STEPS TOWARD DETERMINATION OF THE SIZE AND STRUCTURE OF THE BROAD-LINE REGION IN ACTIVE GALACTIC NUCLEI .9. ULTRAVIOLET OBSERVATIONS OF FAIRALL-9

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has b een conducted with the International Ultraviolet Explorer in an attemp t to obtain reliable estimates of continuum-continuum and continuum-em ission-line delays for a high-luminosity active galactic nucleus (AGN) . While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find gen eral agreement with the earlier results: (1) there is no measurable la g between ultraviolet continuum bands, and (2) the measured emission-l ine time lags are very short. It is especially notable that the Ly alp ha + Nv emission-line lag is about 1 order of magnitude smaller than d etermined from a previous campaign by Clavel, Wamsteker, & Glass (1989 ) when Fairall 9 was in a more luminous state. In other well-monitored sources, specially NGC 5548 and NGC 3783, the highest ionization line s are found to respond to continuum variations more rapidly than the l ower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-dete rmined emission line lag is Ly alpha + N v, for which;we find that the centroid of the continuum-emission-line crosscorrelation function is tau(cent)approximate to 14-20 days. We measure a lag tau(cent)less tha n or similar to 4 days for He II lambda 1640; this result is consisten t with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line l ags measured here cannot rule out similar lags for Ly alpha + N v and He II lambda 1640 at a high level of significance. We are unable to de termine a reliable lag for C IV lambda 1550, but we note that the prof iles of the variable parts of Ly alpha and C N lambda 1550 are not the same, which does not support the hypothesis that the strongest variat ions in these two lines arise in the same region.