Effect of geometrically thin discs on precessing, thick flows: Relevance to type-C QPOs

Abstract: Type-C quasi-periodic oscillations (QPOs) are the low-frequency QPOs most commonly observed during the hard spectral state of X-ray binary systems. The leading model for these QPOs is the Lense-Thirring precession of a hot, geometrically thick accretion flow that is misaligned with respect to the black hole spin axis. However, none of the work done to date has accounted for the effects of a surrounding, geometrically thin disc on this precession, as would be the case in the truncated disc picture of the hard state. To address this, we perform a set of GRMHD simulations of truncated discs misaligned with the spin axes of their central black holes. Our results confirm that the inner-hot flow still undergoes precession, though at a rate that is only 5 percent of what is predicted for an isolated, precessing torus. We find that the exchange of angular momentum between the outer, thin and the inner, thick disc causes this slow-down in the precession rate and discuss its relevance to type-C QPOs.