Soft and hard X-ray variability from the accretion disk of NGC 5548.

EXOSAT observations of the Seyfert galaxy NGC 5548 confirm previous detections of a soft X-ray excess above a power law component. Both components show correlated variability on a typical time scale of half a day. There is evidence for a delay of the hard X-rays with respect to the soft X-rays of 1-2h. The spectral data can be explained in terms of an accretion disk surrounding a 10^7^ M_sun_ black hole. The accretion disk emits locally a modified blackbody spectrum, with a strong maximum in the emissivity at a distance of 5 Schwarzschild radii from the black hole. This radiation is visible as the soft X-ray excess. The hard X-rays are generated by Comptonisation in a hot corona surrounding the innermost edge of the disk (from 3-4 Schwarzschild radii). The observed soft- and hard X-ray variability can be explained by thermal instabilities in the disk, propagating from the region of maximum soft X-ray emissivity inwards, hence causing the observed delay of the power law component as compared to the soft X-ray excess. No evidence for strong Fe line emission is found.