8 Optical variability and the H $\vec \beta$ emission-line lag

The relatively dense sampling of the optical light curves between January and July 1996 allows us to measure the time-delayed response, or ``lag'', of the H$\beta$ emission line to continuum variations by cross-correlation of the light curves shown in Fig. 2. We used both the interpolation method of Gaskell & Sparke (1986) and the discrete-correlation function (DCF) method of Edelson & Krolik (1988), in both cases employing the specific implementation described by White & Peterson (1994). The centroid of the cross-correlation is at 16.7^+5.3_-5.6days. Uncertainties were estimated using the model-independent FR/RSS Monte-Carlo method described by Peterson et al. (1998). The cross-correlation results are shown in Table 3 and Fig. 5. For comparison, Table 3 also lists the results of previous Mrk 279 monitoring campaigns by Maoz et al. (1990) and Stirpe et al. (1994).

Figure 5: Cross-correlation function of the H$\beta$ line intensity versus the 5100 Å optical continuum flux between JD 2450095.1 and JD 2450289.7 (Fig. 2). The interpolation CCF is shown as a solid line and the points with error bars are computed using the DCF method (with a bin size of 8 days). The H$\beta$ line lags behind the continuum by about 16 days