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We present simultaneous X‐ray and optical  B ‐ and  V ‐band light curves of the Seyfert Galaxy NGC 3783 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 d but both  B  and  V  bands lag the X‐ray fluctuations by 3–9 d. 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 of ∼2.9 d while the variability amplitude in the optical bands continues to grow towards the longest time‐scale covered, ∼300 d. We show that the power spectra together with the small value of the time delay are 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 active galactic nuclei and indicates a separate source of long‐term optical variability, possibly accretion rate or thermal fluctuations, in the optically emitting accretion disc.

Correlation and time delays of the X‐ray and optical emission of the Seyfert Galaxy NGC 3783