Abstract
An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-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 general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Ly? + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) 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. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Ly? + N V, for which we find that the centroid of the continuum-emission-line cross-correlation function is ?cent ? 14-20 days. We measure a lag ?cent 4 days for He II ?1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Ly? + N V and He II ?1640 at a high level of significance. We are unable to determine a reliable lag for C IV ?1550, but we note that the profiles of the variable parts of Ly? and C IV ?1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.
Highlights
With the discovery of coordinated continuum and emission-line variability in active galactic nuclei (AGNs), it was realized by many authors that variability might provide a very valuable tool for unveiling the structure of the innermost regions of these extremely luminous objects
NEWSIPS includes a weighted slit extraction method (Kinney, Bohlin, & Neill 1991) and rederived absolute Ñux calibrations (Gonza lez-Riestra, Cassatella, & de la Fuente 1992). All of these improvements result in higher photometric accuracy and higher signal-to-noise ratio spectra, when compared to the data processed with the old software (IUESIPS ; Turnrose & Thompson 1984)
A Ðrst test was made by checking the FES recentering errors after those exposures that were not guided ; a benchmark test made with an International Ultraviolet Explorer (IUE) standard star has shown that up to ^25 units in the X-axis and ^7 units in the Y -axis can be accepted as tolerance limits to ensure that the target did remain in the aperture (Rodr guez-Pascual 1997)
Summary
With the discovery of coordinated continuum and emission-line variability in active galactic nuclei (AGNs), it was realized by many authors that variability might provide a very valuable tool for unveiling the structure of the innermost regions of these extremely luminous objects. The International Ultraviolet Explorer (IUE) is extremely well suited to this task, which motivated the establishment of international consortia in the late 1980s to conduct a series of major observing campaigns using both IUE and ground-based facilities to obtain the necessary spectroscopic observations The largest of these groups, the International AGN Watch (Alloin et al 1994), has completed multiwavelength programs on NGC 5548 (Clavel et al 1991, [Paper I in this series] ; Peterson et al 1991 [Paper II], 1992 [Paper III], 1994 [Paper VII] ; Dietrich et al 1993 [Paper IV] ; Maoz et al 1993 ; Romanishin et al 1995 ; Korista et al 1995 [Paper VIII]), NGC 3783 (Reichert et al 1994 [Paper V] ; Stirpe et al 1994 [Paper VI] ; Alloin et al 1995), and NGC 4151 (Crenshaw et al 1996 ; Kaspi et al 1996 ; Warwick et al 1996 ; Edelson et al 1996). The general results of these AGN monitoring campaigns can be summarized as follows : 1. The UV and optical continua vary simultaneously within the accuracy of the data and the sampling frequency (typically ^2 days or less)
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