X-Ray versus Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness

Abstract

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