The power density spectrum of NGC 5548 and the nature of its variability

We derive power density spectra in the optical and X-ray bands in the time- scale range from several yr down to similar to 1 d. We suggest that the opt ical power density spectrum consists of two separate components: long time- scale variations and short time-scale variations, with the dividing time-sc ale around 100 d, The shape of the short time-scale component is similar to the X-ray power density spectrum, which is consistent with the interpretat ion of short time-scale optical variations being caused by X-ray reprocessi ng. We show that the observed optical long time-scale variability is consis tent with thermal pulsations of the accretion disc.