**Publication date:** Aug 2007

**Abstract:**

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.

**Authors:**

Török, Gabriel; Abramowicz, Marek A.; Stuchlík, Zdeněk; Šrámková, Eva;