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

Abstract

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.