Adam IngramChris Done Michiel van der Klis

The effect of frame dragging on the iron line in X-ray binaries

Low frequency quasi-periodic oscillations (QPOs) seen in many black hole and neutron star binaries have the potential to be very powerful diagnostics of the inner accretion flow. However, this potential cannot be realised without a quantitative model for the QPO. It has recently been shown that the same truncated disc/hot inner flow geometry which is used to interpret spectral transitions can also directly produce the QPO from Lense-Thirring precession of the hot inner flow. This model gives a unique prediction in its iron line signature: a tilted flow illuminates different azimuths of the disc as it precesses. The iron line arising from this rotating illumination is blue shifted when the flow irradiates the approaching region of the spinning disc and red shifted when the flow irradiates the receding region of the disc. This gives rise to a characteristic rocking of the iron line on the QPO frequency which is a necessary and sufficient test of a precession origin. I will talk about the prospects of confirming the precession model by detecting this effect with current and upcoming missions but show that detailed study of this effect, including precision measurements of system parameters, will only be possible with the Large Observatory for X-ray Timing (LOFT).