  We use the XMM-Newton EPIC-pn instrument in timing mode to extend spectral
time-lag studies of hard state black hole X-ray binaries into the soft X-ray
band. We show that variations of the disc blackbody emission substantially lead
variations in the power-law emission, by tenths of a second on variability
time-scales of seconds or longer. The large lags cannot be explained by Compton
scattering but are consistent with time-delays due to viscous propagation of
mass accretion fluctuations in the disc. However, on time-scales less than a
second the disc lags the power-law variations by a few ms, consistent with the
disc variations being dominated by X-ray heating by the power-law, with the
short lag corresponding to the light-travel time between the power-law emitting
region and the disc. Our results indicate that instabilities in the accretion
disc are responsible for continuum variability on time-scales of seconds or
longer and probably also on shorter time-scales.
