Correlation and time delays of the X-ray and optical emission of the
  Seyfert Galaxy NGC3783
  We present simultaneous X-ray and optical B and V band light curves of the
Seyfert Galaxy NGC3783 spanning 2 years. The flux in all bands is highly
variable and the fluctuations are significantly correlated. As shown before by
Stirpe et al. the optical bands vary simultaneously, with a delay of less than
1.5 days but both B and V bands lag the X-ray fluctuations by 3-9 days. This
delay points at optical variability produced by X-ray reprocessing and the
value of the lag places the reprocessor close to the broad line region. A power
spectrum analysis of the light curve, however, shows that the X-ray variability
has a power law shape bending to a steeper slope at a time-scale ~2.9 days
while the variability amplitude in the optical bands continues to grow towards
the longest time-scale covered, ~ 300 days. We show that the power spectra
together with the small value of the time delay is inconsistent with a picture
where all the optical variability is produced by X-ray reprocessing, though the
small amplitude, rapid optical fluctuations might be produced in this way. We
detect larger variability amplitudes on long time-scales in the optical bands
than in the X-rays. This behaviour adds to similar results recently obtained
for at least three other AGN and indicates a separate source of long term
optical variability, possibly accretion rate or thermal fluctuations in the
optically emitting accretion disc.
