Steps Toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei: IX. Ultraviolet Observations of Fairall 9
Rodríguez-Pascual, P.M., et al. 1997, ApJS, 110, 9

An eight-month monitoring campaign on the Seyfert~1 Fairall 9 has been conducted with the International Ultraviolet Explorer (IUE) in order to obtain reliable estimates of continuum--continuum and continuum--emission-line delays for a high-luminosity active galactic nucleus (AGN). The results are similar to those of previous campaigns on lower-luminosity AGNs: (i) there is no measurable lag between ultraviolet continuum bands, and (ii) the measured emission-line time lags are very short. It is especially notable that the Lya+NV emission-line lag is about an order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower-ionization lines, which suggests a radially ionization-stratified broad-line region (BLR). In this case, the results are less certain, since none of the emission-line lags is very well determined. The best-determined emission-line lag is Lya+NV, for which we find that the centroid of the continuum--emission-line cross-correlation function is \tau_{cent} \approx 14-20 days. We measure a lag \tau_{cent} \le 4 days for He II (lambda)1640; this result is consistent with the ionization-stratification pattern seen in lower-luminosity sources, but the relative uncertainties in the emission-line lags measured here cannot rule out similar lags for Lya+NV and He II (lambda)1640 at a high level of confidence. We are unable to determine a reliable lag for C IV (lambda)1550, but we note that the profiles of the variable parts of Lya and C IV (lambda)1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.