**Publication date:** Nov 2006

**Abstract:**

We show that in the equatorial plane of marginally stable thick discs

(with uniformly distributed specific angular momentum l(r, θ) =

const) the orbital velocity relative to the LNRF has a positive radial

gradient in the vicinity of black holes with a > 0.99979. The change

of sign of the velocity gradient occurs just above the center of the

thick toroidal discs, in the region where stable circular geodesics of

the Kerr spacetime are allowed. The global character of the phenomenon

is given in terms of topology changes of the von Zeipel surfaces

(equivalent to equivelocity surfaces in the tori with l(r, θ) =

const). Toroidal von Zeipel surfaces exist around the circle

corresponding to the minimum of the equatorial LNRF velocity profile,

indicating a possibility of development of some vertical instabilities

in those parts of marginally stable tori with positive gradient of the

LNRF velocity. Eventual oscillatory frequencies connected with the

phenomenon are given in a coordinate-independent form.

**Authors:**

Stuchlík, Zdeněk; Slaný, Petr; Török, Gabriel;