Long-term spectral variability of Seyfert galaxies from Rossi X-Ray Timing Explorer color-flux diagrams

We present results from Rossi X-Ray Timing Explorer data obtained during a systematic monitoring program of four Seyfert galaxies (NGC 5548, NGC 5506, MCG-6-30-15, and NGC 4051). We studied the variability of three hardness ratios derived from the light curves in four energy bands (HR1, which describes the continuum variations, and HR2 and HR3, which are sensitive to the iron line and reflection component variations with respect to the continuum, respectively). All the objects show similar spectral variations in all ratios. In order to interpret the results we computed the hardness ratios corresponding to a simple spectral model of a power law plus iron line plus reflection component. In order to derive the model HR2 and HR3 colors, we considered two possibilities: (1) variations with constant line equivalent width and reflection parameter R (the case of a reflecting/reprocessing material that responds with a short delay to the continuum variability) and (2) variations with constant line and reflection flux ( the case of a reprocessor that does not respond to the fast, intrinsic variations). The overall, mean observed trends can be explained by spectral slope variations (DeltaGamma similar or equal to 0.2-0.3, and similar or equal to1 for NGC 4051) and a constant-flux Fe line and reflection component, although the existence of a line component that is variable on short timescales cannot be excluded. Finally, we find that the data are not consistent with an increase of R with flux for individual sources, indicating that, as a single source varies, softer spectra do not correspond to larger R values.