2001MNRAS.325.1527C

We use archival g- and r-band photometry of the gravitational lens system QSO 0957+561A, B to estimate the intrinsic variability of the quasar during 1996 February-June. The light curves span 234d with temporal resolutions of about 2.5d. Both light curves display a single large-amplitude event, of ∼0.1mag (max-to-min) in about 100d, followed by small-amplitude variations of ∼0.02mag on time-scales of tens of days. We find the r-band variations lag those at g by 3.4-1.4+1.5d for the large-amplitude event. This lag is greater than zero at no less than 98 per cent confidence. The delayed coupling of the rest-frame UV intrinsic variations strongly suggests the existence of a stratified reprocessing region extending ∼light-days from the putative central black hole source. The observed lag is consistent with that expected from a reverberation within an irradiated accretion disc structure. However, any definitive statement requires further detailed theoretical modelling and high-quality, signal-to-noise ratio of about 100, optical/IR simultaneous monitoring with about 3-d resolution for approximately 6 months.