Twisted quasar light curves: implications for continuum reverberation mapping of accretion disks

With the advent of high-cadence and multi-band photometric monitoring facilities, continuum reverberation mapping is becoming of increasing importance for the measurement of the physical size of quasar accretion disks. The method is based on measuring the time it takes for a signal to propagate from the center to the outer parts of the central engine, assuming the continuum light curve at a given wavelength has a time shift of the order of a few days with respect to light curves obtained at shorter wavelengths. We show that with high-quality light curves, this assumption is no longer valid and that light curves at different wavelengths are not only shifted in time, but also distorted: in the context of the lamp-post model and thin-disk geometry, the multi-band light curves are, in fact, convolved by a transfer function whose size increases with wavelength. We illustrate the effect with simulated light curves in the Large Synoptic Survey Telescope (LSST) ugrizy bands and examine the impact on the delay measurements when using three different methods, namely JAVELIN, CREAM, and PyCS. We find that current accretion disk sizes estimated from JAVELIN and PyCS are underestimated by ∼30% and that unbiased measurements are only obtained with methods that properly take the skewed transfer functions into account, as the CREAM code does. With the LSST-like light curves, we expect to achieve measurement errors below 5% with a typical two-day photometric cadence.