**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;