Simultaneous X-ray and Ultraviolet spectroscopy of the Seyfert galaxy   NGC 5548. III. X-ray time variability
  The Seyfert 1 galaxy NGC 5548 was observed for a week by Chandra using both the HETGS and LETGS spectrometers. In this paper we study the time variability of the continuum radiation. During our observation, the source showed a gradual increase in flux over four days, followed by a rapid decrease and flattening of the light curve afterwards. Superimposed upon these relatively slow variations several short duration bursts or quasi-periodic oscillations occured with a typical duration of several hours and separation between 0.6-0.9 days. The bursts show a delay of the hard X-rays with respect to the soft X-rays of a few hours. We interprete these bursts as due to a rotating, fluctuating hot spot at approximately 10 gravitational radii; the time delay of the hard X-rays from the bursts agree with the canonical picture of Inverse Compton scattering of the soft accretion disk photons on a hot medium that is relatively close to the central black hole. 
J.S. Kaastra, K.C. Steenbrugge, D.M. Crenshaw, S.B. Kraemer, N. Arav,   I.M. George, D.A. Liedahl, R.L.J. van der Meer, F.B....