Optical monitoring of the BL Lac object ON 231 in 1972-1990

The results of 19-year optical monitoring of ON 231 are presented. Photographic B-band observations were obtained during 123 nights from April 1972 to March 1990 as part of the quasar monitoring program carried out the Astronomical Institute of St. Petersburg State University. The new combined B-band light curve for ON 231 (1892-1999) is appreciably improved by our data, enabling us to identify a number of previously undetected high-amplitude events with time scales of weeks to months. On two nights, intraday variability with amplitudes similar to0.(m)8-0.(m)9 was observed. A number of conclusions are drawn about variability components with time scales from hours to decades. There are cases when very similar behavior on time scales from several weeks to several months was observed in the presence of substantially different base brightness levels. Both the amplitudes (similar to1.(m)5) and the characteristic shapes and overall durations of these events were the same. The constancy of the amplitudes of these events in magnitudes (i.e., the proportionality of the luminosity in rapid outbursts to the underlying level of the slowly varying component) can be explained in models that associate the variability with changes in the Doppler boosting factor due to variations in the inclination of the relativistic jet to the line of sight, if the slow and rapid components of the optical variability are associated with a steady component of the relativistic jet and shock waves (superluminal components) moving in the jet, respectively. Evidence for such a connection has been found for several superluminal sources. At the same time, there is evidence that in typical IDV events for ON 23 1, with durations of similar to1.5-2 hr, the amplitudes depend on the base brightness level of the objects, suggesting that the I DV events and less rapid components of the variability have different natures and/or occur in different locations in the source. (C) 2002 MAIK "Nauka/Interperiodica".