  We show that the low frequency QPO seen in the power density spectra of black
hole binaries (and neutron stars) can be explained by Lense-Thirring
precession. This has been proposed many times in the past, and simple, single
radius models can qualitatively match the observed increase in QPO frequency by
decreasing a characteristic radius, as predicted by the truncated disc models.
However, this also predicts that the frequency is strongly dependent on spin,
and gives a maximum frequency at the last stable orbit which is generally much
higher than the remarkably constant maximum frequency at ~10Hz observed in all
black hole binaries. The key aspect of our model which makes it match these
observations is that the precession is of a radially extended region of the hot
inner flow. The outer radius is set by the truncation radius of the disc as
above, but the inner radius lies well outside of the last stable orbit at the
point where numerical simulations show that the density drops off sharply for a
misaligned flow. Physically motivated analytic estimates for this inner radius
show that it increases with a_*, decreasing the expected frequency in a way
which almost completely cancels the expected increase with spin. This ties the
maximum predicted frequency to around 10Hz irrespective of a_*, as observed.
This is the first QPO model which explains both frequencies and spectrum in the
context of a well established geometry for the accretion flow.
