The nature of X-ray spectral variability in Seyfert galaxies

In many Seyfert galaxies the 2-20 keV X-ray spectrum typically softens with increasing X-ray flux. This effect has been explained by two different models; either pivoting of a single spectral component or relative changes in the normalisations of two components (a hard and a soft) which each have constant shape. We present a model-independent technique to analyse spectral variability, the 'flux-flux' plot, which shows conclusively that the two-component model is correct for some Seyfert galaxies (including the well studied MCC-6-30-15) but in other Seyferts spectral pivoting applies (e.g. NGC 4051). The nature of each of the spectral components in MCG-6-30-15 are briefly examined.