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 to measure the physical size of quasar accretion disks. The method
is based on the measurement of 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 not valid anymore 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
increase with wavelength. We illustrate the effect with simulated light curves
in the 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 $\sim30\%$ and that unbiased measurement 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 typical 2-day photometric cadence.
