An RXTE Survey of Long-Term X-ray Variability in Seyfert 1 Galaxies
  Data from the first three years of RXTE observations have been systematically analyzed to yield a set of 300 day, 2-10 keV light curves with similarly uniform, ~5 day sampling, for a total of nine Seyfert 1 galaxies. This is the first X-ray variability survey to consistently probe time scales longer than a few days in a large number of AGN. Comparison with ASCA data covering a similar band but much shorter (~1 day) time scales shows that all the AGN are more strongly variable on long time scales than on short time scales. This increase is greatest for the highest-luminosity sources. The well-known anticorrelation between source luminosity and variability amplitude is both stronger and shallower in power-law slope when measured on long time scales. This is consistent with a picture in which the X-ray variability of Seyfert 1s can be can be described by a single, universal fluctuation power density shape for which the cutoff moves to longer time scales for higher luminosity sources. All of the Seyfert 1s exhibit stronger variability in the relatively soft 2-4 keV band than in the harder 7-10 keV band. This effect is much too pronounced to be explained by simple models based on either the dilution of the power-law continuum by the Compton reflection component or on the hard X-rays being produced in a static, pair-dominated, plane-parallel Comptonizing corona. 
A. Markowitz, R. Edelson