  XMM-Newton X-ray spectra of the hard state Black Hole X-Ray Binaries (BHXRBs)
SWIFT J1753.5-0127 and GX 339-4 show evidence for accretion disc blackbody
emission, in addition to hard power-laws. The soft and hard band Power-Spectral
Densities (PSDs) of these sources demonstrate variability over a wide range of
time-scales. However, on time-scales of tens of seconds, corresponding to the
putative low-frequency Lorentzian in the PSD, there is additional power in the
soft band. To interpret this behaviour, we introduce a new spectral analysis
technique, the `covariance spectrum', to disentangle the contribution of the
X-ray spectral components to variations on different time-scales. We use this
technique to show that the disc blackbody component varies on all time-scales,
but varies more, relative to the power-law, on longer time-scales. This
behaviour explains the additional long-term variability seen in the soft band.
Comparison of the blackbody and iron line normalisations seen in the covariance
spectra in GX 339-4 implies that the short-term blackbody variations are driven
by thermal reprocessing of the power-law continuum absorbed by the disc.
However, since the amplitude of variable reflection is the same on long and
short time-scales, we rule out reprocessing as the cause of the enhanced disc
variability on long time-scales. Therefore we conclude that the long-time-scale
blackbody variations are caused by instabilities in the disc itself, in
contrast to the stable discs seen in BHXRB soft states. Our results provide the
first observational evidence that the low-frequency Lorentzian feature present
in the PSD is produced by the accretion disc.
