The Disk Reverberation Mapping of the Lensed Quasar Q0957+561

The measurement of continuum time lags in lensed quasars can effectively probe the accretion physics of quasars. This is because microlensing observations of lensed quasars can provide constraints on the half-light radii of quasar accretion disks. By combining the microlensing results with time lag measurements, one can, for the first time, estimate the propagation velocity of the physical process that drives interband time lags and cross correlations among disk emission (i.e., in UV/optical bands). In this study, we perform the disk reverberation mapping study for the well-studied lensed quasar, Q0957+561. The cross correlation between the Zwicky Transient Facility g and r bands was measured; the g variations lead the r ones by 6.4 ± 2.6 days in the rest frame. In combination with the half-light radius from the existing literature, we find that the propagation velocity of the variability mechanism should be 1.7 − 0.7 + 1.5 times the speed of light. We discuss the possible outcomes of this result. Similar studies can be applied to other lensed quasars by utilizing the Legacy Survey of Space and Time observations.