  We present the results from a combined study of the average X-ray spectral
and timing properties of 14 nearby AGN. For 11 of the sources in the sample, we
used all the available data from the RXTE archive, which were taken until the
end of 2006. There are 7795 RXTE observations in total for these AGN, obtained
over a period of ~7-11 years. We extracted their 3-20 keV spectra and fitted
them with a simple power-law model, modified by the presence of a Gaussian line
(at 6.4 keV) and cold absorption, when necessary. We used these best-fit slopes
to estimate the mean spectral slope for each object, while we used results from
the literature to estimate the average spectral slope of the three objects
without archival, monitorin RXTE data. Our results show that the AGN average
spectral slopes are not correlated either with the black hole mass or the
characteristic frequencies that were detected in the power spectra.They are
positively correlated, though, with the characteristic frequency when
normalised to the sources black hole mass. This is similar to the
spectral-timing correlation that has been observed in Cyg X-1, but not the
same.The AGN spectral-timing correlation can be explained if we assume that the
accretion rate determines both the average spectral slope and the
characteristic time scales in AGN. The spectrum should steepen and the
characteristic frequency should increase, proportionally, with increasing
accretion rate. We also provide a quantitative expression between spectral
slope and accretion rate. Thermal Comptonisation models are broadly consistent
with our result, but only if the ratio of the soft photons' luminosity to the
power injected to the hot corona is proportionally related to the accretion
rate.
