Multiwavelength Monitoring of the Narrow-line Seyfert 1 Galaxy Akn 564. Iii. Optical Observations and the Optical–uv–x-ray Connection

We present the results of a two-year long optical monitoring program of the narrow-line Seyfert 1 galaxy Akn 564. The majority of this monitoring project was also covered by X-ray observations (RXTE) and for a period of ∼ 50 days, we observed the galaxy in UV (HST) and X-rays (RXTE & ASCA) simultaneously with the groundbased observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly time-scale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (one day) or long (several months) time-scales. The only significant optical variations can be described as two 2–4 day events with ∼ 10% flux variations. We detect no significant optical line variations and thus cannot infer a reverberation size for the broad-line region. Similarly, the large X-ray variations seem to vanish when the light curve is smoothed over a period of 30 days. The UV continuum follows the X-rays with a lag of ∼ 0.4 days, and the optical band lags the UV band by ∼ 2 days. No significant correlation was found between the entire X-ray dataset and the optical band. Focusing on a 20-day interval around the strongest optical event we detect a significant X-ray–optical correlation with similar events seen in the UV and X-rays. Our data are consistent with reprocessing models on the grounds of the energy emitted in this single event. However, several large X-ray flares produced no corresponding optical emission. Subject headings: galaxies: active – galaxies: individual (Akn 564) – galaxies: nuclei – galaxies: Seyfert – X-rays: galaxies