Multiwavelength monitoring of the narrow-line Seyfert 1 galaxy Arakelian 564. III. Optical observations and the optical-UV-X-ray connection

We present the results of a 2 yr long optical monitoring program of the nar row-line Seyfert 1 galaxy Ark 564. The majority of this monitoring project was also covered by X-ray observations (RXT E), and for a period of similar to 50 days, we observed the galaxy in UV (HST) and X-rays (RXT E and ASCA) simultaneously with the ground-based observations. Rapid and large-amplitu de variations seen in the X-ray band, on a daily and hourly timescale, were not detected at optical and UV wavelengths, which in turn exhibited much l ower variability either on short (1 day) or long (several months) timescale s. The only significant optical variations can be described as two 2-4 day events with similar to 10% flux variations. We detect no significant optica l 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 similar to0.4 days, and the optical band lags the UV band by similar to2 days. No significant correlation was found between t he entire X-ray data set and the optical band. Focusing on a 20 day interva l 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 con sistent with reprocessing models on the grounds of the energy emitted in th is single event. However, several large X-ray flares produced no correspond ing optical emission.