Abstract
The time delays between brightness variations in different optical bands have been measured for a large number of Seyfert galaxies and low-redshift quasars. These time delays represent the travel time of light between different emission regions of the source and can be used to test variability scenarios. Based on multiwavelength observations of the lensed quasars: Q2237+0305, SBS1520+530, HE2149-2745, HE1104-1805 and UM673, we explore a variability mechanism in the redshift range 1.4z2.7 . From cross-correlation analysis of the light curves of the quasars we find that the brightness variations at longer wavelengths may follow the brightness variations at shorter wavelengths by a few days. An independent analysis of the multiband light curves of nonlensed quasars from the same redshift interval confirms the evidence of a time delay. A comparison of the observed time delays with the delays expected from a model of an accretion disc irradiated by the central variable source shows that reprocessing might be the primary mechanism responsible for the observed variability on the considered timescales.
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