We present the results from the spectral analysis of more than 7,500 RXTE spectra of 10 AGN, which have been observed by RXTE regularly over a long period of time ~ 7-11 years. These observations most probably sample most of the flux and spectral variations that these objects exhibit, thus, they are ideal for the study of their long term X-ray spectral variability. We modelled the 3-10 spectrum of each observation in a uniform way using a simple power-law model (with the addition of Gaussian line and/or edge to model the iron Kalpha emission/absorption features, if necessary) to consistently parametrize the shape of the observed X-ray continuum. We found that the average spectral slope does not correlate with source luminosity or black hole mass, while it correlates positively with the average accretion rate. We have also determined the (positive) spectral slope-flux relation for each object, over a larger flux range than before. We found that this correlation is similar in almost all objects. We discuss this global spectral slope-flux trend in the light of current models for spectral variability. We consider (i) intrinsic variability, expected e.g. from Comptonization processes, (ii) variability caused by absorption of X-rays by a single absorber whose ionization parameter varies proportionally to the continuum flux variations, (iii) variability resulting from the superposition of a constant reflection component and an intrinsic power-law which is variable in flux but constant in shape, and, (iv) variability resulting from the superposition of a constant reflection component and an intrinsic power-law which is variable both in flux and shape. Our final conclusion is that scenario (iv) describes better our results.
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