Broad Emission Line Variability as a Constraint upon the Physical Conditions within the Broad Emission Line Region of NGC 5548
Using a Gaussian fitting procedure, we have constructed an accurate UV narrow emission line template spectrum for the Seyfert 1 galaxy NGC 5548, from the low-state Hubble Space Telescope/Faint Object Spectrograph UV spectrum taken on 1982 July 5 with the 1.0" circular aperture. This template spectrum is similar in form to that determined for the prototypical narrow-line Seyfert 2 galaxy NGC 1068.The narrow emission line template spectrum of NGC 5548 has for the first time enabled us to isolate the narrow and broad UV emission lines and thereby to determine firm estimates for the intensities of the broad emission lines for both the low-state spectrum and the average HST/FOS archival spectrum obtained from the intensive monitoring campaign undertaken by ``AGN Watch'' on 1993 April 19, when the continuum was a factor of ~4.8 brighter. A comparison of the low-state and high-state spectra show that while the narrow UV emission lines are nonvariable over timescales of ~10 months, the broad UV emission lines exhibit large variations in both their strength and shape. By combining a photoionization code with a robust global optimization routine, we have determined global best-fit parameters for the ``average'' physical conditions within the broad emission line region gas for both the low- and high-state spectra. By using a ``best guess'' estimate for the shape of the ionizing continuum of this source, we find that a single zone photoionization model of the broad emission line region (BLR) cannot simultaneously fit both the emission-line ratios of the strongest UV lines and their variability timescales. However, the line ratios and variability timescales can be reproduced if we assume a stratified BLR, i.e., a BLR that has strong gradients in density (NH ~ 1/r2, 1011.3-1010.0). These models also suggest that the BLR gas is in a moderately high state of ionization with log10 U ~ -0.6 in the low-state spectrum, rising to ~0.0 in the high state. We find that the observed differences in the broad emission line fluxes and their ratios, between the low- and high-state spectra, are not solely a consequence of changes in the ionizing continuum source luminosity. Rather, they imply in addition, a change in the spectral energy distribution of the ionizing continuum, although changes in either the covering fraction, or composition of the broad emission line region gas, cannot necessarily be ruled out. By constructing a simple two-zone model for the high-ionization lines, we find that in order to reproduce the observed line ratios and line equivalent widths, the gas covering fraction for this source must necessarily be high, ~38% at a radial distance of 2 lt-days and decreasing outward to ~32% at 10 lt-days. This is considerably larger than the typical value of ~10% quoted for active galactic nuclei, derived from the incidence of Lyman edges in high-redshift quasars. Although the statistics for the incidence of Lyman edges in Seyfert 1 galaxies is poorly determined, our derived covering fraction is broadly consistent with the 25% covering fraction estimate obtained from observations with the Hopkins Ultraviolet Telescope.