Exploring variability timescales in the X-ray emisison of active galaxies with EXOSAT and Ginga.

The authors have examined the long EXOSAT medium energy detector array observations of six well-known active galaxies and found no evidence for any periodic or quasi-periodic behaviour. Neither is there strong evidence for a shortest timescale that they could associate with, for example, the light-travel time or dynamical timescale around a massive black hole. The consequences of these findings are discussed in detail.