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

Abstract

During 1997 MarchÈJuly, the Rossi X-Ray T iming Explorer observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every D12.8 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 D4.3 days. These data are used to construct the Ðrst well-determined X-ray Ñuctuation 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 Ñattening 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 cuto † in the PDS. The characteristic variability timescale corresponding to this cuto † temporal frequency is D1 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 di †erent luminosities and putative black hole masses (di †ering by more than a factor of 105) may have similar X-ray generation processes and structures.