Publication date: Feb 2008
Abstract:
Change of sign of the LNRF-velocity gradient has been found for
accretion discs orbiting rapidly rotating Kerr black holes with spin a
> 0.9953 for Keplerian discs and a > 0.99979 for marginally stable
thick discs. Such a "humpy" LNRF-velocity profiles occur just above the
marginally stable circular geodesic and could be related to oscillations
of accretion discs. The frequency of such "hump"-induced oscillations
can be identified with the maximal rate of change of the orbital
velocity within the "humpy" profile. Therefore, we introduce an extended
orbital resonance model (EXORM) of quasiperiodic oscillations (QPOs)
assuming non-linear resonant phenomena between oscillations with the
orbital epicyclic frequencies and the humpy frequency defined in a fully
general relativistic way. The EXORM is developed for both Keplerian
discs and perfect-fluid tori where the approximation of oscillations
with epicyclic frequencies is acceptable. Clearly, the EXORM could be
applied to the near-extreme Kerr black hole systems exhibiting
relatively complex QPO frequency patterns. Assuming a Keplerian disc, it
can be shown that in the framework of the EXORM, all the QPOs observed
in the microquasar GRS 1915+105 could be explained, while it is not
possible in the case of QPOs observed in the Galactic Centre source Sgr
A*.
Authors:
Stuchlik, Zdenek; Slany, Petr; Torok, Gabriel;