Thirty Years of Continuum and Emission‐Line Variability in NGC 5548

Abstract

We analyze a total of 827 optical spectra of NGC 5548 obtained over the 30 year period 1972-2001 for the purpose of studying the long-term behavior of the broad Hβ line profile. The variability characteristics, such as mean continuum and emission-line fluxes, rms variability amplitude, autocorrelation functions, and cross-correlation results, are found to be similar for CCD spectra obtained during the period 1989-2001 and for archival pre-CCD spectra from 1972 to 1988. The only significant difference between these two sets of data is that the mean Hβ flux is higher for 1972-1988. While the Hβ profile can vary dramatically on timescales of months to years, the mean profiles for these two periods are similar, but not identical. We searched for correlations among Hβ profile parameters (such as line width, line centroid, blue-to-red ratio of the line wing fluxes) and investigated their relationship to the continuum flux. Only trivial correlations were found, specifically the correlation between continuum and line fluxes and the anticorrelation between line width and line flux, the latter following from the former and from the inverse correlation between line time lag and line width expected for gravitationally dominated motion.