STEPS TOWARD DETERMINATION OF THE SIZE AND STRUCTURE OF THE BROAD-LINE REGION IN ACTIVE GALACTIC NUCLEI .10. VARIABILITY OF FAIRALL-9 FROM OPTICAL-DATA

The results of an optical monitoring campaign on the active nucleus in the luminous Seyfert 1 galaxy Fairall 9 are presented. This campaign was undertaken in parallel with ultraviolet spectroscopic monitoring w ith the IUE satellite which is described in a separate paper. The prim ary purpose of this program is to measure the response times (or ''lag s'') of the emission lines to continuum variations and thus to extend the range in luminosity of active galactic nuclei (AGNs) for which suc h measurements have been made. The main conclusions of this work are a s follows: 1. Continuum (at 5340 Angstrom) variations of amplitude sim ilar to 12% are detected on timescales as short as similar to 20 days. These variations are much larger than the typical uncertainties in th e measurements, which are of order similar to 2%. Over similar to 94 d ays, a factor of 2 change in the nuclear continuum was observed. 2. Th e optical continuum light curve resembles that of the UV continuum, sh owing two ''events'' of low-amplitude variations with a duration of si milar to 70 days and with no measurable lag between the UV and optical continuum light curves. The UV data show a third larger amplitude eve nt that occurred after the optical monitoring had terminated and unfor tunately went unobserved in the optical. 3. The H beta emission-line f lux also underwent significant, low-amplitude (greater than or equal t o 20%) variations. Crosscorrelation analysis reveals that H beta lags behind the UV continuum by about 23 days, a value much smaller than wh at was previously suggested by earlier variability studies. However, t his small lag is consistent with the lags for the UV lines during this campaign in the sense that the H beta lag is approximately 50% larger than that of Lyx lambda 1216, as it has been found for lower luminosi ty AGNs. 4. The H beta difference profile produced by subtracting the low-state from the high-state data can be described as a two-component structure with blue and red components of similar width (similar to 2 500 km s(-1)) and that appear to vary in phase.