X-Ray versus optical variations in the Seyfert 1 nucleus NGC 3516: A puzzling disconnectedness

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516 obtained at Wise Observatory from 1997 March to 2002 March, contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data we increase the temporal baseline of this data set to 5 years, more than triple the coverage we have previously presented for this object. Analysis of the new data does not confirm the 100 day lag of X-ray behind optical variations tentatively reported in our previous work. Indeed, excluding the first year's data, which drove the previous result, there is no significant correlation at any lag between the X-ray and optical bands. We also find no correlation at any lag between optical flux and various X-ray hardness ratios. We conclude that the close relation observed between the bands during the first year of our program was either a fluke or perhaps the result of the exceptionally bright state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results of published joint X-ray and UV optical Seyfert monitoring programs, we speculate that there are at least two components or mechanisms contributing to the X-ray continuum emission up to 10 keV: a soft component that is correlated with UV optical variations on timescales greater than or similar to1 day and whose presence can be detected when the source is observed at low enough energies (similar to1 keV), is unabsorbed, or is in a sufficiently bright phase, and a hard component whose variations are uncorrelated with the UV optical.