On the Reliability of Cross‐Correlation Function Lag Determinations in Active Galactic Nuclei

Many AGN exhibit a highly variable luminosity. Some AGN also show apronounced time delay between variations seen in their optical continuum and intheir emission lines. In effect, the emission lines are light echoes of thecontinuum. This light travel-time delay provides a characteristic radius of theregion producing the emission lines. The cross correlation function (CCF) isthe standard tool used to measure the time lag between the continuum and linevariations. For the few well-sampled AGN, the lag ranges from 1-100 days,depending upon which line is used and the luminosity of the AGN. In the bestsampled AGN, NGC 5548, the H_beta lag shows year-to-year changes, ranging fromabout 8.7 days to about 22.9 days over a span of 8 years. In this paper it isdemonstrated that, in the context of AGN variability studies, the lag estimateusing the CCF is biased too low and subject to a large variance. Thus theyear-to-year changes of the measured lag in NGC 5548 do not necessarily implychanges in the AGN structure. The bias and large variance are consequences offinite duration sampling and the dominance of long timescale trends in thelight curves, not due to noise or irregular sampling. Lag estimates can besubstantially improved by removing low frequency power from the light curvesprior to computing the CCF.Comment: To appear in the PASP, vol 111, 1999 Nov; 37 pages; 10 figure