Publication date: Dec 2008
Abstract:
High-frequency twin peak quasiperiodic oscillations (QPOs) are observed
in four microquasars, i.e., Galactic black hole binary systems, with
frequency ratio very close to 3:2. In the microquasar GRS 1915+105 the
structure of QPOs exhibits additional frequencies and more than two
frequencies are observed in the Galaxy nuclei Sgr A* or in some
extragalactic sources (NGC 4051, MCG-6-30-15 and NGC 5408 X-1). The
observed QPOs can be explained by a variety of the orbital resonance
model versions assuming resonance of oscillations with the Keplerian
frequency νK or the vertical epicyclic frequency
νθ, and the radial epicyclic frequency νr, or some
combinations of these frequencies. Generally, different resonances could
arise at different radii of an accretion disk. However, we have shown
that for special values of dimensionless black hole spin a strong
resonant phenomena could occur when different resonances can be excited
at the same radius, as cooperative phenomena between the resonances may
work in such situations. The special values of a are determined for
triple frequency ratio sets
νK:νθ:νr=s:t:u with s,t,u being small
integers. The most promising example of such a special situation arises
for black holes with extraordinary resonant spin a=0.983 at the radius
r=2.395 M, where νK:νθ:νr=3:2:1. We
also predict that when combinations of the orbital frequencies are
allowed, QPOs with four frequency ratio set 4:3:2:1 could be observed in
the field of black holes with a=0.866,0.882 and 0.962. Assuming the
extraordinary resonant spin a=0.983 in Sgr A*, its QPOs with observed
frequency ratio ≍3:2:1 imply the black hole mass in the interval
4.3×106 Msolar< M< 5.4×106
Msolar, in agreement with estimates given by other,
independent, observations.
Authors:
Stuchlík, Z.; Kotrlová, A.; G. Török;