The swan song in context: long-time-scale X-ray variability of NGC 4051

On 1998 May 9-11, the highly variable, low-luminosity Seyfert I galaxy NGC 4051 was observed in an unusual low-flux state by BeppoSAX, RXTE and EUVE. We present fits of the 4-15 keV RXTE spectrum and BeppoSAX MECS spectrum ob tained during this observation, which are consistent with the interpretatio n that the source had switched off, leaving only the spectrum of pure refle ction from distant cold matter We place this result in context by showing t he X-ray light curve of NGC 4051 obtained by our RXTE monitoring campaign o ver the past two and a half years, which shows that the low state lasted fo r similar to 150 d before the May observations (implying that the reflectin g material is >10(17) cm from the continuum source) and forms part of a lig ht curve showing distinct variations in long-term average flux over time-sc ales > months. We show that the long-time-scale component to X-ray variabil ity is intrinsic to the primary continuum and is probably distinct from the variability at shorter time-scales. The long-time-scale component to varia bility maybe associated with variations in the accretion flow of matter on to the central black hole. As the source approaches the low state, the vari ability process becomes non-linear. NGC 4051 may represent a microcosm of a ll X-ray variability in radio-quiet active galactic nuclei (AGNs), displayi ng in a few years a variety of flux states and variability properties which more luminous AGNs may pass through on time-scales of decades to thousands of years.