A possible 100 day X-ray-to-optical lag in the variations of the Seyfert 1 nucleus NGC 3516

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516, obtained at Wise Observatory from 1997 March to 1998 September contemporaneously with the X-ray 2-10 keV measurements of RXTE. The cross-correlation function shows a positive peak when the optical variations lead the X-rays by ∼100 days, and anticorrelation peaks at various leads and delays between the X-rays and the optical. We show that the putative correlation signal at 100 days is entirely due to the slow (≳30 day) components of the light curves. During the first year of this monitoring, smoothed versions of the light curves are nearly identical copies of each other, but scaled in amplitude and shifted in time. However, for the next 200 days, the X-ray and optical variations are clearly different. During the whole period, the more rapidly changing components of the light curves are uncorrelated at any lag. We consider the detection of these lags tentative and the significance of the correlations uncertain. If the 100 day delay is real, however, one interpretation is that the slowly varying part of the X-ray emission is an echo of the optical emission, Compton scattered from a medium located at, or extending, ∼50-100 lt-days from the optical source. We point out that a possibly analogous phenomenon, a lag between hard and soft X-rays for a given variability timescale, exists in Galactic stellar-mass accretors. Remarkably, in both cases the lag corresponds to a light-travel distance of the order of 10⁴ gravitational radii. Alternatively, the lag may not represent a physical size, but some other timescale. For example, it may be the manifestation of an instability propagating inward in an accretion flow, appearing first in the optical and then in the X-rays. In any event, we observe no strong correlation at zero lag, or at the small positive lags expected if the optical continuum were produced by reprocessing of X-rays. An energetically significant reprocessed component in the optical emission of NGC 3516 is thus ruled out by our data.