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

During 1997 March-July, the Rossi X-Ray Timing Explorer observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every ~12.8 hours for 4.5 months and nearly continuously (with interruptions due to South Atlantic Anomaly passage but not Earth occultation) for a 4.2 day period in the middle. These were followed by ongoing monitoring once every ~4.3 days. These data are used to construct the first well-determined X-ray fluctuation power density spectrum (PDS) of an active galaxy to span more than four decades of usable temporal frequency. The PDS shows no signs of any strict or quasi-periodicity but does show a progressive flattening of the power-law slope from -1.74 at short timescales to -0.73 at longer timescales. This is the clearest observation to date of the long predicted cutoff in the PDS. The characteristic variability timescale corresponding to this cutoff temporal frequency is ~1 month. Although it is unclear how this timescale may be interpreted in terms of a physical size or process, there are several promising candidate models. The PDS appears similar to those seen for Galactic black hole candidates such as Cyg X-1, suggesting that these two classes of objects with very different luminosities and putative black hole masses (differing by more than a factor of 10⁵) may have similar X-ray generation processes and structures.