A comparative study of the discrete and cross correlation techniques : an application to the NGC 5548 IUE light-curve.

Abstract

Over the past few years, cross-correlation techniques have become of increasing importance in the context of active galactic nuclei (AGN) variability studies. Inference of time delays in the response of the emission line intensities with changes in the ionizing continuum flux have the potential to yield the length scale of the broad line emitting region (BLR). Here we review the capabilities of the two techniques most commonly used namely the Discrete Correlation Function (Edelson and Krolik, 1988) and the more familiar Cross-Correlation Function (CCF; see e.g. Gaskell and Peterson, 1986). Both techniques are applied to the well sampled ultraviolet continuum light-curve of the Seyfert galaxy NGC 5548, and the results are compared. We conclude that the CCF and the DCF yield essentially the same results, and that the limiting factor is the frequency and the regularity of the observations rather than the details of the technique being used to infer time delays.