Publication date: Aug 2007
In Galactic microquasars with double-peak kHz quasi-periodic
oscillations (QPOs), the ratio of the two frequencies is 3:2. This
supports the suggestion that double-peak kHz QPOs are due to a
non-linear resonance between two modes of accretion disk oscillations.
For the microquasars with known mass, we briefly compare the black hole
spin estimates based on the orbital resonance model with the recently
reported spin predictions obtained by fitting the spectral continua.
Results of these two approaches are not in good agreement. We stress
that if the spectral fit estimates are accurate and can be taken as
referential (which is still questionable), the disagreement between the
predicted and referential values would represent a rather generic
problem for any relativistic QPO model, as no spin influence would
appear in the observed 1/M scaling of the QPO frequencies. The epicyclic
frequencies relevant in these models are often considered to be equal to
those of a test particle motion. However modifications of the
frequencies due to disc pressure or other non-geodesic effects may play
an important role, and the inaccuracy introduced in the spin estimates
by the test particle approximation could be crucial.
Török, Gabriel; Abramowicz, Marek A.; Stuchlík, Zdeněk; Šrámková, Eva;