SIMBAD references

We present the results of a systematic search for evidence of temporal changes (i.e., non-stationarity) associated with spectral variations in 3C 390.3, using data from a two-year intensive RXTE monitoring campaign of this broad-line radio galaxy. In order to exploit the potential information contained in a time series more efficiently, we adopt a multi-technique approach, making use of linear and non-linear techniques. All the methods show suggestive evidence of non-stationarity in the temporal properties of 3C 390.3 between 1999 and 2000, where the characteristic time-scale of variability decreases as the energy spectrum of the source softens. However, only the non-linear, ``scaling index method'' is able to conclusively show that the temporal characteristics of the source do vary, although the physical interpretation of this result is not clear at the moment. Our results indicate that the variability properties of 3C 390.3 may vary with time, in the same way as they do in Galactic black holes in the hard state, strengthening the analogy between the X-ray variability properties of the two types of object. This is the first time that such a behavior is detected in an AGN X-ray light curve. Further work is needed to investigate whether this is a common behavior in AGN, as it is in the Galactic binaries.