Publication date: Dec 2007
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:
Stuchlík, Zdeněk; Slaný, Petr; Török, Gabriel;