The soft and medium-energy X-ray variability of NGC 5548: A reanalysis of EXOSAT observations

We present a detailed cross-correlation (CCF) and power spectrum reanalysis of the X-ray light curves of the bright Seyfert 1 galaxy NGC 5548 obtained with EXOSAT. The 0.05-2 keV and 1-4 keV light curves are cross-correlated with the 1-9 keV and 4-9 keV light curves, respectively. We discuss how spurious time lags can be introduced by systematic effects related to detector swapping as well as the switching on and off of the instruments. We also find strong evidence that one of the ME detectors was not working normally during the second part of the 1986 March observation. When these effects are taken into account, the CCF peaks are in all cases consistent with the absence of delays between X-ray variations at different energies. This is unlike the results found by several authors based on the same data. The power spectra of the 1-9 keV light curves are calculated, and a detailed search for quasi-periodic oscillations (QPOs) is carried out on these spectra by using a new technique for the detection of periodic (or quasi-periodic) signals even in the presence of source noise variability. No significant peaks are found above the 95% confidence detection threshold, except during the second part of the 1986 March observation, most probably as a consequence of the ME detector malfunctioning. We discuss and compare our results with those published by Papadakis & Lawrence in 1993.