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.;