  Accreting stellar-mass black holes often show a `Type-C' quasi-periodic
oscillation (QPO) in their X-ray flux, and an iron emission line in their X-ray
spectrum. The iron line is generated through continuum photons reflecting off
the accretion disk, and its shape is distorted by relativistic motion of the
orbiting plasma and the gravitational pull of the black hole. The physical
origin of the QPO has long been debated, but is often attributed to
Lense-Thirring precession, a General Relativistic effect causing the inner flow
to precess as the spinning black hole twists up the surrounding space-time.
This predicts a characteristic rocking of the iron line between red and blue
shift as the receding and approaching sides of the disk are respectively
illuminated. Here we report on XMM-Newton and NuSTAR observations of the black
hole binary H 1743-322 in which the line energy varies systematically over the
~4 s QPO cycle (3.70 sigma significance), as predicted. This provides strong
evidence that the QPO is produced by Lense-Thirring precession, constituting
the first detection of this effect in the strong gravitation regime. There are
however elements of our results harder to explain, with one section of data
behaving differently to all the others. Our result enables the future
application of tomographic techniques to map the inner regions of black hole
accretion disks.
