On uncertainties in cross-correlation lags and the reality of wavelength-dependent continuum lags in active galactic nuclei

We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from the light curves of active galactic nuclei (AGNs) and use this method to investigate the reality of lags between UV and optical continuum variations in well-studied AGNs. Our results confirm the existence of such lags in NGC 7469, We find that the continuum variations at 1825, 4845, and 6962 Angstrom follow those at 1315 Angstrom by 0.22(-0.13)(+0.12), 1.25(-0.35)(+0.48), and 1.84(-0.94)(+0.93) days, respectively, based on the centroids of the cross-correlation functions; the error intervals quoted correspond to 68% confidence levels, and each of these lags is greater than zero at no less than 97% confidence. We do not iind statistically significant interband continuum lags in NGC 5548, NGC 3783, or Fairall 9. Wavelength-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelength-dependent continuum lags in sources other than NGC 7469 are not expected to have been detectable in previous experiments. We also confirm the existence of a statistically significant lag between X-ray and UV continuum variations in the blazer PKS 2155 - 304.