Abstract

Black hole X-ray binary systems consist of a black hole stripping material from its stellar companion to generate a large X-ray flux. Quasi-periodic oscillations (QPOs) with a period in the range 0.1 to 10 seconds are routinely observed in the X-ray flux from these systems. Despite being strong signals that have been detectable for around 40 years, the physical origin of these QPOs is still debated. The leading theory associates the QPO with the relativistic effect of Lense-Thirring precession. This model requires the black hole spin axis to be moderately misaligned with the binary rotation axis, in which case the frame dragging effect is expected to induce precession in the inner accretion flow. I will review the precession model and the observational evidence in support of it, as well as discussing alternative theories. I will focus on two key predictions of the precession model. First, I will show that the iron line emitted from the inner accretion disk oscillates between red and blue shifted with QPO phase. Second, I will describe our work to determine whether the X-ray polarization angle oscillates with QPO phase, which has recently been made possible by the launch of the Imaging X-ray Polarimetry Explorer (IXPE).