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 g alactic nuclei (AGNs) and use this method to investigate the reality o f lags between UV and optical continuum variations in well-studied AGN s. 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. Waveleng th-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelen gth-dependent continuum lags in sources other than NGC 7469 are not ex pected to have been detectable in previous experiments. We also confir m the existence of a statistically significant lag between X-ray and U V continuum variations in the blazer PKS 2155 - 304.