The effect of frame dragging on the iron K alpha line in X-ray binaries. (arXiv:1208.0728v1 [astro-ph.HE])

The effect of frame dragging on the iron K alpha line in X-ray binaries. (arXiv:1208.0728v1 [astro-ph.HE]):
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.