High-rate spectroscopic active galactic nucleus monitoring at the wise observatory. II. NGC 5548

We present further results of the spectroscopic monitoring of AGNs, described in Paper I of this series, for the Seyfert galaxy NGC 5548. The high sampling rate (44 spectra during a 149 day period) and measurement accuracy (3%-4% in absolute flux) of our observations allow the resolution of a small pulse in Hα, Hβ, and the optical continuum in this object during May-June of 1988. Cross-correlation of the line and continuum light curves yields a very significant peak at a lag of 7 days in the emission line response to continuum variations. Using Monte-Carlo simulations, we estimate the size of the broad line region (BLR) in this galaxy to be 4-10 It-days with a probability of 68%. Our results rule out, at the 95% significance level, a BLR 14 It-days or larger for several different assumed gas distributions. This is an additional BLR size measurement discrepant by an order of magnitude with the predictions of "standard" photoionization theory. Several possible solutions to this problem are briefly discussed.