Abstract

The accretion disc is a key component in the X-ray spectra of black hole X-ray binaries, but until now the disc has not made itself heard above the noise variations of the power-law continuum component on time-scales of seconds or less. At least within the soft states, the disc appears to be relatively stable on these short time-scales. Here, I will present evidence for accretion disc variability in the hard states of black hole X-ray binaries. I will show how reflection signatures can be used to disentangle intrinsic disc variations from the variable blackbody emission due to heating by the power-law. Using this approach and including new data on GX 339 -4 from XMM -Newton, I will demonstrate that the disc shows intrinsic variations on time-scales of seconds, corresponding to the low-frequency Lorentzian seen in the power spectrum. This correspondence suggests that the disc itself produces this timing signature, and hence connects the disc to all other timing signatures whose frequencies correlate with that of the low-frequency Lorentzian. I will also describe possible links between the unstable disc and the production of jets in the hard states, and discuss the energy-dependent time-lags associated with the intrinsic disc variability, which can constrain the radial drift time-scales in the inner disc.