Two-dimensional structure of thin transonic discs: Observational manifestations

We study the two-dimensional structure of thin transonic accretion discs in the vicinity of a nonspinning black hole within the framework of the hydrodynamical version of the Grad-Shafranov equation. Our analysis focuses on the region inside the marginally stable orbit (MSO), r < r_ms. We show that all components of the dynamical force in the disc become significant near the sonic surface and (especially) in the supersonic region. Under certain conditions, the disc structure can be far from radial, and we review the affected disc properties, in particular the role of the critical condition at the sonic surface. Finally, we present a simple model aimed at explaining quasi-periodical oscillations that have been observed in the infra-red and the X-ray radiation of the Galactic Centre.