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

We present optical broadband (B and R) observations of the Seyfert 1 nucleu s 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-c orrelation function shows a positive peak when the optical variations lead the X-rays by similar to 100 days, and anticorrelation peaks at various lea ds 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 (greater than o r similar to 30 day) components of the light curves. During the first year of this monitoring, smoothed versions of the light curves are nearly identi cal copies of each other, but scaled in amplitude and shifted in time. Howe ver, for the next 200 days, the X-ray and optical variations are clearly di fferent. During the whole period, the more rapidly changing components of t he light curves are uncorrelated at any lag. We consider the detection of t hese lags tentative and the significance of the correlations uncertain. If the 100 day delay is real, however, one interpretation is that the slowly v arying part of the X-ray emission is an echo of the optical emission, Compt on scattered from a medium located at, or extending, similar to 50-100 It-d ays from the optical source. We point out that a possibly analogous phenome non, a lag between hard and soft X-rays for a given variability timescale, exists in Galactic stellar-mass accretors. Remarkably, in both cases the la g corresponds to a light-travel distance of the order of 10(4) gravitationa l radii. Alternatively, the lag may not represent a physical size, but some other timescale. For example, it may be the manifestation of an instabilit y propagating inward in an accretion flow, appearing first in the optical a nd then in the X-rays. In any event, we observe no strong correlation at ze ro lag, or at the small positive lags expected if the optical continuum wer e produced by reprocessing of X-rays. An energetically significant reproces sed component in the optical emission of NGC 3516 is thus ruled out by our data.