Continuous rapid X-ray variability and spectral changes in NGC 4051

The observation of rapid X-ray variability in active galaxies is of great potential interest. In Seyfert galaxies, variability on a time-scale of days seems common, but fast (minutes-hours) variability is apparently very rare. In a variability survey with the HEAO-1 A2 experiment, Tennant and Mushotzky (1983) found only one object to vary on short time-scales. This was NGC 6814. The only other clear demonstration of rapid variability in a Seyfert galaxy was provided by Marshall et al. (1983), who saw the soft X-ray flux from NGC 4051 double in approximately 2000 s. An unusual IPC color ratio for NGC 4051 was also reported. This observation suggests a much steeper spectrum than is normal for Type 1 Seyfert galaxies. The present investigation with the aid of the Exosat observatory had the objective to confirm and extend these observations of NGC 4051. The long orbit of Exosat permitted the collection of 8.5 uninterrupted hours of data in two independent instruments. The significance of the obtained data is evaluated.