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 reana lysis of the X-ray light curves of the bright Seyfert 1 galaxy NGC 554 8 obtained with EXOSAT. The 0.05-2 keV and 1-4 keV light curves are cr oss-correlated with the 1-9 keV and 4-9 keV light curves, respectively . We discuss how spurious time lags can be introduced by systematic ef fects 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 d etectors was not working normally during the second part of the 1986 M arch observation. When these effects are taken into account, the CCF p eaks 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-pe riodic oscillations (QPOs) is carried out on these spectra by using a new technique for the detection of periodic (or quasi-periodic) signal s even in the presence of source noise variability. No significant pea ks are found above the 95% confidence detection threshold, except duri ng the second part of the 1986 March observation, most probably as a c onsequence of the ME detector malfunctioning. We discuss and compare o ur results with those published by Papadakis & Lawrence in 1993.