The long term X-ray spectral variability of AGN
  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.
