Intensive HST, RXTE, and ASCA monitoring of NGC 3516: Evidence against thermal reprocessing

During 1998 April 13-16, the bright, strongly variable Seyfert 1 galaxy NGC 3516 was monitored almost continuously with HST for 10.3 hr at ultraviolet wavelengths and 2.8 days at optical wave-lengths and simultaneous RXTE and ASCA monitoring covered the same period. The X-ray fluxes were strongly variable with the soft (0.5-2 keV) X-rays showing stronger variations (∼65% peak to peak) than the hard (2-10 keV) X-rays (∼50% peak to peak). The optical continuum showed much smaller but still highly significant variations: a slow ∼2.5% rise followed by a faster ∼3.5% decline. The short ultraviolet observation did not show significant variability. The soft and hard X-ray light curves were strongly correlated, with no evidence for a significant interband lag Likewise, the optical continuum bands (3590 and 5510 Å) were also strongly correlated with no measurable lag, to 3 σ limits of ≲0.15 day. However, the optical and X-ray light curves showed very different behavior and no significant correlation or simple relationship could be found. These results appear difficult to reconcile with previous reports of correlations between X-ray and optical variations and of measurable lags within the optical band for some other Seyfert 1 galaxies. These results also present serious problems for "reprocessing" models in which the X-ray source heats a stratified accretion disk, which then reemits in the optical/ultraviolet: the synchronous variations within the optical would suggest that the emitting region is ≲0.3 It-day across, while the lack of correlation between X-ray and optical variations would indicate, in the context of this model, that any reprocessing region must be ≳1 It-day in size. It may be possible to resolve this conflict by invoking anisotropic emission or special geometry, but the most natural explanation appears to be that the bulk of the optical luminosity is generated by some mechanism other than reprocessing.