  In X-ray observations of hard state black hole X-ray binaries, rapid
variations in accretion disc and coronal power-law emission are correlated and
show Fourier-frequency-dependent time lags. On short (~0.1 s) time-scales,
these lags are thought to be due to reverberation and therefore may depend
strongly on the geometry of the corona. Low-frequency quasi-periodic
oscillations (QPOs) are variations in X-ray flux that have been suggested to
arise because of geometric changes in the corona, possibly due to General
Relativistic Lense-Thirring precession. Therefore one might expect the
short-term time lags to vary on the QPO time-scale. We performed novel
spectral-timing analyses on NICER observations of the black hole X-ray binary
MAXI J1820+070 during the hard state of its outburst in 2018 to investigate how
the short-term time lags between a disc-dominated and a coronal
power-law-dominated energy band vary on different time-scales. Our method can
distinguish between variability due to the QPO and broadband noise, and we find
a linear correlation between the power-law flux and lag amplitude that is
strongest at the QPO frequency. We also introduce a new method to resolve the
QPO signal and determine the QPO-phase-dependence of the flux and lag
variations, finding that both are very similar. Our results are consistent with
a geometric origin of QPOs, but also provide evidence for a dynamic corona with
a geometry varying in a similar way over a broad range of time-scales, not just
the QPO time-scale.
