2023A&A...674A.168T

Context. We study the geometry of the transition layer (TL) between the classical Keplerian accretion disk (the TL outer boundary) and the compact object at the TL inner boundary.
Aims. Our goal is to use the hydrodynamical formalism to demonstrate that the TL is created, together with a shock, in response to a discontinuity and to adjust the Keplerian disk motion to a central object (CO).
Methods. We apply hydrodynamical equations to describe a plasma motion near a CO in the TL.
Results. We point out that, before matter accretes to a CO, the TL cloud is formed between an adjustment radius and the TL inner boundary, which is probably a site where the emergent Compton spectrum originates. Using a generalization of the Randkine-Hugoniot relation and a solution of the azimutal force balance equation, we are able to reproduce the geometric characteristics of the TL.