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

TL;DR

This paper proposes a model where Lense-Thirring precession causes a characteristic rocking of the iron K alpha line in X-ray binaries, providing a test for disc truncation and precession theories.

Contribution

It introduces a novel prediction of iron line rocking due to precession, linking spectral features with QPOs and disc geometry in X-ray binaries.

Findings

The model predicts a blue-red shift rocking of the iron line at QPO frequency.

Archival RXTE and XMM data could potentially observe this effect.

The effect tests Lense-Thirring precession and disc truncation models.

Abstract

The clear characteristic timescale picked out by the low frequency quasi-periodic oscillations (QPOs) seen in many black hole and neutron star binaries has the potential to provide a very powerful diagnostic of the inner regions of the accretion flow. However, this potential cannot be realised without a quantitative model for the QPO. We have recently shown that the same truncated disc/hot inner flow geometry which is used to interpret the spectral transitions can also directly produce the QPO from Lense-Thirring (vertical) precession of the hot inner flow. This correctly predicts both the frequency and spectrum of the QPO, and the tight correlation of these properties with the total spectrum of the source via a changing truncation radius between the disc and hot flow. This model predicts a unique iron line signature as a vertically 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 probably sufficient) test of a Lense-Thirring origin. This is also an independent test of disc truncation models for the low/hard state, as vertical precession cannot occur if there is a disc in the midplane. We show that it may be possible to observe this effect using archival data from the Rossi X-ray timing explorer (RXTE) or XMM Newton. However, a clean test requires a combination of moderate resolution and good statistics, such as would be available from a long XMM-Newton observation or with data from the proposed ESA mission LOFT.