Confronting the models of 3:2 quasiperiodic oscillations with the rapid spin of the microquasar GRS 1915+105

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

Astronomy & Astrophysics, (2011), vol. 531, 07/2011, p. 6


The spectral fitting estimates of the spin a ≡ cJ/GM 2 in the microquasar GRS 1915+105 indicate a value higher than a = 0.98. However, there are certain doubts concerning this (nearly) extremal number. Resolving this issue and confirming the high value a > 0.9 would have a significant impact on the theory of high-frequency quasiperiodic oscillations (HF QPOs). Here we discuss its possible implications assuming several often quoted orbital models of 3:2 HF QPOs. We show that the estimate of a > 0.9 rather excludes two hot-spot (relativistic precession and tidal disruption) models and the warped disc resonance model. On the contrary the epicyclic resonance and discoseismic model assuming the c- and g- modes are favoured. Finally we extend the discussion on these findings also to other two microquasars that display the 3:2 HF QPOs. Frequencies of these QPOs scale roughly inversely to the microquasars masses, and the differences in the individual spins, such as a = 0.9 vs. a = 0.7, represent a generic problem for most of the discussed geodesic 3:2 QPO models. In order to explain the observations of all the three microquasars by one unique mechanism, these models would be forced to accommodate very large non-geodesic corrections.