It is highly probable that a non-linear resonance between some modes of oscillations in the accretion discs around black holes and neutron stars can play a crucial role in exciting detectable modulations of the X-ray flux. Detailed studies of the resonance models revealed that several of such non-linear resonances are possible in nearly Keplerian discs in strong gravity. Moreover, this idea seems to be strongly supported by observations - in all four microquasars showing twin peak QPOs (quasiperiodic oscillations), the ratio of frequency peaks is 3:2. In principle, using known frequencies of the twin peaks and the known mass of the central black hole, the black-hole spin can be determined. This was already done for the presently known sources and few miscellaneous resonance models. Details of excitation mechanisms of eventual resonances are still not fully explained, nevertheless one can imagine that not only one resonance could be excited in the accretion disc. Thus, if two such different resonances are present (by an accident or because of some causal connection), the black hole spin can be precisely determined independently of the knowledge of the black hole mass, for some specific cases discussed here.
RAGtime 6/7: Workshops on black holes and neutron stars
- Pub Date:
- December 2005
- compact objects;
- X-ray variability;