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, similar to5 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 (less than or similar to 1day) 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.