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 nucleusNGC 3516, obtained at Wise Observatory from 1997 March to 1998 September,contemporaneously with the X-ray 2-10 keV measurements of RXTE. The crosscorrelation function shows a positive peak when the optical variations lead theX-rays by about 100 days. We show that the putative correlation signal at 100days is entirely due to the slow ($\gtorder 30$ days) components of the lightcurves. During the first year or this monitoring, smoothed versions of thelight curves are nearly identical copies of each other, but scaled in amplitudeand shifted in time. However, for the next 200 days, the X-ray and opticalvariations are clearly different. During the whole period, the faster-changingcomponents of the light curves are uncorrelated at any lag. The detection ofthis lag is tentative and the significance of the correlation uncertain. If the100-day delay is real, however, one interpretation is that that theslowly-varying part of the X-ray emission is an echo of the optical emission,Compton scattered from a medium located about 50-100 lt days from the opticalsource. We point out that a possibly analogous phenomenon, of a lag betweenhard and soft X-rays for a given variability timescale, exists in Galacticstellar-mass accretors. Remarkably, in both cases the lag corresponds to alight travel distance of order $10^4$ gravitational radii. Alternatively, thelag may not represent a physical size, but for example, the timescale of aninstability propagating inward in an accretion flow, appearing first in theoptical and then in the X-rays. In any event, there is no strong correlation atzero, or small positive, lag, ruling out an energetically-significant X-rayreprocessed component in the optical emission.Comment: To appear in AJ, January 2000 issue. AAS LaTex, 14 pages, 6 figure