  We model the energy dependence of a quasi periodic oscillation (QPOs)
produced by Lense-Thirring precession of a hot inner flow. We use a fully
3-dimensional Monte-Carlo code to compute the Compton scattered flux produced
by the hot inner flow intercepting seed photons from an outer truncated
standard disc. The changing orientation of the precessing torus relative to the
line of sight produces the observed modulation of the X-ray flux.
  We consider two scenarios of precession. First, we assume that the precession
axis is perpendicular to the plane of the outer disc. In this scenario the
relative geometry of the cold disc and the hot torus does not change during
precession, so the emitted spectrum does not change, and the modulation is
solely due to the changing viewing angle. In the second scenario the precession
axis is tilted with respect to the outer disc plane. This leads to changes in
the relative geometry of the hot flow and cold plasma, possibly resulting in
variations of the plasma temperature and thus generating additional spectral
variability, which combines with the variations due to the viewing angle
changes.
