Rapid X-Ray Amplitude and Spectral Variability in AGN Evidence of Non-Stationary Power Spectra from ASCA Observations

We examine the X-ray properties of several of the most rapidly variable active galactic nuclei observed by ASCA. The sources exhibit large amplitude variability on timescales of hundreds of seconds and some of the sources show associated spectral variability. With reference to X-ray observations with previous missions we can also assess the longer-term behaviour. The results are discussed in terms of the implications for physical models of the central engines and how future monitoring missions can help to constrain such models.