On the UV/optical variation in NGC 5548: new evidence against the
  reprocessing diagram
  The reprocessing scenario is widely adopted in literature to explain the
observed tight inter-band correlation and short lags in the UV/optical
variations of active galactic nuclei (AGNs). In this work we look into the
color variability of the famous Seyfert galaxy NGC 5548 with high quality Swift
multi-band UV/optical light curves. We find the color variation of NGC 5548 is
clearly timescale-dependent, in a way that it is more prominent at shorter
timescales. This is similar to that previously detected in quasar samples, but
for the first time in an individual AGN. We show that while a reprocessing
model with strict assumptions on the driving source and the disk size can
apparently match the observed light curves and inter-band lags, it fails to
reproduce the observed timescale dependency in the color variation. Such
discrepancy raises a severe challenge to, and can hardly be reconciled under
the widely accepted reprocessing diagram. It also demonstrates that the
timescale dependency of the color variation is uniquely powerful in probing the
physics behind AGN UV/optical variations.
