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We model the energy dependence of a quasi periodic oscillation (QPOs)produced by Lense-Thirring precession of a hot inner flow. We use a fully3-dimensional Monte-Carlo code to compute the Compton scattered flux producedby the hot inner flow intercepting seed photons from an outer truncatedstandard disc. The changing orientation of the precessing torus relative to theline of sight produces the observed modulation of the X-ray flux.  We consider two scenarios of precession. First, we assume that the precessionaxis is perpendicular to the plane of the outer disc. In this scenario therelative geometry of the cold disc and the hot torus does not change duringprecession, so the emitted spectrum does not change, and the modulation issolely due to the changing viewing angle. In the second scenario the precessionaxis is tilted with respect to the outer disc plane. This leads to changes inthe relative geometry of the hot flow and cold plasma, possibly resulting invariations of the plasma temperature and thus generating additional spectralvariability, which combines with the variations due to the viewing anglechanges.

Energy spectra of X-ray quasi-periodic oscillations in the Lense-Thirring precession model