**Publication date:** Jul 2011

**Abstract:**

Spectral fitting of the spin a ≡ cJ/GM2 in the

microquasar GRS 1915+105 estimate values higher than a = 0.98. However,

there are certain doubts about this (nearly) extremal number. Confirming

a high value of a > 0.9 would have significant concequences for the

theory of high-frequency quasiperiodic oscillations (HF QPOs). Here we

discuss its possible implications assuming several commonly used orbital

models of 3:2 HF QPOs. We show that the estimate of a > 0.9 is almost

inconsistent with two hot-spot (relativistic precession and tidal

disruption) models and the warped disc resonance model. In contrast, we

demonstrate that the epicyclic resonance and discoseismic models

assuming the c- and g-modes are favoured. We extend our discussion to

another two microquasars that display the 3:2 HF QPOs. The frequencies

of these QPOs scale roughly inversely to the microquasar masses, and the

differences in the individual spins, such as a = 0.9 compared to a =

0.7, represent a generic problem for most of the discussed geodesic 3:2

QPO models. To explain the observations of all the three microquasars by

one unique mechanism, the models would have to accommodate very large

non-geodesic corrections.

**Authors:**

Török, G.; Kotrlová, A.; Šrámková, E.; Stuchlík, Z.;