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During 1997 March-July, XTE observed the bright, strongly variable Seyfert 1galaxy NGC 3516 once every ~12.8 hr for 4.5 months and nearly continuously(with interruptions due to SAA passage but not Earth occultation) for a 4.2 dayperiod in the middle. These were followed by ongoing monitoring once every ~4.3days. These data are used to construct the first well-determined X-rayfluctuation power density spectrum (PDS) of an active galaxy to span more than4 decades of usable temporal frequency. The PDS shows no signs of any strict orquasi-periodicity, but does show a progressive flattening of the power-lawslope from -1.74 at short time scales to -0.73 at longer time scales. This isthe clearest observation to date of the long-predicted cutoff in the PDS. Thecharacteristic variability time scale corresponding to this cutoff temporalfrequency is 1 month. Although it is unclear how this time scale may beinterpreted in terms of a physical size or process, there are several promisingcandidate models. The PDS appears similar to those seen for Galactic black holecandidates such as Cyg X-1, suggesting that these two classes of objects withvery different luminosities and putative black hole masses (differing by morethan a factor of 10^5) may have similar X-ray generation processes andstructures.Comment: 21 pages, incl. 5 figures, AASTe

A Cutoff in the X‐Ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516