Optical Observations of Lensed Quasars: Multiwavelength Correlationsand Interband Time Delays

摘要

The time delays between brightness variations in different optical bands have been measured for a largenumber of Seyfert galaxies and low-redshift quasars. These time delays represent the travel time of light between differentemission regions of the source and can be used to test variability scenarios. Based on multiwavelength observations of thelensed quasars: Q2237+0305, SBS1520+530, HE2149-2745, HE1104-1805 and UM673, we explore a variabilitymechanism in the redshift range 1.4 ≤ z ≤ 2.7 . From cross-correlation analysis of the light curves of the quasars we findthat the brightness variations at longer wavelengths may follow the brightness variations at shorter wavelengths by a fewdays. An independent analysis of the multiband light curves of nonlensed quasars from the same redshift interval confirmsthe evidence of a time delay. A comparison of the observed time delays with the delays expected from a model of anaccretion disc irradiated by the central variable source shows that reprocessing might be the primary mechanismresponsible for the observed variability on the considered timescales.