Evidence for accretion disc reprocessing in QSO 0957+561

We use archival g- and r-band photometry of the gravitational lens system Q SO 0957+561A, B to estimate the intrinsic variability of the quasar during 1996 February-June. The light curves span 234 d with temporal resolutions o f about 2.5 d. Both light curves display a single large-amplitude event, of similar to0.1 mag (max-to-min) in about 100d, followed by small-amplitude variations of similar to0.02 mag on time-scales of tens of days. We find th e r-band variations lag those at g by 3.4(-1.4)(+1.5) d for the large-ampli tude event. This lag is greater than zero at no less than 98 per cent confi dence. The delayed coupling of the rest-frame UV intrinsic variations stron gly suggests the existence of a stratified reprocessing region extending si milar to light-days from the putative central black hole source. The observ ed lag is consistent with that expected from a reverberation within an irra diated accretion disc structure. However, any definitive statement requires further detailed theoretical modelling and high-quality, signal-to-noise r atio of about 100, optical/IR simultaneous monitoring with about 3-d resolu tion for approximately 6 months.