2001ApJ...547..684M

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 probe consistently timescales longer than a few days in a large number of active galactic nuclei (AGNs). Comparison to ASCA data covering a similar band but much shorter (≲1 day) timescales shows that all the AGNs are more strongly variable on long timescales than on short timescales. 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 timescales. This is consistent with a picture in which the X-ray variability of Seyfert 1 galaxies can be can be described by a single, universal fluctuation power-density shape for which the cutoff moves to longer timescales for higher luminosity sources. All of the Seyfert 1 galaxies 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 either on 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.