On Gravitomagnetic Precession and Qpo in Black Hole Candidates
  The X-ray observation of black hole candidates provides a valuable tool to probe regions very close to the central black hole, where strong-field relativistic effects become important. Recent studies have shown that these effects seem to manifest themselves in the observed X-ray spectrum, in terms of the shape of X-ray continuum and the profile of emission lines, and in the X-ray light curves, in terms of certain quasi-periodic oscillations (QPOs). The latter will be the focus of this paper. We will review the proposed observational evidence for gravitomagnetic precession in black hole candidates, in light of new observational data and alternative models. Quantitative comparison will be made between the data and the models. We will comment on recent theoretical and observational efforts to address whether the gravitomagnetic precession of accreted, orbiting matter around a rotating black hole is a viable process for producing the observed QPOs. The results are inconclusive. We will mention a few areas where further progress can be made to possibly shed more light on the issue. 
Wei Cui (MIT), Wan Chen (GSFC/Umd), and S. N. Zhang (UAH)