Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. X. Variability of Fairall 9 from optical data

Abstract

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 with the IUE satellite which is described in a separate paper. The primary purpose of this program is to measure the response times (or "lags") of the emission lines to continuum variations and thus to extend the range in luminosity of active galactic nuclei (AGNs) for which such measurements have been made. The main conclusions of this work are as follows : 1. Continuum (at 5340 Å ) variations of amplitude ~12% are detected on timescales as short as ~20 days. These variations are much larger than the typical uncertainties in the measurements, which are of order ~2%. Over ~94 days, a factor of 2 change in the nuclear continuum was observed. 2. The optical continuum light curve resembles that of the UV continuum, showing two "events" of low-amplitude variations with a duration of ~70 days and with no measurable lag between the UV and optical continuum light curves. The UV data show a third larger amplitude event that occurred after the optical monitoring had terminated and unfortunately went unobserved in the optical. 3. The Hβ emission-line flux also underwent significant, low-amplitude (≥20%) variations. Crosscorrelation analysis reveals that Hβ lags behind the UV continuum by about 23 days, a value much smaller than what was previously suggested by earlier variability studies. However, this small lag is consistent with the lags for the UV lines during this campaign in the sense that the Hβ lag is approximately 50% larger than that of Lya λ1216, as it has been found for lower luminosity AGNs. 4. The Hβ 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 (~2500 km s-ˡ) and that appear to vary in phase.