accreting black hole.
• We use the slim-disc approximation (SDA) to illustrate this point.