Steps Toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei: XVI. A Thirteen-Year Study of Spectral Variability in NGC 5548
Peterson, B.M., et al. 2002, ApJ, in press

We present the final installment of an intensive 13-year study of variations of the optical continuum and broad H-beta emission line in the Seyfert 1 galaxy NGC 5548. The data base consists of 1530 optical continuum measurements and 1248 H-beta measurements. The H-beta variations follow the continuum variations closely, with a typical time delay of about 20 days. However, a year-by-year analysis shows that the magnitude of emission-line time delay is correlated with the mean continuum flux. We argue that the data are consistent with the simple model prediction between the size of the broad-line region and the ionizing luminosity, r proportional to L_ion^1/2. Moreover, the apparently linear nature of the correlation between the H-beta response time and the nonstellar optical continuum F_opt arises as a consequence of the changing shape of the continuum as it varies, specifically F_opt proportional to F_UV^0.56.