STScI Preprint #1201

Broad Emission-Line Variability as a Constraint upon the Physical Conditions within the BLR 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 HST/FOS UV spectrum taken on July 5th 1992 with the 1.0 arc-second 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 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 April 19th 1993, when the continuum was a factor of ~4.8 brighter. A comparison of the low-state and high-state spectra show that whilst the narrow UV emission-lines are non-variable 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 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 which has strong gradients in density (N_H  1/r², 10^11.3-10^10.0). These models also suggest that the BLR gas is in a moderately high state of ionization with log₁₀U ~-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 light-days, and decreasing outward to ~32% at 10 light-days. This is considerably larger than the typical value of ~10% quoted for AGN, 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 (HUT).

Status:

Appeared in: The Astrophysical Journal, 495:718-739, 1998

Affiliations:

Space Telescope Science Institute 3700 San Martin Drive Baltimore, MD 21218 USA